Addition of Hydrogen and Hydrogen Cyanide to Carbon–Carbon Double and Triple Bonds

51 Addition of Hydrogen and Hydrogen Cyanide to Carbon-Carbon Double and Triple Bonds B. R. JAMES University of British Columbia

51.1 ACTIVATION AND ADDITION OF HYDROGEN 51.1.1 Introduction 51.1.2 Historical Overview 51.1.3 Overview of Mechanisms 51.1.3.1 Activation of hydrogen and hydridometal catalysts 51.1.3.2 Activation of the unsaturated organic substrate and hydrogen transfer 51.1.3.3 Hydrogen transfer from ligands and solvents 51.1.3.4 Transition metal clusters, dimers and hydrogenases 51.1.4 Practical Homogeneous Catalysts 51.1.4.1 General: monoenes, dienes and polyenes 51.1.4.2 Ziegler systems 51.1.4.3 Hydrogenation of unsaturated fats 57.7.5 Hydrogenation of Aromatic Hydrocarbons 51.1.6 Hydrogenation of Alkynes 57.7.7 Photocatalysis 51.1.8 Supported Catalysts, Membrane Systems, Phase-transfer Catalysis and Molten Salt Systems 57.7.9 Concluding Remarks 51.2 ACTIVATION AND ADDITION OF HYDROGEN CYANIDE 57.2.7 Introduction 51.2.2 Hydrocyanation of Alkenes 51.2.3 Mechanism of Alkene Hydrocyanation 51.2.4 Hydrocyanation of A Ikynes 57.2.5 Concluding Remarks REFERENCES

51.1 51.1.1

285 285 286 289 290 305 323 326 332 332 340 342 345 347 350 351 352 353 353 354 357 359 360 360

ACTIVATION AND ADDITION OF HYDROGEN Introduction

Expansion in the field of homogeneous catalysis in solution has been phenomenal in the last 20 years, and this has been particularly so in the case of carbon-bonded substrates due to major developments in organometallic chemistry. All types of homogeneous catalysis are concerned with reactions of coordinated ligands. The organometallic area, excluding metal carbonyls, involves coordination of organic molecules such as alkenes, alkynes and aromatic and saturated hydrocarbons. The organic moieties frequently undergo reactions not observed for the free species; alkene isomerization, oligomerization, polymerization and disproportionation (metathesis) provide familiar examples (see Chapters 52 and 54). Concomitant activation at the metal centre of small inorganic molecules (e.g. dihydrogen, carbon monoxide and dioxygen) along with the organic substrate can lead, in these cases, to catC.O.M.C. VOL. 8—J*

285

286

Addition ofH2 and HCN to C=C and C=C Bonds

alytic hydrogenation, carbonylation/hydroformylation and oxygenation/oxidation, respectively, of the substrate. The relative simplicity of testing systems for hydrogenation {e.g. C2H2 —> C2H4 -* C2H6) must certainly be one of the reasons for the vast literature on the subject. The simplicity may be contrasted, for example, with the activation of dioxygen which is a much more diverse and complex subject because of the many pathways available for O2 reactions.1'2 The monograph 'Homogeneous Hydrogenation',3 which attempted to cover the literature exhaustively to the end of 1970 and incorporated some 1971 and 1972 references, cited nearly 2000 references, and the volume of literature on the subject has at least doubled since that time. The intense research effort has advanced the knowledge of some catalytic hydrogenations to a very sophisticated level: the homogeneous system involves a discrete molecular catalyst, usually a monomer, that can be studied in detail by the usual spectroscopic techniques under catalytic conditions, since the catalysts usually operate at mild conditions of temperature and pressure, which also leads to greater selectivity in product formation. Geometrical and stereochemical features of organometallic intermediates have been fully elucidated in certain cases. The kinetics and thermodynamics of many individual steps within an overall complex system have been determined. Such detailed understanding aids in catalyst design in which the associated ligands, solvent, conditions, etc., can be varied in a controlled manner, usually in attempts to control selectivity. Some brief general comments on the hydrogenation of organic compounds catalyzed heterogeneously at metal surfaces seem in order, since this remains one of the most useful and versatile tools for the synthetic organic chemist. Good thermal stability, few solvent restrictions and ease of separation from reaction products all represent significant advantages for heterogeneous catalysts, particularly in engineering and economic aspects of commercial chemical processes. The systems (supported and unsupported), however, are difficult to study, a major limitation being reproducibility of the active surface, and there remains controversy over even quite fundamental steps. For example, a recently suggested mechanism4 for alkene hydrogenation involves hydrogen transfer between an absorbed carbonaceous residue [M—C2HX] and adsorbed alkene, rather than the more classical addition of adsorbed hydrogen to adsorbed alkene. The heterogeneous systems require a study of surfaces, both in the absence and presence of the reacting molecules; low-energy electron diffraction (LEED), Auger and other forms of electron- and photo-emission spectroscopy, extended X-ray absorption fine structure (EXAFS) and electron microscopy are being used principally,5"8 but the techniques are usually applicable at low pressure conditions, very different to those used in the catalysis. Nevertheless, despite the complexity and mystique of catalytic heterogeneous hydrogenation, use of the mass of accumulated, largely empirical data, dating back even to the nineteenth century, has allowed for selective reductions with a high degree of stereochemical control and with considerable predictability.9 The long list of reviews on hydrogenation, including specialized texts and book sections, that have appeared to the end of 1977 may be traced through references 3 and 10. Most senior inorganic texts published since the mid-1960s and organic texts since about 1972 contain a section on homogeneous hydrogenation. Quite recent additions to this literature are found in reviews, 1120 inorganic texts 21 and more specialized texts in catalysis and organometallic chemistry. 9 ' 2227 It is worth listing separately recent books28'29 and reviews30*31 that have been aimed at presenting to practising organic chemists the principles and applications of transition metal complexes as hydrogenation catalysts; the continuing series by Fieser and Fieser32 is useful for tracing current applications of such catalysts in organic syntheses. Some of the continuing Specialist Periodical Reports 33 and the Annual Reports of The Royal Society of Chemistry, 34 as well as the MTP series,35 are useful for keeping abreast of advances in the field. 51.1.2 Historical Overview Despite the now very extensive literature on homogeneous hydrogenation, prior to 1954 only two systems had been claimed to involve homogeneous activation of molecular hydrogen by metal complexes. The first was that reported in 1938 by Calvin on the copper(I) acetate-catalyzed reduction by H 2 of copper(II) to copper (I), or benzoquinone to quinhydrone, in quinoline solution. 36 ' 37 The second pertained to the hydroformylation (oxo) reaction discovered by Roelen38 (equation 1) and related hydrogenation reactions catalyzed by cobalt carbonyl complexes; an insensitivity of the systems to sulphur compounds, which typically poison heterogeneous systems, seemed consistent with a homogeneous process, and considerable work was done later39 to establish fully that [CoH(CO)4] was the solution phase catalyst; interest continues in this system and, for

Addition ofH2 and HCN to C=C and C=C Bonds

287

example, quite recently the often proposed coordinatively unsaturated species [CoH(CO) 3 ] was detected in a low temperature matrix. 40 RCH=CH 2

+

H 2 •+

CO

C 2(CO)8

°

>

RCH2CH2CHO

+

RCH(CHO)Me

(1)

It should be noted that some of the systems reported as early as 1909 by Ipatieff and Werchowsky41 on the precipitation of metals or metal oxides by H2 reduction of dissolved metal salts were later shown to involve homogeneous activation of H2, particularly with copper(II) in aqueous solution.42 Iguchi's work in 193943 on hydrogenation of inorganic and organic substrates using rhodium(III) complexes in aqueous solution probably involved heterogeneous catalysis via trace metal; 44 his early observations on the absorption of H2 by cobalt(II) cyanide solutions and their hydrogenating ability were not followed up until much later, when the potential of the very widely studied pentacyanocobaltate(II) catalyst was developed.45 Some principal foundations of the homogeneous hydrogenation field were laid by Halpern's group during 1953-61. 18 The work, stemming from hydrometallurgical studies involving precipitation of metals from solution using H2, initially led to detailed kinetic studies that showed activation of H2 at a single copper(II) centre in solution; other metal ions and complexes including, in chronological order, those of silver(I), mercury(I), mercury(II), permanganate, palladium(II), rhodium(III), ruthenium(III) and ruthenium(II) were shown to react homogeneously with H 2 . 18 The ruthenium(II) system, first reported in 1961, catalyzed the hydrogenation of activated alkenes (maleic acid) at 80 °C/1 atm H2, and appears to be the first well characterized example of homogeneous hydrogenation of an alkenic substrate, although Zeise's dimer [{PtCl2(C2H4)2)2] had been reported somewhat earlier to convert ethylene-hydrogen mixtures to ethane at low temperatures (<—15 °C). 4 6 Addition of tin(II) chloride to the platinum(II) systems led in the early 1960s47 to the first effective catalyst for reduction of ethylene at ambient conditions, while during the same period, boranes were found to catalyze under more severe conditions the hydrogenation of other terminal alkenes.48 It was during the mid- to late-1950s also that stabilized transition metal hydrides, e.g. [ReHCp2] and /ra«s-[PtHCl(PEt 3 ) 2 ], although initially not formed from H 2 , were first being discovered and characterized,49 which added credence to the postulates of reactive hydridometal intermediates in the solution hydrogen activation studies of that era. Previously known carbonyl hydrides such as [HCo(CO) 4 ] and [H 2 Fe(CO)4] had been difficult to characterize because of thermal instability.50 Nevertheless, very important contributions to the field of hydrogenation came from studies during the 1950-60 period on cobalt carbonyl-catalyzed hydroformylation and hydrogenation reactions, 39 in particular the important mechanistic work of Heck and Breslow.51 The discovery in 1962 by Vaska and Diluzio52 of the reversible binding of H2 (and other gas molecules) by an iridium(I) complex to give the iridium(III) dihydride (equation 2) was significant in that it led to the reaction classification of oxidative addition to square planar complexes, and the reverse, reductive elimination from octahedral complexes, processes that are now clearly recognized as being critical in hydrogenation and homogeneous catalytic reactions in general. fAWW-IrCl(CO)(PPh3)2

+

H2

^=

IrH2Cl(CO)(PPh3)2

(2)

It is worth noting the report in 1963 by a group at Hercules53 on the use of Ziegler-type catalysts, e.g. [R 3 A1/Cr(acac) 3 ] combinations, as considerable interest has since developed in the use of these systems commercially for partial hydrogenation of polymers (Section 51.1.4.2). Many of the reports during 1955-62 on the more active hydrogenation catalysts and the isolated hydrides involved platinum metal complexes, and it is in this area that subsequent intense activity has resulted. The discovery of 'Vaska's compound', together with that of the very efficient hydrogenating ability of [RhCl(PPh 3 ) 3 ] in 1965 by both Coffey 54 and Wilkinson'sgroup, 55 and that of [RuHCl(PPh 3 ) 3 ] in the same year again by Wilkinson's group, 56 has led to a deluge in the literature reporting on rhodium, iridium and ruthenium complexes with tertiary organophosphine-type ligands, sometimes together with one or more carbonyl ligands. Rhodium carbonyl complexes containing phosphines and phosphites were found to be very active under mild conditions for selective formation of the desired linear aldehydes in hydroformylation of alkenes (equation

288

Addition ofH2 and HCN to C=C and C=C Bonds

1), and a commercial process based on such findings was developed by Union Carbide and became operational in 1976.57'58 The truly remarkable success story of the development of catalysts for asymmetric hydrogenation from Wilkinson-type systems (equation 3) using chiral phosphine ligands, particularly with rhodium, for the commercial production of optically active amino acids 1019 ' 26 ' 59 ' 60 is summarized in Chapter 53 of this volume. CO 2 H +*

.

CO 2 H

Cnir3.lc3.t3.lyst

CO 2 H

^^ ^^^.^. ^^^S

NHCOR

^-^ ^-^^^ s~+^^'

NHCOR

/ o \

NH 2

Attempts to utilize commercially some of the extremely effective homogeneous catalysts discovered in the 1960s led to a 'heterogenizing' of such species and development of the area of supported transition metal complex catalysts.61 The idea of binding homogeneous catalysts generally to solid-phase supports via ionic or covalent bonds was first reported in the patent literature by Mobil workers in 1969-70,62 although the use of complexes adsorbed onto silica gel and other supports (van der Waals interactions) had been documented earlier.63 Systems for hydrogenation have been reviewed,10 and the area of polymer-supported catalysis is considered more generally in Chapter 55 of this volume. Enzyme systems can be considered as heterogenized homogeneous catalysts, the support being a large, semi-ordered protein polymer, and an interesting recent development is the incorporation of an achiral rhodium(I) phosphine moiety, covalently bound to biotin, at a specific site in the protein avidin. This system effects asymmetric hydrogenation by way of chirality within the biotin-avidin 'support',64 and such studies on semi-synthetic enzymes and the general modelling of metalloenzymes continue to attract interest. Catalytic hydrogenation using [RhCl(PPh3)3] within a bilayer now offers potential for studying properties of cell membranes (Section 51.1.8), and such work relates indirectly to the very promising methods now being developed for catalyst separation using various phase transfer techniques (Section 51.1.8). Studies involving comparisons of homogeneous and heterogeneous catalysis have always been popular,65 but these intensified in the 1970s,66 partly as a result of developments in the supported catalysis area, but mainly because of an upsurge of interest in metal clusters and their potential as catalysts.67 These discrete molecular species (Section 51.1.3.4) potentially provide a bridge between the heterogeneous multimetal-site systems and the homogeneous, usually single-site, systems. The resurgence of interest in clusters (and hydrogenation catalysts generally) resulted, to a large extent, from the arrival of the energy crisis. A partial solution involves a greater utilization of coal for the production of gaseous and liquid fuels and petrochemicals; conversion of coal to synthesis gas (CO and H2), and subsequent reduction of the CO by H2 to methane, alcohols and hydrocarbons via commercial Fischer-Tropsch-type syntheses, presently use heterogeneous catalysts.68"72 More selective homogeneous catalysts appear attractive, and clusters are being used10'68'69'73 in attempts to emulate the heterogeneous catalysts which, simply because of geometrical considerations at the surface, must involve more than one metal atom. Nevertheless, mononuclear homogeneous complexes such as [CoH(CO)4], [MnH(CO)s] and [Ru(CO)5] have been found effective for methanol synthesis from CO and H 2 . 74 ' 75 Interest in the transition metal .cyclopentadienyl hydrides of Groups IV and V grew in the 1970s, due in part to the potential of the very hydridic hydride ligand (compared, for example, with the more acidic ones of Group VIII metal hydrides) to reduce CO.68 A catalyzed H2 reduction of carbon dioxide offers an alternative route to methanol.76'77 A further important old reaction to be rejuvenated since 1975 is the water-gas shift reaction (equation 4), which provides a way of increasing H 2 : CO ratios in synthesis gas, or of producing H2.68 Some extremely effective homogeneous catalysts have been found recently, and CO in aqueous media can be used with catalysts for the hydrogenation of alkenes and other substrates based on the stoichiometry of equation (4). 10 ' 78 CO

+

H2O ^

CO2 +

H2

(4)

Hydrogenation via the water-gas shift reaction involves an overall net hydrogen transfer from solvent water to substrate (Section 51.1.3.3). A mechanistically quite different hydrogen transfer catalysis occurs with a wide range of organic solvents, the net stoichiometry being simply that of equation (5). The topic seems to be attracting more attention, due mainly to increasing diversity in effective solvents (e.g. alcohols, aldehydes, amides, acids, ethers, amines and aromatic hy-

Addition ofH2 and HCN to C=C and C=C Bonds

289

drocarbons) rather than any novelty in the transition metal catalysts, again predominantly those discovered in the 1960s. Solvent

+

Substrate (S) —•

(e.g. isopropanol)

Dehydrogenated solvent

+

SH2

(5)

(e.g. acetone)

The energy crisis has also contributed to renewed interest in the photochemistry of metal complexes, one aim, for example, being a chemical photosynthesis: the light-induced splitting of water molecules to give H2 and O2. 79 ' 80 This has further encouraged photochemical studies on transition metal hydrides (and complexes known to activate H2) and has led to several new photoassisted catalytic hydrogenations;10'81 see Section 51.1.7. The number of really 'new' hydrogenation catalysts (particularly for addition to carbon-carbon double and triple bonds and excluding chiral ligand systems), following the proliferation during the 1960s, has been relatively small, although notable advances have been made in the reduction of aromatic compounds, 10 ' 82 which could again be important for coal-based chemical processes. Some very detailed mechanistic studies have now firmly established reaction pathways postulated for some hydrogenation catalysts of the 1960s, while other recent studies83 suggest that free radical mechanisms may have been overlooked in some earlier systems. Finally, at the other end of the scale in terms of characterized systems, the enzyme hydrogenase is beginning to reattract attention following isolation of purified forms of the enzyme.10 Extensive reference is made to a 1973 book,3 and a recent review that contains references to 1977-78. 10 The book references are broken down to within individual chapters and it should be straightforward using these and the extensive index to trace the original references on any particular system. The review references are similarly fragmented to within the various sections so that individual systems can again be traced quite readily. Complexes within tables are listed within Group triads from I to VII followed by the Group VIII triads.

51.1.3 Overview of Mechanisms The uncatalyzed addition of H 2 to an alkenic or alkynic bond, although thermodynamically favourable, is symmetry forbidden in the ground state via a concerted cis addition (Figure I). 8 4 8 5 Electrons cannot flow from H 2 to the empty 7r*-orbital because there is no net overlap. However, the J-orbitals of a transition metal have the correct symmetry to react directly with H 2 , and electron flow from the filled ^/-orbital into the empty (7*-orbital of H 2 dissociates the H—H bond and forms two metal-hydride bonds; the hydrogen atoms can now be transferred, at least in principle, in one step to an alkene (Figure 1). One role of the catalyst is certainly to circumvent symmetry restrictions, although the mechanism of simultaneous transfer of two H atoms to a non-coordinated substrate as envisioned in Figure 1 has not been proven thus far (see Section

H+

H

Figure 1 Symmetry-forbidden addition of H2 to a double bond and metal-catalyzed allowed concerted addition of two H atoms to a double bond

An uncatalyzed stepwise process involving successive addition of H atoms is clearly unfavourable because of the high bond dissociation of H 2 both in the gaseous phase (435 kJ mol" 1 ) and in water (ca. 420 kJ mol" 1 ). 86 Of interest, the energy required for the heterolytic splitting of H 2 in water (equation 6) is only about 155 kJ mol" 1 , indicating plausible hydrogen activation via such a heterolytic mechanism particularly in polar solvents, and work has been reported on metal-free base-catalyzed isotope exchange (H 2 -D 2 O) and hydrogenation of aromatic ketones (but not C = C bonds) via hydride attack and subsequent protonation (equation 7). 87

290

Addition ofH2 and HCN to C=C and C=C Bonds H2(aq)

H2

+

OH-

—*

H"

H"

+

—•* H+(aq)

+

H"(aq)

(6)

+ H2O; R2CO

^=^

R 2 CHO"

^ 2 -

R2CHOH

+

OH~

(7)

The catalyzed hydrogenation of unsaturated carbon-carbon bonds inevitably involves the formation of intermediate metal hydrides and this will now be considered.

51.1.3.1 Activation of hydrogen and hydridometal catalysts

(/) Oxidative addition (homolytic splitting) of hydrogen A common and readily demonstrated mode of H 2 activation is by oxidative addition. This is shown generally in equation (8) for addition to an initial univalent metal complex, while equation (2) gives a specific example. Such reactions have been discussed extensively88"90 and are written as a direct concerted H 2 addition via a three-centre transition state (see Section 51.1.3). Examples are particularly common for addition to a single ds metal centre in a square planar complex (especially Rh1 and Ir1) since this results in the favoured octahedral d6 configuration. M1

+

H2

MinH2

^

(8)

Table 1 gives examples of more well-characterized dihydrides that have been used for catalytic hydrogenation of unsaturated organic substrates, although the [PtH 2 (PEt 3 ) 3 ] complex, an example of a formal dl0-*d* synthesis, decomposes at sub-zero temperatures and its potential for catalysis was not reported. The transient species listed seem very plausible intermediates in dihydride formation, e.g. during the synthesis of [MoH 2 Cp 2 ] from [MoCl 2 Cp 2 ] (equation 9). In the complexes listed under Simple addition and Transients, the dihydride formula is given simply by adding H 2 to the listed complex; thus [IrH 3 L 2 ] forms the pentahydride [IrHsL 2 ]. In some cases, dihydride formation is accompanied by loss of one ligand, for example, during H 2 addition to a five-coordinate ds system. In the examples given in Table 1, the dissociated ligand appears in bold type; thus, [RuH 2 (PPh 3 ) 4 ] forms [RuH 4 (PPh 3 ) 3 ], while [IrO(CO)(PMe 2 Ph) 3 ] gives [IrH 2 (CO)(PMe 2 Ph) 3 ]+.

MoCl2Cp2

^ ^

'MoCp2'

-^

MoH2Cp2

(9)

Considering a hydride ligand as a formally - 1 unit (but see below), the examples listed in Table 1 include, as well as the more familiar d]0 -* ds and ds - * d6 processes, d9 -+ d1, d6 -* d4, d4 - * d2, d3 -* d2 (counting cleavage of the metal-metal bond in [{Mo(77-C5Me5)2}2]) and d2 - * d° oxidative addition processes. It should be noted that with the exception of d6 complexes containing hydride ligands, e.g. [RuH 2 (PPh 3 ) 4 ], examples of further H 2 addition to such systems are rare; such ad6 —* d4 step is commonly invoked during catalytic hydrogenation of alkenes using Ru(II) complexes (Section 51.1.3.2,1). An indirect example, however, comes from the synthesis of the catalytically inactive complexes [OsH 2 Cl 2 (PR 3 ) 3 ] via amalgam reduction of mer[OsCl 3 (PR 3 ) 3 under H 2 , which probably involves H 2 addition to an Os(II) intermediate.91 Many other mononuclear complexes directly form dihydrides that are catalytically inactive at least under mild conditions, presumably because the metal-hydride bonds are thermodynamically or kinetically too stable. Some examples, mainly involving ds systems, are given in Table 2. A d1 metal complex can attain the d6 octahedral configuration via a net H 2 addition to two such centres, a classic case being that of pentacyanocobaltate(II) (equation 10). This example of oxidative addition of H 2 at two metal centres is included in Table 3 together with other systems 2Co H (CN) 5 3 - or Co2n(CN)io6-

+

H2 ^

2Co m H(CN) 5 3-

(10)

Addition ofH2 and HCN to C=C and C=C Bonds

291

Table 1 Oxidative Addition of H 2 at a Single Metal Centre to give Catalytically Active Dihydrides: M + H2^MH2a Dihydride precursor (ref.) Simple addition Ti(T7-C5Me5)2] (1); [FeH2L3] (2, 3); [CoHL3] (4, 5); [RhXL3]b, [|RhXL2}2] (7, 8); RhCl(PBu5)2] (9); [RhCl(PPh 3 ) 2 (CH 2 =CHCN)] (10); [Rh{PhNC(S)NMe2j(PPh3)2] (11); Rh(dppp)2]+, [Rh(dppb) 2 ] + , [Rh(diop)2]+ (12); [RhXL2(diene)]c (13); ML2S2]+ d, M = Rh, Ir (14-16); [M(phen)S2]+ e, M = Rh, Ir (20-22); [MCl(PCy3)2], M = Rh, Ir (23); {MCl}2-M-(p-{(Ph2PCH2CH2)2P}C6H4)], M = Rh, Ir (24); trans-[\xX(CO)L2Y (16, 25); IrH 3 L 2 ], [Ir(diphos)2]+ (15, 16); [Ir(PPh3)(py)S2]+, [Ir(cod)2]+, [Ir(Ph2PMe)2(cod)] + (27); IrCl(PPh3)2(cod)]8 (28); [Ir2(M-S)(CO)2(dppm)2] (29); [Pt(PEt3)3] (30); [Pt|PBu 2 (CH 2 ) 3 PBu 2 }] 2 h(31) With loss of one ligand (indicated in bold) [Cr{P(OR)3}6] (32); [{Mo(7?-C5Me5)2}2]i (33, 34); [FeH2(N2)(PEtPh2)3] (2, 35); Ru(CO) 2 (PPh 3 ) 3 ] (36); [RuH 2 (N 2 )(PPh 3 ) 3 ], [RuH2(PPh3)4] (16, 37); CoH(N 2 )(PPh 3 ) 3 ], [Co(N2)(PPh3)3] (4,16); [IrCl(CO)(PMe2Ph)3] (38); [IrH(CO)3(PPr3)] (39); [IrH(CO)(PPh 3 ) 3 ] (15, 16,40) Transients [Zr(77-C5Me5)2] (41); [NbHCp2]J (42); [TaHCp2] (43); [MCp2], M = Mo, W (33, 34); [Fe(CO)4]? (44, 45); [RuL4] (16, 37, 46); [Ru(diphos)2] (47); [Os3(CO)10] (48); [CoL3(r;i-C3H5)] (49); [Pd(PPh3)w] (50); [Pt(diphos)] (31) a Unless stated otherwise, X = anion (usually halide); L = monodentate tertiary phosphine, arsine or phosphite; S = solvent. b Mixed phosphine ligand systems have also been used.6 c Product is [RhH2XL2], L = P(/?-ClC6H4)3. d Formed from diene precursors such as [ML2(cod)]+; with M = Rh, L2 also = (RCN)(PR3),17'18 (py)(PR3);16>19 S = solvent or L. Dihydrides well substantiated only with bis(phosphine) systems. e Formed from diene precursors such as [ML 2 (cod)] + ; with M = Rh, L2 also = bipy, (amine)2, (nitrile)2;17'18 S = solvent. Dihydrides not well substantiated. f X here includes halides, NCO, NCS, N 3 , NO 3 , SnCl3, GeR3, cr-carboranes.26 « Product is [(cod)Ir(M-Cl)2IrH2(PPh3)2]. h Product is d5-[PtH2{PBu2(CH2)3PBu2j]. * Product is [MoH^-CsMeshl; process visualized as involving dissociation of a [Mo(77-C5Me5)2] ligand. J Forms not only the trihydride by addition of H 2 but also loses H 2 reversibly to give [|NbHCp(C5H4)}2], which probably contains bridging rj5:775-fulvalene (cf structure 1). 1. J. E. Bercaw, R. H. Marvich, L. G. Bell and H. H. Brintzinger, J. Am. Chem. Soc, 1972, 94,1219. 2. V. D. Bianco, S. Doronzo and M. Aresta, /. Organomet. Chem., 1972, 42, C63. 3. W. E. Newton, J. L. Corbin, P. W. Schneider and W. A. Bulen, J. Am. Chem. Soc, 1971, 93, 268. 4. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter X, Section G. 5. M. C. Rakowski and E. L. Muetterties, /. Am. Chem. Soc, 1977,99, 739. 6. R. L. Augustine and R. J. Pellet, J. Chem. Soc, Dalton Trans., 1979, 832. 7. Ref. 4, Chapter XI, Section B. 8. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIA. 9. T. Yoshida, S. Otsuka, M. Matsumoto and K. Nakatsu, Inorg. Chim. Ada, 1978, 29, L257. 10. Y. Ohtani, A. Yamagishi and M. Fujimoto, Bull. Chem. Soc Jpn., 1978, 51, 2562. 11. A. W. Gal and F. H. A. Bolder, /. Organomet. Chem., 1977,142, 375. 12. B. R. James and D. Mahajan, Can. J. Chem., 1979, 57, 180. 13. L. A. Oro and J. V. Heras, Inorg. Chim. Acta, 1979, 32, L37. 14. Ref. 4, Chapter XI, Section G. 15. Ref. 4, Chapter XII, Section B. 16. Ref. 8, Sections IIB, IIC. 17. R. Uson, L. A. Oro, J. Artigas and R. Sariego, /. Organomet. Chem., 1979,179, 65. 18. R. Uson, L. A. Oro, M. C. Carmen and P. Lahuerta, Transition Met. Chem., 1979, 4, 55. 19. R. H. Crabtree and H. Felkin, /. Mol. Catal, 1979, 5, 75. 20. Ref. 8, Section XI. 21. G. Mestroni, G. Zassinovich and A. Camus,J. Organomet. Chem., 1977,140, 63. 22. A. Camus, G. Mestroni and G. Zassinovich, /. Mol. Catal, 1979, 6, 231. 23. S. Hietkamp, D. J. Stufkens and K. Vrieze, J. Organomet. Chem., 1978,152, 347. 24. M. M. T. Khan, S. S. Ahmed and Rafeequnnisa, in 'Proceedings of the 16th International Conference on Coordination in Chemistry', 1974, Dublin, Abstract R39 {Chem. Abstr., 1976, 85, 40 280). 25. Ref. 4, Chapter XII, Section A. 26. B. Longato, F. Morandini and S. Bresadola, Inorg. Chem., 1976,15, 650. 27. R. Crabtree, Ace. Chem. Res., 1979,12, 331. 28. M. Gargano, P. Giannoccaro and M. Rossi, J. Organomet. Chem., 1977,129, 239. 29. C. Kubiak, C. Woodcock and R. Eisenberg, Inorg. Chem., 1980,19, 2733. 30. D. H. Gerlach, A. R. Kane, G. W. Parshall, J. P. Jesson and E. L. Muetterties, J. Am. Chem. Soc, 1971,93, 3543. 31. T. Yoshida, T. Yamagata, T. H. Tulip, J. A. Ibers and S. Otsuka, J. Am. Chem. Soc, 1978,100, 2063. 32. S. D. Ittel and C. A. Tolman, U. S. Pat. 4 155 925 (1979) (Chem. Abstr., 1979, 91, 59 608). 33. J. L. Thomas and H. H. Brintzinger, J. Am. Chem. Soc, 1972, 94, 1386. 34. J. L. Thomas, J. Am. Chem. Soc, 1973, 95, 1838. 35. E. Koerner von Gustorf, I. Fischler, J. Leitich and H. Dreeskamp, Angew. Chem., Int. Ed. Engl., 1972,11,

292 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

Addition ofH2 and HCN to C=C and C=C Bonds

1088. F. Porta, S. Cenini, S. Giordano and M. Pizzotti, J. Organomet. Chem., 1978,150, 261. Ref. 4, Chapter IX, Section B. J. Y. Chen and J. Halpern, J. Am. Chem. Soc, 1971, 93,4939. R. Whyman, J. Organomet. Chem., 1975, 94, 303. M. G. Burnett and C. J. Strugnell, J. Chem. Res. (S), 1977, 250. J. E. Bercaw, Adv. Chem. Ser., 1978,167,136. F. N. Tebbe and G. W. Parshall, / . Am. Chem. Soc, 1971, 93, 3793. E. K. Barefield, G. W. Parshall and F. N. Tebbe, J. Am. Chem. Soc, 1970, 92, 5234. Ref. 4, Chapter XIV, Section B. M. A. Schroeder and M. S. Wrighton, J. Am. Chem. Soc, 1976, 98, 551. S. Komiya and A. Yamamoto, J. Mol. Catal, 1979, 5, 279. P. Pertici, G. Vitulli, W. Porzio and M. Zocchi, Inorg. Chim. Ada, 1979, 37, L521. J. B. Keister and J. R. Shapley, / . Am. Chem. Soc, 1976,98,1056. Ref. 8, Section VII. A. S. Berenblyum, L. I. Lakhman, 1.1. Moiseev and E. D. Radchenko, Koord. Khim., 1976, 2, 841. Table 2 Complexes that form Catalytically Inactive Dihydrides via Oxidative Addition of H2: M + H? — MH9 Dihydride precursor {ref.) Simple addition + [RuCl(NO)(PPh 3 ) 2 ] a (1); [Co(diphos) 2] + (2); [M(m-Ph 2 PCH=CHPPh 2 ) 2 ] , M = Co, Ir (3); [Rh(Me 2 PCH 2 CH 2 PMe 2 ) 2 ] + , [Rh(m-Ph 2 AsCH=CHAsPh 2 ]+ (4); [IrCl(PPh3)3] (5); [IrCl{P(C6H9)Cy2}(PCy3)] (6); Pt(PR 3 ) 2 b (7) With loss of one ligand (indicated in bold) [Ru(CO)3(PPh3)2] (8,9); [Os(CO)5], [Os(CO)4(PPh3)], [Os(CO)3(PPh3)2] (10); [Ir(CO)(m-Ph 2 PCH=CHPPh 2 ) 2 ] + (11); [Ir(CO)2(CS)(PCy3)2]+ (12); [Ir(CO)2L3]+, [Ir(CO)3L2]+, [IrH(CO)3L]< (13) a

Dihydride not isolated. b R = C6H) 1 or Bul; product is fra«s-[PtH2(PR3)2]. c L = tertiary phosphine, arsine, or phosphite. 1. M. H. B. Stiddard and R. E. Townsend, Chem. Commun., 1969, 1372. 2. C. F. Nobile, M. Rossi and A. Sacco, Inorg. Chim. Acta, 1971, 5, 698. 3. L. Vaska, L. S. Chen and W. V. Miller, J. Am. Chem. Soc, 1971,93, 6671. 4. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter XI, Section G. 5. Ref. 4, Chapter XII, Section Bl. 6. B. R. James, M. Preece and S. D. Robinson, Adv. Chem. Ser., 1982,196, 145. 7. T. Yoshida, T. Yamagata, T. H. Tulip, J. A. Ibers and S. Otsuka, J. Am. Chem. Soc, 1978,100, 2063. 8. Ref. 4, p. 99. 9. B..E. Cavitt, K. R. Grundy and W. R. Roper, /. Chem. Soc, Chem. Commun., 1972, 60. 10. F. L'Eplattenier and F. Calderazzo, Inorg. Chem., 1968, 7,1290. U . S . Doronzo and V. D. Bianco, Inorg. Chem., 1972,11, 466. 12. M. J. Mays and F. P. Stefanini, J. Chem. Soc (A), 1971, 2747. 13. Ref. 4, Chapter XII, Section B3. that give rise to monomeric monohydride products via equation (11). It is only recently that oxidative addition of H2 to a dimer has been shown to result in a dimeric product with one hydrogen atom bound to each metal (equation 12). 92 In this reaction, the formally Ir(II) d1 atoms in the product maintain an 18-electron-rule configuration by formation of a metal-metal bond. This system shows contrasting behaviour with the formally analogous chloride-bridged d& complex [{RhCl(PPh3)2)2], which adds H2 at just one metal to generate the mixed-valence product [(Ph3P)2Rh(M-Cl)2RhH2(PPh3)2].93 The A-frame complex [Ir2(/u-S)(CO)2(dppm)2] similarly adds H 2 at one metal centre. 94 The reported H 2 addition to the titanocene dimer to give a dimeric monohydride (equation 13) is more complex in that the 'titanocene' probably has structure (1), l^-(r]:r] fulvalene)-di-M-hydrido-bis(cyclopentadienyltitanium); the [{TiHCp2(2] product also has bridging hydrides.

2M(orM2) But Ph3P. ^ S ^ / C O >< >< Bul

+

+

H2

H2

-+

^

2MH

[IrHGu-SButXCOXPPl^k

(11)

(12)

Addition ofH2 and HCN to C=C and C=C Bonds

293

Table 3 Oxidative Addition of H 2 at Two Metal Centres: M2 (or 2M) + H 2 ^ 2MH Complex (ref.) Active catalyst systems [{TiCP2}2]a, [Ti(PPh3)Cp2] (1-3); [|Cr(CO)3Cp|2] (4); [Mn2(CO),0] (5); 2[Co(CN) 5 p- or [Co 2 (CN) 10 ] 6 - (6, 7); [(CN) 4 Co( M -en)Co(CN) 4 ] 4 - b (8, 9); 2[Co(diphos)2], [{Co(P(OR)3)4|2] (10); [Co2(CO)8] (11, 12); [{Co(CO)3(PR3)h] (13); 2[Co(dmgh)2]c, [{Co(dmgh)2py|2], 2[Co(dmgh)2(PBu31)] (7, 14-17); [{Rh(dmgh)2(PPh3)}2] (18); [{Rh(CO)(PPh3)2}2], [{Rh(CO)2(PPh3)2}2] (19) Inactive precursors r(Rh(PF3)4bl (20); MKCOWPPhObl (21)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

a See text. b Hydride formed in situ; other Co(II)/cyanide/amine solutions also absorb H2.8'9 c dmgh = dimethylglyoxime. J. E. Bercaw, R. H. Marvich, L. G. Bell and H. H. Brintzinger, J. Am. Chem. Soc, 1972, 94,1219. A. Davison and S. S. Wreford, /. Am. Chem. Soc, 1974, 96, 3017. G. P. Pez and S. C. Kwan, /. Am. Chem. Soc, 1976, 98, 8079. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter VII, Section A. T. A. Weil, S. Metlin and I. Wender, J. Organomet. Chem., 1973, 49, 227. Ref. 4, Chapter X, Section A. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIC. Ref. 4, Chapter X, Section D. Y. Ohgo, K. Kobayashi, S. Takeuchi and J. Yoshimura, Bull. Chem. Soc. Jpn., 1972,45,933. Ref. 4, Chapter X, Section G. Ref. 4, Chapter X, Section E. F. Ungvary, J. Organomet. Chem., 1972, 36, 363. Ref. 4, Chapter X, Section F. Ref. 4, Chapter X, Section H. G. N. Schrauzer and R. J. Holland, J. Am. Chem. Soc, 1971, 93,4060. T. H. Chao and J. H. Espenson, J. Am. Chem. Soc, 1978,100,129. L. I. Simandi, E. Budo-Zahonyi, Z. Szeverenyi and S. Nemeth, / . Chem. Soc, Dalton Trans., 1980, 276. Ref. 4, Chapter XI, Section G. Ref. 4, Chapter XI, Section D. M. A. Bennett and D. J. Patmore, Inorg. Chem., 1971,10, 2387. L. Malatesta, M. Angoletta and G. Caglio, /. Organomet. Chem., 1974, 73, 265.

'[TiCp2]2'

+

H2

=^

[TiHCp2]2

(13)

(1) Mechanistically, it is difficult to prove whether a reaction such as that shown in equation (10) involves a direct termolecular step, H 2 addition to undetectable amounts of dimer, or even the process outlined in equation (14) via an undetectable dihydride intermediate. All three have been considered for the [Co(CN)5] 3 ~ and cobalt(II)/dmgh (cobaloxime) systems. M

+

H2

^

MH2

-^

2MH

(14)

There are many other systems active for hydrogenation of organic compounds in which oxidative addition of H 2 at one or two metal centres has been invoked as the initiating step prior to any reaction with the organic substrate: the (di)hydride is not detected and the evidence for its formation is usually based on one or more of the following: kinetic arguments; mechanistic considerations based on products formed, especially on using D 2 for the catalysis; or by analogy to systems listed in Tables 1-3. Representative systems in which dihydride formation has been invoked include the following species or intermediates: [Cr(CO) 3 ], 95 [Ru 1 ], 96 [Fe(CO) 3 ] 97 and [Rh 2 (O 2 CMe) 4 ] (see Section 51.1.3.2,ii).98 Hydrogen addition across several metal centres is exemplified49 by treatment of carbonyls with H 2 ; [Re3H3(CO)i2] and [Re4H4(CO)i2] are formed from

294

Addition ofH2 and HCN to C=C and C=C Bonds

[Re 2 (CO)i 0 ], [H 4 Ru 4 (CO) 12 ] from [Ru 3 (CO)i 2 ] or [Ru 2 Fe(CO 12 ], [FeRu 3 H 4 (CO)i 2 ] from [FeRu 3 H 2 (CO)i 3 ] and [Os 4 H 4 (CO)i 2 ] from [Os 3 H 2 (CO)i 0 ] or [Os 3 (CO) 12 l. The rhodium cluster [Rh 4 (CO)i 2 ] is considered to be active in hydroformylation via an H2-generated lRhH(CO) 3 ] intermediate, although neither this nor [RhH(CO) 4 ], that could be formed under CO, has been detected." The clusters [Ir{M[P(p-tolyl)2](CO)2(i?-Cp)}2]+ (M = Fe, Ru) bind one mole of H 2 , presumably at the iridium.100 Except where noted, the dihydride products formed from the complexes listed in Tables 1 and 2 have cis geometry for the two hydrogens, although in principle a trans concerted addition is allowed.101 Data have been presented for a contributing direct trans mode of addition to [IrD(CO)(PPh 3 ) 3 ] via the four-coordinate intermediate frww-[IrD(CO)(PPh3)2], although some intramolecular interchange processes, which were not ruled out, could give rise to a trans dihydride product. 102 Cis-trans rearrangements have been observed in complexes of the type [MH 2 L 4 ](M = Fe, Ru). 1 0 3 Of interest here is the formation (via a rare d6 —» d4 oxidative addition of H 2 ) of a trans-dihydride via reaction (15), although no mechanistic data were present.104 The formation of trans-dihydrides with [Pt(PR 3 ) 2 ] systems, where R is a bulky substituent, is well documented (Table 2, footnote b). rnms-Mo(N2)2(diphos)2

~^

trans-MoH2(diphos)2 +

2N2

(15)

Oxidative addition of H 2 is often reversible and the forward reaction is usually considered (see below) to be promoted by low initial oxidation state, high metal basicity and unsaturation in the coordination sphere.89 The first two factors reflect the fact that complexes with greater electron density are more likely to undergo reactions which decrease such density. The loss of electrons by oxidation is compensated for by a gain of electrons through an increased coordination number. For this reason, the early transition metals (dx to d4) tend to form complexes of higher coordination number than those of the later dn-d™ systems (particularly Group VIII metals), and so for steric reasons show less tendency to undergo oxidative addition generally, although such a constraint should not be severe for the small hydrogen ligands. Studies on addition of H 2 to Vaska-type compounds [IrX(CO)L 2 ] (Table 1) are generally consistent with the model noted above. Steric and donor properties of L, and the electronegativity of X, have all been examined;105'106 reactivity (kinetic and thermodynamic) usually increases with metal basicity and decreases with ligand size. Similarly, the non-reactivity of frafts-[RhCl(CO)(PPh3)2] towards H 2 at ambient conditions, in contrast to *ra«s-[IrCl(CO)(PPh 3 ) 2 ] and [RhCl(PPh 3 ) 3 ], is attributed to the weaker basicity of Rh compared with Ir, and the introduction of the 7r-acid CO ligand, respectively.107 Correspondingly, introduction of the stronger 7r-acid CS renders [IrCl(CS)(PPh 3 ) 2 ] unreactive toward H 2 , while [Ir(CO) 2 (CS)(PCy 3 ) 2 ]+ forms a dihydride [IrH 2 (CO)(CS)(PCy 3 ) 2 ] + and the weaker base analogue with PPh 3 does not.108 The stronger 7r-acid ligands, e.g. CS, PF 3 and including some alkenic and alkynic substrates, are said to decrease reactivity towards H 2 by increasing the 'promotional energy' for its oxidative addition.105 The nitrosyl ligand appears potentially important in catalysis in that its ability to function as a three- or one-electron donor (with linear or bent M—N—O geometries, respectively) should allow for the creation of a vacant coordination site. However, relatively few nitrosyl complexes have been reported active for catalytic hydrogenation (see Tables 6, 7 and 12 and Sections 51.1.3.1,iv and 51.1.3.4), and the role of the nitrosyl group remains to be established. Its strong 7r-acceptor ability may be detrimental to catalytic activity. Of importance, the dissociation energy of a metal-hydride bond may be deduced from the temperature dependence and/or position of the equilibria exemplified by equations (8) and (11). 105 ' 109 ' 110 In Vaska-type complexes, the dissociation energy is about 260 kJ mol" 1 , while in [MnH(CO) 5 ], [CoH(CN) 5 ] 3 -, [CoH(CO) 4 ], [CoH(dmgh) 2 (PBu?)] and [CoH 3 (PPh 3 ) 3 ], the values are >250, 242, 229, ~220 and 247 kJ mol" 1 , respectively. The value for the Vaska systems is comparable, for example, with that for H 2 chemisorbed on iridium metal, 1 '' and clearly such values are quite compatible for catalytic hydrogenation of organic substrates, a property demonstrated by the iridium, manganese and cobalt hydride complexes. It should be noted that in the much earlier studies on H 2 activation by copper(I) and silver(I) systems, oxidative addition was frequently invoked based on detailed kinetic studies [usually for the catalyzed reduction of benzoquinone or inorganic substrates such as copper(II), silver (I), chromium(VI) or manganese(VII)]. The work has been extensively reviewed.18'112 Reactions invoked are exemplified by equations (16)-(18), and more recent spectroscopic studies have detected the hydrido species [Ag ]I H] 113 and [Cu ]I H]. 114 Direct termolecular steps have been invoked

Addition ofH2 and HCN to C=C and C=C Bonds

295

for equations (16) and (18); the kinetic and energetic arguments appear reasonably strong, at least for the silver system, although the mechanism outlined in equation (14) has also been invoked here. 2Cu'

+

[CuHquinoline)^

H2 +

2AgJ

+

H2 H2

2CunH

^

(16)

^=^ [Cu(quinoline)H]2 ^

2Ag"H

(17) (18)

In reactions such as equations (16)—(18), the H2 was described in the earlier literature as being activated by homolytic splitting, in contrast to the heterolytic cleavage (—»-H~-+ H + ) to be discussed in the following section. Homolytic cleavage of H2 formally becomes an oxidative addition only when electrons are transferred from the metaj to the hydrogen atoms; such cleavage may be formulated in three ways exemplified in equation (19) for a divalent complex. The formulations of the products differ only in the position of the electron originally associated with the hydrogen atom. Several hydrides {e.g. [MnH(CO) 5 ], [CoH(CO) 4 ] and [CoH(CN) 5 ] 3 -) act as hydrogenation catalysts via free radical pathways involving H atom transfer (Section 51.1.3.2,1), implying that the hydrogen is better pictured as a stabilized atom M n ( • H) rather than hydride M m ( : H ) , at least during the approach of a reducible substrate. Addition of H2 to [Ir(cod)L 2 ] + and [Ir(cod)2] + (Table 1) has been considered to be reductive rather than oxidative in character (equation 20). 115 Indeed, hydrides of the late transition metals tend to be covalent or acidic in character, while those of the earlier transition metals show hydridic behaviour.68'116 Thoughout this Chapter, however, coordinated hydrogen will be assigned the classical - 1 oxidation state, but the shortcomings in the description in terms of the polarity of the metal-hydrogen bond have to be realized. 2Mn(-H) 2M n

or

(M n ) 2

+

2M!(H)

H:H

(19)

2M ni (:H) M

+

H2

—

M

(20)

The detailed mechanism of the H 2 addition remains problematic. As well as the picture involving electron flow from metal ^/-orbitals to the antibonding er*-orbital of H 2 (Section 51.1.3), a reverse electron flow from the bonding orbital of H 2 to a vacant metal orbital has been invoked;117 the pictures correspond to the oxidative or reductive addition dichotomy.

(ii) Heterolytic splitting of Hi There are many examples where a metal hydride is formed via a. net heterolytic splitting of H2. Equation (21) typifies such a process for a divalent metal complex; there is no change in the formal oxidation state of the metal and the reaction is overall a substitution reaction with hydride usually replacing an initially coordinated ligand X which is usually anionic. The released proton is often stabilized by a base; this may be the ligand X, the solvent or more commonly an externally added base such as NEt3. There are many examples of isolated, catalytically active monohydrides formed via such routes (Table 4), but again, as with dihydride formation, the first substantial evidence for heterolytic splitting came from early kinetic studies on copper and silver systems involving inorganic substrates. 18 ' 112 Thus, the observed rate law, shown in equation (22), for the Cu 2+ -catalyzed H 2 reduction of [Cr 2 O 7 ] 2 ~ in perchloric acid solutions offers convincing evidence for the mechanistic sequence of equations (23)-(25) involving steady state concentrations of [CuH] + . M»X

+

H2

^

MnH

+

H+

+

X-

(21)

296

Addition ofH2 and HCN to C=C and C=C Bonds Table 4 Monohydride Formation via a Net Heterolytic Cleavage of H2: MX + H 2 ^ MH + H+ + X- a Monohydride precursor, MX (ref.) RuX2L3], [R11X2L2S2], [RuX3L2]b (1-3); [RuHCI(PPh3)3] (4); RuCl2(PPh3)(r?-C6Me6)], [{RuCl2(r7-C6Me6)}2]c (5); RUC1 2 (DMSO)(T;-C 6 H 6 )] (1, 6, 7); m-[RuCl 2 (DMSO) 4 ] (3); [OsCl2(PPh3)3] (8); CoX{P(OEt)3Ud (9); fra«5-[RhCl(CO)(PPh3)2] (10); [RhHCl2(PBu2Me)2] (11); RhCl(dmgh)2(PPh3)] (12); [Rh(HSO 4 )(C 2 B 9 H n )(PPh 3 ) 2 ] (13); [{MX2(r7-C5Me5)j2]e, M = Rh, Ir (14-16); [PdCl(SnCl3)(PPh3)2] (17); m-[PtX 2 L 2 ] f , m-[PtX 2 L 2 ]/SnCl 2 f 'g, [Pt(SnCl3)s]3- (18-20)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

a X = halide, unless stated otherwise. b X = usually halide but with [RuX2L3] is also carboxylate or a-hydroxycarboxylate; L = tertiary phosphine, usually PPh3, but also chiral phosphines; S = solvent. c Product is [{Ru(^C6Me6)j2(M-H)2(M-Cl)]+. dn = 3,4. e Product is [{MCl(T7-C5Me5)j2(/i-H)(Ai-Cl)]. f X = halide, L = tertiary phosphine. g Product is, for example, [PtH(SnCl 3 ) 2 (PEt 3 ) 2 ]-. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter IX, Section B2. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIA. B. R. James, R. S. McMillan, R. H. Morris and D. K. W. Wang, Adv. Chem. Ser., 1978,167,122. T. I. Eliades, R. O. Harris and M. C. Zia, Chem. Commun., 1970, 1709. M. A. Bennett, T. N. Huang and T. W. Turney, J. Chem. Soc, Chem. Commun., 1979, 312. R. Iwata and I. Ogata, Tetrahedron, 1973, 2753. A. G. Hinze, Reel. Trav. Chim. Pays-Bas, 1973, 92, 542. A. Oudeman, F. Van Rantwijk and H. Van Bekkum, /. Coord. Chem., 1974, 4,1. M. E. Volpin and I. S. Kolomnikov, Russ. Chem. Rev. (Engl. Transl.), 1969, 38, 273. Ref. 1, Chapter XI, Section E. C. Masters, W. S. McDonald, G. Raper and B. L. Shaw, Chem. Commun., 1971, 210. B. G. Rogachev and M. L. Khidekel, Bull. Acad. Sci. USSR, Div. Chem. Sci., 1969,127. W. C. Kalb, R. G. Teller and M. F. Hawthorne, J. Am. Chem. Soc, 1979,101, 5417. M. R. Churchill and A. S. Julis, Inorg. Chem., 1979,18, 1215. P. M. Maitlis, Adv. Chem. Ser., 1979,173, 31. D. S. Gill, C. White and P. M. Maitlis, /. Chem. Soc, Dalton Trans., 1978, 617. Ref. 1, Chapter XIII, Section B. Ref. 1, Chapter XIII, Section C. F. Van Rantwijk and H. Van Bekkum, /. Mol. Catal., 1976,1, 383. J. C. Bailar, Jr., Adv. Chem. Ser., 1979,173, 1.

-d[H 2 ] dt

2Cu+

=

fcifc2[Cu2+HH2] &-i[H+] + &2[Cu2+]

+

H2

CuH +

+

Cu2+

-^>

2Cu+

+

H+

(24)

substrate(CrVI)

^V

2Cu2+

+

product(Crin)

(25)

Y^

+

H+

}

Cu2+

+

CuH+

(

(23)

Additional evidence for reaction (23) was provided by observations of H2-D2O exchange when the reaction was carried out in D2O. Similar kinetic and isotope exchange data (H2-D2O or D2~solvent) have provided evidence for reactions such as equation (21) giving non-detectable catalytically active hydrides in a wide range of other systems. Such 'hydrides' include: [Cu'H], [Ag J H], 112 [Mo 2 H(0 2 )(SnCl 3 )2]-, 118 [RuHCls] 3 ", [RuHCl 4 (CO)(H 2 O)] 2 -, Ru(II) and Ru(III) chlorohydrido species in DMA, [RuHCl(x-arene)(solvent)], [RuHCl(nitrile),/], [RuHCl 3 (bipy)] 2 ", 1 1 9 hydrides formed from ruthenium acetate trimers such as [RU3O(CO2Me)6(DMF) 3 ] + , 120 [RhHCb] 3 ", 44 [RhHBr 3 (H 2 O)2]-, 121 Rh(III) chlorohydrido complexes containing sulphides, sulphoxides and carboxylates containing a phenyl ring (including amino acids), 122 [NiHCl(PPh 3 ) 2 ], 123 [PdHCl 2 (PPh 3 )]-, 124 [PdHCb] 2 ", 1 2 5 [PdH(salen)]" 126 and [PtHCl2(SnCl 3 )2p~. 127 Some hydrogenase systems are also considered to involve heterolytic cleavage of H2 (Section 51.1.3.4,1). Demonstration of a coordinate ligand (X) stabilizing the released proton came from early studies on Cu(II) salts and Ag(I) ammine complexes. Correlations between rates of hydrogen activation and ligand basicity were demonstrated and plausible transition states exemplified by formula (2) were invoked.112 Similar pictures involving a polarized H2 molecule could be written with X = solvent or added base. In reaction (26), added base also becomes a coordinated ligand.128

Addition ofH2 and HCN to C=C and C=C Bonds

297

—M---X H----H+ (2) Ru(PPh 3 ) 3 2 +

+

MeC 2

H2

° ">

RuH(CO 2 Me)(PPh 3 ) 3

+

CH 3 CO 2 H

(26)

The reversible nature of reaction (21) has been demonstrated for only a few of the hydride systems listed in Table 4, namely [RuHCl(PPh 3 ) 3 ], [RuHBr 2 (AsPh 3 ) 2 ] and trans[PtHCl(PEt3)2]. However, the protonolysis of a metal hydride with loss of H 2 (the back reaction) is more generally well known, e.g. equations (27) and (28), and is considered a useful way of creating a vacant coordination site at the metal. 129

NiH[P(OR) 3 ] 4 IrH 3 (PPh 3 ) 3

Ni 2 +

- ^ -^

+

4P(OR) 3

IrH 2 (PPh 3 ) 3 +

+

+

H2

(27)

H2

(28)

A net heterolytic H 2 cleavage can also result via oxidative addition of H 2 , followed by reductive elimination of HX which again may be base-assisted. Some well-documented examples, where the intermediate dihydrides are characterized, are given in equations (29)-(32). 1 3 0 ' 1 2 7 1 3 1 1 3 2 Hydrogenations catalyzed by /ra«s-[RhCl(CO)(PPh 3 ) 2 ] complexes 107132133 may involve J/ww-[RhH(CO)(PPh 3 ) 2 ] species formed according to equation (29); in some aminophosphine analogues 134'134a a coordinated nitrogen atom can play the role of proton acceptor.135 It is difficult in some cases to distinguish between the two-stage process and a real heterolytic cleavage via a transition state such as (2). With metals in higher oxidation states, e.g. Ru(III) and Rh(III), an initial oxidative addition seems less feasible, but with Pt(II) and Ru(II) systems the situation is more equivocal, and both routes seen plausible and could depend on the ligands. There are many examples in platinum chemistry of reactions akin to equation (30). 127 Dihydrogen can certainly oxidatively add to Ru(II) to generate seven-coordinate Ru(IV) species, e.g [RuH 4 (PPh 3 ) 3 ] (Table 1), and formation of [RuHCl(PPh 3 ) 3 ] (Table 4) could well proceed via the intermediate [RuH2Cl2(PPh3)rt](« = 2 or 3). Corresponding intermediates could be written for the other Ru(II) systems listed in Table 4.

//ww-IrCl(CO)(PPh 3 ) 2

+

H2

^

IrH 2 Cl(CO)(PPh 3 ) 2

^ */ww-IrH(CO)(PPh 3 ) 2

c«-PtCl 2 (PEt 3 ) 2

+

H2

^^

PtH 2 Cl 2 (PEt 3 ) 2

^

^«5-PtHCl(PEt 3 ) 2 RhHL 3

RhClL 3

+

H2

=5=*

RhH 2 ClL 3

^ >

RhL2S2+

+

H2

^

RhH 2 L 2 S 2 +

^ ^

+

RhHL 2 S 2

+

HC1

base-HCl +

base-H+

(29) (30) (31) (32)

(L = tertiary phosphine; S = solvent)

Considerable kinetic data are available for formation of transition metal hydrides via the oxidative addition and net heterolytic pathways. Some representative data are given in Table 5. Oxidative addition of H 2 to give isolable dihydrides commonly proceeds with A//* values of up to 60 kJ mol" 1 and AS* values of - 1 5 5 to - 6 0 J mol" 1 K" 1 , and activation parameters determined in, or close to, these ranges for a net heterolytic H 2 cleavage can offer indirect support for the two-stage process exemplified in equations (29)-(32). Thus Ru(II) hydrides and [PdHCl 3 ] 2 ~ may well be formed by this route, and certain workers have favoured such a mechanism for some of the heterolytic Cu(I) and Ag(I) systems. However, there are cases where a genuine heterolytic cleavage seems more realistic chemically, e.g. with some Rh(III) systems, even when the parameters are still within the oxidative addition ranges. Nevertheless, the remarkably limited data

Addition ofH2 and HCN to C=C and C=C Bonds

298

Table 5 Kinetic Data for Activation of H2 by Metal Complexesa Metal complexes/solvent Oxidative addition

A//,* (kJmol" 1 )

Ref.

M + H 2 ^ =i M H 2

Chlororuthenate(I)/DMAb Chloro(alkene)rhodate(I) complexes0/DMA [Co(2=phos) 2 ] +d /PhCl [Rh(diphos)(alkene)]+ e /MeOH fra/w-[IrX(CO)L2]/various' [Ir(diphos)2]+/PhCl [Ir(2=phos) 2 ] + /PhCl Oxidative addition

AS,* (Jmol" 1 K"1)

2M (or M2)

Cu(OCOMe)/quinoline AgC104/aq. acid [Co(CN) 5 p-/aqueous [Co2(CO)8]

T

46 75

-84 -28

1 2

15 27 25-60 10 21

-96 -118 -145 t o - 6 0 -155 -100

3 4 5 6 3

112

^

54-67 63-67 -0 72

MH -84 t o - 2 1 -104 t o - 8 4

7 8 9 10

I + H+ + X-

Net heterolytic cleavage MX + Cu(II) salts/aq. acid Cu(I) salts/acids Ag(I) salts/acids Ag(OCOMe)/pyridine Ag(I) amines/aqueous RuCl r 3H 2 O/aq. HC1 or DMA Chlororuthenate(I I)/DM A Chloro(alkene)ruthenium(II) complexesc/aq. HC1 [RuX3L2]e/DMA [RuCl2(PPh3)3]/DMA Rh(III) halides/aq. acids Chlororhodate(III)/DMA cw-[RhCl3(Et2S)3]/DMA RhCl3(DMSO)3/DMSO [PdCl 4 ] 2 -/aq. HC1

^

92-113 63-84 75-100 52-67 58-117 79-100 69 42-75

-88 to-21 -146 t o - 6 3 - 4 2 to - 2 5 -104 -84 to+33 +8 to +29 -54 -109 t o - 1 3

7 7 8 8 8 11 11 12

63-71 54 96-104 71 54 113 81

-25 to 0 -50 +37 to +46 -42 -90 +75 -29

13 14 15 2 2 2 16

a

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Hydrides detected directly or isolated only in systems containing phosphines (refs. 3-6, 12, 13). b DMA = 7V,7V'-dimethylacetamide. c Alkene = unsaturated carboxylic acids. d (2=phos) = cw-PPh2CH=CHPPh2. e Alkene = methyl (Z)-a-acetamidocinnamate (Scheme 4, Section 51.1.3.2.2). f X = halide, L = tertiary phosphine or phosphite; solvent mainly toluene. * X = halide, L = PPh3 or AsPh3. B. C. Hui and B. R. James, Can. J. Chem., 1974, 52, 3760. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter XI, Section H. L. Vaska, L. S. Chen and W. V. Miller, /. Am. Chem. Soc, 1971,93, 6671. A. S. C. Chan, J. J. Pluth and J. Halpern, /. Am. Chem. Soc, 1980,102, 5952. Ref. 2, Chapter XII, Section A. L. Vaska, Inorg. Chim. Ada, 1971, 5, 295. Ref. 2, Chapter II, Section C. Ref. 2, Chapter II, Section D. Ref. 2, Chapter X, Section A. N. H. Alemdaroglu, M. L. Johannes and E. Oltay, Monatsh. Chem., 1976,107, 1043. Ref. 2, Chapter IX, Section Bl. Ref. 2, Chapter IX, Section B2. D. K. W. Wang, Ph.D. Dissertation, University of British Columbia, 1978. L. D. Markham, Ph.D. Dissertation, University of British Columbia, 1973. B. R. James and G. Rosenberg, Can. J. Chem., 1976, 54, 313. Ref. 2, Chapter XIII, Section B.

available for well-established H 2 oxidative addition reactions all reveal relatively low A//* values compared with most Cu(II), Cu(I), Ag(I), Ru(III) and Rh(III) systems, which are thus probably genuinely heterolytic. However, conclusions about the mechanism of H 2 activation based on ac-

Addition ofH2 and HCN to C=C and C=C Bonds

299

tivation enthalpies and entropies must always be somewhat tentative. Activation volumes have been reported only for oxidative addition to trans-[IrC\(CO)(PPh3)2], A F * = - 2 0 ml mol" 1 . 136 Kinetic isotope effects, when using gaseous deuterium in place of hydrogen, have been studied for a wide range of metal complex systems but in most cases are small (&H/&D = 0.9-1.5) and have contributed little to distinguish between the different mechanisms for H 2 activation.137 The low values, however, are consistent with metal-hydride bond formation and hydrogen-hydrogen bond breaking occurring to some extent at the same time, i.e. via a concerted process. Larger kinetic isotope effects of about 3.0 have been recorded for a postulated oxidative addition of H 2 at a rhodium(II) centre in a rhodium(II) acetate system in DMF, 138 and for a net heterolytic splitting of H 2 by ruthenium(II) chlorides in DMA, although this latter system could involve a rate-determining oxidative addition step. 96 The small isotope effects measured for systems that definitely involve dihydride formation have always involved addition to a square planar ds system. Further data on dihydride formation from other geometries and configurations (cf Table 1) would be valuable. A few rates of reaction of H 2 with water-soluble species (cf Table 5) have been measured in both H 2 O and D 2 O, but the &H 2 OMD 2 O values (0.9-1.25) give little insight into mechanistic details. 137 Active systems for catalytic hydrogenation are sometimes generated via initial hydride formation followed by proton elimination, for example, equation (33). The net reaction is a twoequivalent reduction of a metal complex by H 2 , and is exemplified by several precursor Rh(III) systems that lead to Rh(I) species which are catalytically active, sometimes via subsequent oxidative addition of H 2 (equation 8), but more generally via an 'unsaturate route' when the organic substrate is activated (bound) prior to reaction with H 2 (Section 51.1.3.2). 122 The proton loss from coordinated hydride in reaction (33) is governed by the pKa of the hydride complex, which will depend partly on the nature of other coordinated ligands, in particular their ability to stabilize the lower oxidation state via x-acceptor properties. Representative values are given for the following complexes: 39 ' 44 ' 139 - 142 [MnH(CO) 5 ], ~ 7 ; [FeH 2 (CO) 4 ], - 5 . 0 ; [Ru 4 H 4 (CO)i 2 ], 11.7; [OsH 2 (CO) 4 ], 12.8; [CoH(CO) 4 ], - 0 . 0 ; [CoH(CO) 3 {P(OPh) 3 |], 5; [CoH(CO) 3 (PPh 3 )], 7; [CoH(CO) 3 (PBu§)],-9; [CoH(dmgh)2(PBu§)], 10.5; [CoH(CN) 5 ] 3 -,-18; [RhH(NH 3 ) 5 ] 2 + , - 1 0 ; [RhH(en) 2 OH]+, - 1 2 . MmXn

+

H2

^ ^

MIIIHXrt_1

^

MiX^-2

+

HX

(33)

Generation of [RI1H2CIL3] catalysts via the base-promoted H 2 reduction of [RI1CI3L3] complexes (L = tertiary phosphine) is readily rationalized in terms of reaction (33) followed by reaction (8), although an alternative mechanism involving a second hydride substitution/HC1 elimination has been suggested (equation 34). 143 Such a mechanistic choice is likely to be dominated by the p ^ a of the monohydride. RhHCl 2 L 3

+

H2

^

RhH 2 ClL 3

+

HC1

(34)

A net one-equivalent reduction of a metal complex to generate hydrogenation catalysts occurs when the initially generated hydride reacts with the starting metal complex. This is shown in equation (35) for the formation of an active Ru(II) system from a precursor trivalent system. The Ru(II) dimer subsequently forms an active hydride via heterolytic splitting of H 2 (equation 21 ). 93 The second stage reaction of equation (35) incorporates a mechanistically similar ambiguity to that noted for the Rh(III) system, in that the Ru(III) hydride, RuHX 2 L 2 , could deprotonate to yield a Ru(I) intermediate (equation 33), which then reduces the RuX 3 L 3 complex. RuX 3 L 2

+

H2

^ ^

RuHX 2 L 2

Ru 3L2

*

>

[RuX 2 L 2 ] 2

+

H+

(35)

(X = halide; L = PPh 3 , AsPh 3 )

Net one- or two-equivalent reductions of metal ions by H 2 without the intermediary of hydridometal species have been invoked in the case of the catalyzed reduction of inorganic substrates

300

Addition ofH2 and HCN to C=C and C=C Bonds

using Hg(II), Hg(I) and Mn(VII); the mercury systems probably involve Hg atom intermediates. 144 ' 145 (//*) Hydrogenolysis and hydrogenation of coordinated ligands Net hydrogenolysis reactions exemplified by equations (36)-(39) 105 ' 127 ' 131 may be considered formally analogous to a net heterolytic splitting of H2, and the Pt(II) system with an activation energy of only 38 kJ mol" 1 proceeds via a dihydride intermediate. 127 There are many examples of hydrogenolysis of such metal-'metalloid' bonds, and extension of such systems, for example, up the Group IV elements to carbon leads to hydrogenolysis of metal-alkyl bonds, an important step in the catalytic hydrogenation of alkenes (Section 51.1.3.2). Hydrogenolysis of metal-alkyl or -aryl bonds has also been used for the generation of active monohydride catalyst. Equation (40) shows an example for an aryl system,131 while a key step in the widely used Ziegler hydrogenation catalysts (transition metal halide with an alkyl of aluminium, alkaline earth or alkali metal, Section 51.1.4.2) almost certainly involves cleavage of the transition metal alkyl to generate an active hydride (equation 41). Catalytically active dihydrides have been formed similarly (equations 41a 146 and 41b). 146 ' 147 The Group IV cyclopentadienyl alkyl and aryl derivatives [ZrCl(Me)Cp 2 ] and [TiPh 2 Cp 2 ] that catalyze alkene hydrogenation presumably operate via similarly formed hydrides,148 while hydrogenolysis of the tetrabenzyl complexes [M(CH2Ph)4](M = Zr, Ti, HO in the presence of Me 2 P(CH 2 )2PMe 2 leads to the active [MH3JMe2P(CH2)2PMe2}2] hydrides. 149 Pt(GePh3)2(PEt3)2

+

H2

—* PtH(GePh3)(PEt3)2

+

Rh(SiPh3)(PF3)4

+

H 2 —•

RhH(PF3)4

IrH2(GeEt3)(CO)(PPh3)2

+

H2

IrH3(CO)(PPh3)2

ClHg-IrCl 2 (CO)(PPh 3 ) 2

+

H2

—

(36)

SiHPh3

(37)

+

(38)

GeHEt3

—• IrHCl2(CO)(PPh3)2

Rh(Ph)(diene)(PPh3)

ZrH(R)Cp2

+

GeHPh3

+

H2

RMX«

+

+

—•

H2

H2

+

Hg°

+

—** RhH(diene)(PPh3) —

RH

+

[ZrH3(R)Cp2]

H+ +

+

C6H6

MHXn

—-• ZrH2Cp2

Cl"

(39) (40) (41)

+

RH

(41a)

(R = alkyl) MMe2Cp2

+

2H2

—• MH2Cp2

+

2CH4

(41b)

(M = Zr, Hf) Closely related is the hydrogenolysis of metal- a- or -7r-allyl bonds, which has been used extensively for isolation or in situ formation of metal hydride catalysts (equation 42). Examples include M = [TiCp 2 ], 151 [Co(CO) 2 L], 139 [C0L3], 150 [Rh(PPh 3 ) 2 ], [Rh{P(OMe)3}3], [RhX 2 ] (X = Cl, Br) in the presence of added phosphines, sulphides and amines,126'152 [Ir(CO)(PPh 3 ) 3 ], 153 [NiClL 2 ] 123 and [PdClL n ] (n = 1,2),125'154 where L throughout is a tertiary phosphine or phosphite. The [Rh(PF3)3(773-C3H5)] complex yields propene and inactive [RhH(PF 3 ) 4 ], 126 while [Co{P(OPri)3)3(T?3-C3H5)] reacts with the two moles of H 2 to yield the catalytically active trihydride [CoH 3 {P(OPr i ) 3 i 3 ]; 150 some TT-allylmanganese(I) carbonylphosphine complexes also appear to give catalytically active hydrides.155 The ruthenium allyl [RuCl(CO)3(r73-C3H5)] with H 2 yields bridged alkyloxycarbene species arising by hydrogenation of the allyl group followed by CO insertion reactions. 126 A plausible mechanism for reaction (42) is via dihydride addition to an M(r;1-C3H5) intermediate; 150 see also equation (72) in Section 51.1.3.2,i. M(r?3-allyl)

+

H2

—• MH

+

alkene

(42)

The removal of an unsaturated ligand by its complete hydrogenation has been used to generate systems active for catalytic hydrogenation. Hydrogenation of a diene to free alkane is the most

Addition ofH2 and HCN to C=C and C=C Bonds

301

common example (equation 43, M = Rh, Ir; L = tertiary phosphine type ligand, phen or bipy; S = solvent). 132 The systems generated here subsequently operate catalytically via dihydride formation (cf. Table 1), but the initial stoichiometric reaction giving an alkane is clearly germane to the topic of catalytic hydrogenation of dienes and monoenes. Other examples include formation of [IrH 2 (SnCl 3 )(PR3)2] from [Ir(SnCl 3 )(PR 3 )2(cod)], 156 and [IrH 3 (PR 3 )2] from [IrH(cod)(PR 3 ) 2 ], 1 5 7 while active [CoH{P(OR)3}3(MeCN)] catalysts have been prepared by hydrogen treatment of 7r-cyclooctenyl-7r-cycloocta-l,5-dienecobalt(I) in the presence of triaryl phosphites.158 M(diene)Ln+

+

2H2

—+

ML n S x +

+

alkane

(43)

1* MH2US Unsaturated phosphine ligands (L) of the type [Ph 2 P(CH 2 ),jCH=CH 2 ] may be catalytically hydrogenated within [RhCl(CO)L] complexes.159 In essence the system involves reduction of a chelated alkenic substrate via a postulated dihydride intermediate, and provides a useful mechanistic model for such a system (Section 51.1.3.2,ii). Stoichiometric hydrogenation of one o-styryldiphenylphosphine ligand (L) of the [RhL 2 ] + [BPh 4 ]~ complex leads to loss of the reduced ligand and formation of [RhL(r7 6 -C6H5BPh 3 )]. 160 Hydrogenation of aryldiazenido (—N=N—Ar) complexes is of more interest in model nitrogenase studies, but hydrido complexes have been prepared by such routes, e.g. equation (44). 126

Pt(HNNAr)(PPh3)32+

+

H2

-+

PtH(PPh3)3+

+

N2

+

ArH

(44)

Hydrogen probably plays a key role in removing oxygen-containing ligands from species described as active hydrogenation catalysts, formed from the O 2 oxidation of phosphine complexes that are themselves hydrogenation catalysts. Enhancement of alkene hydrogenation rates by trace amounts of O 2 (or H 2 O 2 ) in reactions catalyzed especially by Rh(I) and Ir(I) complexes containing tertiary phosphine type ligands is due to the formation of the more weakly coordinating phosphine oxide (formed via free or coordinated phosphine); 93 ' 161 ' 162 in essence, assistance is being given to coordination of the alkene at a 'more free' site at the metal. Studies on more complete O 2 oxidation of rhodium(I) phosphine complexes, notably [RhClL3] and [RhCl(CO)L 2 ], L = tertiary phosphine or phosphite,93 have led to isolated products with a wide variety of suggested formulations (including bridging mono- and di-oxygen moieties and r;2-peroxide, as well as the oxide of the phosphorus ligand); nevertheless, at least one phosphine ligand is usually removed in the process, e.g. equation (45). There is evidence that, under hydrogen, the various forms of oxygen are reduced, probably at the metal, via hydrido species to water. 163 In essence, the O 2 and subsequent H 2 treatment have resulted in the net 'burning off of a coordinated ligand from the original complex to give the new catalyst, although possible coordination of, or decomposition by, the water detracts from this potentially interesting method of creating a catalytically favourable vacant coordination site. 2RhCl(PPh 3 ) 3

+

3iO 2

—>

Rh 2 Cl 2 (PPh 3 ) 2 O 5

+

2OPPh 3

(45)

It should also be noted that several widely used hydrogenation catalysts (e.g. [RhClL 3 ] and [IrCl(CO)L 2 ], Table 1; RuX 2 L 3 , Table 4; L = PPh 3 ) are reactive towards hydroperoxides (and peroxides); the reactions are generally complex involving, for example, oxygen transfer from coordinated hydroperoxide to give the phosphine oxide and/or CO 2 (from liganded CO). 164 Under H 2 , such complexes can also catalyze the decomposition of peroxides in reactions such as that shown in equation (46). 165 Careful kinetic and mechanistic studies on the catalytic hydrogenation of unsaturated organic substrates require removal of such peroxides that are often present, for example, in liquid alkenes and alkynes.

302

Addition ofH2 and HCN to C=C and C=C Bonds RCO2CMe3

+

H2

—»

RCH(OH)

+

Me2CO

+

Me-

(46)

(iv) Hydridometal catalysts from syntheses not utilizing H^ Transition metal hydrides have been prepared by many methods not involving the use of H2, including the following.49'166 (a) Oxidative addition of HX as in equation (47), where HX may be water, hydrochloric, sulphuric, perchloric, acetic and hydrocyanic acids as well as species such as sulphides, silanes, germanes or hydrocarbons. There are also examples of boron-hydrogen, nitrogen-hydrogen and phosphorus-hydrogen bonds adding oxidatively to metal complexes. Hydrogenation catalysts containing carborane ligands have been made by such a route (Table 6a). M1

+

HX —*

[MinHX]

(47)

(b) The related protonation of a species to give a monohydride, shown in equation (48) again for a univalent precursor; this is still an oxidative addition process but the X~ anion remains uncoordinated. Corresponding protonation of a monohydride can lead to a dihydride, while subsequent loss of H2 from this can generate a species of reduced coordination number compared with the precursor monohydride (cf. equations 21, 27, 28 and 30). Related to this, protonation of carboxylate complexes often removes the carboxylate ligand, and a range of platinum metal catalysts, especially Ru(II) and Rh(I) species also containing tertiary phosphine ligands, have been made in this way.167 M1

+

H + X-

—•

[MmH]+

+

X-

(48)

(c) The use of alcohol or water as the source of hydride via non-oxidative addition processes. Reactions of alcoholic solutions of a metal halide or halide complex (particularly of the platinum metals) in the presence of tertiary phosphorus or arsenic ligands have yielded a very wide range of hydrido or hydridocarbonyl complexes if CO abstraction also occurs; bases often aid the reaction by neutralizing the hydrohalic acid formed (equation 49). Trace water is possibly the source of the hydride ligand in some syntheses, while hydride formation via reaction of a carbonyl complex, usually a cation, with water is well documented (equation 50);127 in the related reaction with alcohols, hydridocarboxylate complexes can be formed (equation 51). 168

EtOH

+

Cl—M

M—CO

+

H2O

M-CO

MeCHO

+

CH4

+

MH(CO)

M—CO2H —• MH

+

CO2

., „ „ / V MeCH^ "M H ^

-HCI

-^±

- ^ +

ROH

MH (49)

—*• MH(CO2R)

(50) (51)

(d) The use of [BFLi]" or [A1H4]~ (or alkyl and alkoxy derivatives) as the hydride source. Many important hydrogenation catalysts have been synthesized using such reagents, often using metal halides in the presence of ancillary ligands such as phosphines; concomitant carbonyl abstraction from solvent (or added aldehyde) can also be accomplished. The reactions are mechanistically complex and may involve, for example, complexes with coordinated borohydride, which have been isolated.169'170 Borohydrides, and occasionally hydrazine, have also been used to generate undetectable species, possibly monohydrides or low valent intermediates (see Section 51.1.3.2, 'reduction-protonation' mechanisms) that are active for hydrogenation under H2 atmospheres. Active species have been generated in this way by reaction with, for example, [RuCl2(CO)(PPh 3 )2DMF], 119 [CoX 2 (PR 3 )2], 171 [MCl(NO)(PPh 3 ) 2 ](M = Co, Rh, Ir),132<172 [PdCl 2 (PR 3 ) 2 ], 125 ' 173 ' 174 [PdCl(PR3)(?73-C3H5)],175 [Pd(naphtholato)2],176 [PdCl 2 (DMSO) 2 ], [RhCl 3 (DMSO) 3 ] and [RhCl(DMSO)(PPh 3 ) 2 ], 177 as well as simple salts of Co(II) and Ni(II) (Section 51.1.4.1). (e) The reaction of alkyl-lithium, -magnesium and -aluminium compounds with metal salt/ added ligand mixtures. Hydrogen transfer within an intermediate alkyl seems likely, e.g. equation (52). MCH2CH2R

—

MH(CH 2 =CHR)

—>- MH

+

RCH=CHj

(52)

Addition ofH2 and HCN to C=C and C=C Bonds

303

(f) The use of reducing agents such as hydrazine and formic acid (equations 53 and 54), alkali metals (usually as amalgams or naphthalenide) and other reducing agents in the presence of a hydride source (water, alcohol).

cw-[PtCl 2 (PEt 3 )2]

^!^

NzH4

[PtCl(N 2 H 4 )(PEt3)2]Cl

PtHCl(PEt 3 )2 M—Cl

+

HCO 2 H

—*

MHC1(CO 2 H)

^^>

> +

NH 4 C1

MH 2 C1

—•

+

N2 MH

+ +

NH3 HC1

(53) (54)

(g) Intramolecular hydrogen transfer from a coordinated phosphine or phosphite-type ligand, in which an aryl or aliphatic C—H bond of the ligand oxidatively adds to the metal, forming a chelate ring with a metal-carbon bond (the so-called orthometallation reaction),178 e.g. equation (55).

IrCl(PPh 3 ) 3

^

(PPh 3 ) 2 Ir^H

(55)

(h) Intramolecular hydrogen transfer from organic ligands, and hydrogen abstraction from uncoordinated organic reagents or solvents. Intramolecular hydrogen transfer within metal alkyls is well documented and the establishment of the reversibility of reactions such as that shown in equation (52) is important for catalysis involving alkenes (Section 51.1.3.2,i). Hydrogen transfer from a x-allyl ligand is exemplified by equations (56) and (57) involving 773-cyclooctenyl complexes. The 'titanocene' dimer (1) reveals hydrogen transfer from coordinated *7-Cp. Conversion of carboxylate complexes into hydrides with liberation of CO2 is documented for the heavier platinum metals (cf. equation 54). Conversion of formyl complexes to hydrido complexes is also known (equation 58); the carbonyl may or may not remain coordinated.68

Co(773-C8H13)(cod) Ir(773-C8H13)(cod)

- ^

M-CHO

-^

CoHLw

+

IrH(cod)(PR 3 ) 2 —*

MH(+CO)

2 cod +

(56) cod

(57) (58)

There are also many reactions in which hydrides are formed, but the hydride source is obscure. Considering that catalyzed hydrogen-transfer reactions (Section 51.1.3.3) have used alcohols, glycols, aldehydes, amides, acids, ethers, amines and even aromatic hydrocarbons as donor solvents, and that intermediate metal hydrides are usually invoked, the hydride sources in the synthetic procedures may be of wide origin. Table 6 shows a representative list of hydrides, synthesized by methods (a)-(h) outlined above, that have been used for catalytic hydrogenation of organic substrates; once in the catalytic cycle, at least when using molecular H 2 , the hydride is regenerated by steps involving the H 2 . In summary, Section 51.1.3.1 has reviewed how metal complexes can activate molecular hydrogen to generate detectable or non-detectable hydrido species, and how catalytically active hydrides have been formed by other routes not involving H 2 .

304

Addition ofH2 and HCN to C=C and C=C Bonds Table 6 Catalytically Active Hydrides Synthesized by Methods (a)-(h) not utilizing H2 (see text)3

Hydride complex {ref.) Method (a) RuH(C2B8H9)(PPh3)3], [ R u H ^ B o H n X P P h s h ] (1); [RhH(C2B10H8)(PEt3)2] (1); MH(C2B9Hn)(PPh3)2]b, M = Rh, Ir (2); [MH(SBi0Hi0)(PPh3)2], M = Rh, Ir (3); RhH2(O2CR)(PPh3)2] (4); [RhHCl2L3] (5, 6); RhH2(O2COH)(PCy3)2 (7); [IrHCl2LM]c, [IrH2(O2CCH3)(PPh3)j] (8). Method (b) [MnH(CO)5] (9); [OsH 3 L 3 ] + (10); [CoH(CO)4] (11); [RhHL4] (12,13); [RhHCl{PhP(CH2CH2CH2PPh2)2}]+ (14); [NiHL 4 ] + (15). Method (c) [MoH4(diphos)2] (16); [FeH(CO)4]~ (17); [RuH(NO)L3] (18); [RuHCl(CO)L3] (19); RuH 2 (CO)L 3 ] (19, 20); [OsHCl2L3], [OsHCl(CO)L3] (10); [RhHLw]d (12,13); [RhHCl2L2], RhH 2 ClL 2 ] e (21); [RhH(CO)L3] (13, 20, 22, 23); [RhH(C 2 B 9 Hi,)(PPh 3 ) 2 ] (24); ;irH2ClL2] (25); [IrHCl2(DMSO)3] (26); fra/w-[PtHXL2] (27, 28). Method {d) [FeH2L3] (17, 29); RuH2L4, RuHClL3 (13,19, 30); [OsH3L3] (10); OsH4L3 (31); OsHClL3 (32); [CoH(CN) 5 p- (33, 34); [C0HL4], [C0H3L3] (34-36); [CoH(BH4)(PCy3)] (37); [RhH(NH 3 ) 5 ] 2+ (38); [RhH(dmgh)2L]g (12, 39); [RhHL4] (12,13, 40); [RhH(diphos)2] (12); [RhH(CO)L3] (13, 20, 22, 23); [IrH 3 L 3 ], [IrH 2 ClL 3 ], [IrHCl2L3], [IrH(CO)L3], [IrH(CO) 2 L 2 ], [IrH3(CO)L2] (8, 34); fra/w-[NiHClL2] (15); [PdHCl{P(OR)2(OH)}2] (41);/ra/w-[PtHClL2] (27). Method (e) [RuH 2 L 3 ], [RuH2L4] (13,19); [CoH(N2)L3] (34, 35); [RhHL3], [RhHL4] (12, 13); [IrHL2(cod)] (8, 42); f/ww-[PtHClL2] (27, 43) Method (f) [ZrH(dmpe)2(r/5-C6H7)]h (44); RuH(O2CCF3)L3 (19); [CoH(CN) 5 ] 3 " (33, 34, 45); [CoHL4] (35); [RhH(NH 3 ) 5 ] + (38); [RhHL4] (12, 13, 46); [RhH(CO)L3] (13, 22, 47); [IrH(CO)L 3 ] (8, 34, 47); mz«HIrH 2 Cl(CO)(PMe 2 Ph) 2 ] (48); fra/w-[PtHClL2] (27, 43). Method (h) [C0HL3], [CoHL4] (35, 49); [CoH(CO)L3] (50, 51), [IrHL2(cod)] (8, 52); [{TiHCp}2(M-C10H8)], see (1) (53). a L is usually a tertiary phosphine, and sometimes a tertiary arsine or phosphite. b Related rhoda- and iridacarboranes also reported. c n = 2,3. d n - 3,4. e L = tertiary f-butylphosphines. f X = halide, NO3-. 8 dmgh2 = dimethylglyoxime. h dmpe = l,2-bis(dimethylphosphino)ethane. 1. C. W. Jung and M. F. Hawthorne, J. Am. Chem. Soc, 1980,102, 3024. 2. T. E. Paxson and M. F. Hawthorne, J. Am. Chem. Soc, 1974, 96,4674. 3. D. A. Thompson and R. W. Rudolph, J. Chem. Soc, Chem. Commun., 1976, 770. 4. Z. Nagy-Magos, B. Heil and L. Marko, Transition Met. Chem., 1976,1, 215. 5. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter XI, Section Bl 1. 6. G. M. Intille, Inorg. Chem., 1972,11, 695. 7. T. Yoshida, D. L. Thorn, T. Okano, J. A. Ibers and S. Otsuka, /. Am. Chem. Soc, 1979,101,4212. 8. Ref. 5, Chapter XII, Section B. 9. Ref. 5, Chapter VIII. 10. Ref. 5, Chapter IX, Section C. 11. Ref. 5, Chapter X, Section E. 12. Ref. 5, Chapter XI, Section G. 13. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIB. 14. J. A. Tiethof, J. L. Peterson and D. W. Meek, Inorg. Chem., 1976,15,1365. 15. Ref. 5, Chapter XIII, Section A. 16. T. Tatsumi, H. Tominaga, M. Hidai and Y. Uchida, Chem. Lett., 1977, 37. 17. Ref. 5, Chapter IX, Section A. 18. S. T. Wilson and J. A. Osborn, /. Am. Chem. Soc, 1971, 93, 3068. 19. Ref. 5, Chapter IX, Section B. 20. N. Ahmed, J. J. Levison, S. D. Robinson and M. F. Uttley, Inorg. Synth., 1974,15, 45. 21. C. Masters, W. S. McDonald, G. Raper, and B. L. Shaw, Chem. Commun., 1971, 210. 22. Ref. 5, Chapter XI, Section D. 23. J. Hjortkjaer and Z. Kulicki, /. Catal., 1972, 27,452. 24. W. C. Kalb, R. G. Teller and M. F. Hawthorne, J. Am. Chem. Soc, 1979,101, 5417. 25. D. H. Empsall, E. M. Hyde, E. Mentzer, B. L. Shaw and M. F. Uttley, J. Chem. Soc, Dalton Trans., 1976, 2069. 26. Ref. 5, Chapter XII, Section C. 27. Ref. 5, Chapter XIII, Section C2. 28. H. C. Clark, C. Billard and C. S. Wong, J. Organomet. Chem., 1979,193, 341. 29. V. D. Bianco, S. Doronzo and M. Aresta, J. Organomet. Chem., 1972, 42, C63. 30. Ref. 13, Section IIA. 31. B. Bell, J. Chatt and G. J. Leigh, /. Chem. Soc, Dalton Trans., 1973, 997.

Addition ofH2 and HCN to C=C and C=C Bonds 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.

305

A. Oudeman, F. Van Rantwijk and H. Van Bekkum, / . Coord. Chem., 1974, 4, 1. Ref. 5, Chapter X, Sections A, B. Ref. 13, Section IIC. Ref. 5, Chapter X, Section G. L. W. Gosser, Inorg. Chem., 1975,14, 1453. M. Nakajima, H. Moriyama, A. Kobayashi, T. Saito and Y. Sasaki, J. Chem. Soc, Chem. Commun., 1975, 80. Ref. 5, Chapter XI, Section A. J. H. Weber and G. N. Schrauzer, J. Am. Chem. Soc, 1970, 92, 726. Z. Nagy-Magos, S. Vastag, H. Balint and L. Marko, Transition Met. Chem., 1978, 3, 123. T. S. Kukhareva, I. D. Rozhdestvenskaya and E. E. Nifant'ev, Koord. Khim., 1977, 3, 241. H. Yamazaki, M. Takesada and N. Hagihara, Bull. Chem. Soc. Jpn., 1969, 42, 275. J. Chatt, R. S. Coffey, A. Gough and D. T. Thompson, J. Chem. Soc. {A), 1968, 190. M. B. Fischer, E. J. James, T. J. McNeese, S. C. Nyburg, B. Posin, W. Wong-Ng and S. S. Wreford, J. Am. Chem. Soc, 1980,102,4941. R. G. S. Banks and J. M. Pratt, / . Chem. Soc (A), 1968, 854. K. C. Dewhirst, W. Keim and C. A. Reilly, Inorg. Chem., 1968, 7, 546. S. S. Bath and L. Vaska, J. Am. Chem. Soc, 1963,85, 3500. J. Van Doom, C. Masters and C. Van der Woude, J. Organomet. Chem., 1977,141, 231. M. Rossi and A. Sacco, Chem. Commun., 1969, 471. Ref. 5, Chapter X, Section F. S. Otsuka and M. Rossi, / . Chem. Soc. (A), 1969, 497. M. Lavecchia, M. Rossi and A. Sacco, Inorg. Chim. Acta, 1970, 4, 29. A. Davison and S. S. Wreford, / . Am. Chem. Soc, 1974, 96, 3017.

51.1.3,2 Activation of the unsaturated organic substrate and hydrogen transfer In cases where the hydridometal complex has been preformed, activation of the organic substrate (mono- or poly-alkene, alkyne or aromatic) generally involves its simple coordination. Catalytic cycles operating via such initial steps have been termed the 'hydride' route, implying prior formation of a mono- or di-hydride; in contrast, binding of the unsaturated organic substrate followed by activation of H2 as the initial steps in the catalysis has been labelled an 'unsaturate' route. 143 In nearly all cases studied, the overall addition of H2 to carbon-carbon double and triple bonds appears to be cis.ul This is readily demonstrated for alkene reduction using D2 addition experiments, and was first demonstrated for a homogeneous catalyst with a chlororuthenate(II) system using maleic and fumaric acids as substrates. 119 The former gives meso-2,3-dideuterosuccinic acid (equation 59), while the latter gives (i)-dideuterosuccinic acid, cis Addition to alkynes is usually demonstrated by the hydrogen reduction of diphenylacetylene to ds-stilbene, simple internal hexynes to the c/s-alkenes or deuteration of phenylacetylene to a s - P h C D = C D H . HO2C

CO2H C=C

H

(1) Monohydride

- ^ H

^ V D» C—C D \T ^H

(59)

catalysts

Proposed mechanistic pathways for monohydride catalysts (see Tables 3-6, and p. 296) are summarized in Scheme 1 for the hydrogenation of monoalkenes.22'137 Step (a) defines the hydride route and step (b) the unsaturate route. Step (b), a net heterolytic cleavage of H 2 (Section 51.1.3.1,ii) by a metal alkene complex, has been proposed, for example, for chlororuthenate(II) species based on kinetic and deuterium tracer and exchange studies.119 Similar studies with trichlorostannate(II) complexes of Pt(II) were initially interpreted in terms of the same mechanism, but heterolytic cleavage of H2 prior to alkene coordination, step (a), was later preferred. 127 ' 179 " 181 A key alkyl intermediate (4) is usually assumed to be formed via hydride migration (alkene insertion) within the hydridoalkene complex (3). The formation of metal alkyls via reaction of alkene with metal hydride and the reverse process (/3-hydride elimination) are very well documented, but there appears to be no case within a catalytic hydrogenation system where step (c; 3 - * 4) has been observed directly. The hydride migration is facile and other steps within the catalytic cycle are found to be rate determining (see below). Monohydrido-alkene or -diene complexes that have been isolated (e.g. [MoH(C 2 H 4 )(r7-Cp) 2 ] + , 22 [RuHCl(PPh 3 ) 2 -

306

Addition ofH2 and HCN to C=C and C=C Bonds M-H MH

a. >!<

• w

-—

M4

H.

(3) (g) (c)

*H

M—H

H

>

M

^H

M (5)

(4) (f) MH

M 2 (or2M) Scheme 1

Mechanistic pathways for monohydride catalysts

(nbd)], 119 [RhH(PMe 3 )(C 2 H 4 )(r?-Cp)] + , 22 [IrH(CO)(PPh 3 ) 2 (fumarate)], 182 [(IrHCl2(cod)hL 183 [IrH(PPh 3 ) 2 (cod)] (Table 6e), [PtH(PEt 3 ) 2 (C 2 H 4 )] + , [PtH(CN)(PEt 3 ) 2 (C 6 N 4 )] and [Pt 2 H 2 (SnCl 3 ) 2 (PPh 3 ) 2 (/Li-cod)]) 127 are not effective in catalytic cycles (see below). The migration-insertion reaction (3) - * (4) is highly stereospecific, the four-centre transition state (3a) requiring a coplanar arrangement of metal, hydride and alkene ?r-bond and leads to exclusive cis addition of the metal hydride.

ft (3a)

Some cobalt catalysts operate via alkyls that appear to be formed without prior coordination of the unsaturated substrate. Thus [CoH(CN) 5 ] 3 ~ forms alkyls with rates independent of the cyanide concentration, implying the absence of any ligand dissociation step,172'184 while a presumed [CoH(dmgH)2py] catalyst formed from [Co(dmgH)2py] is also considered unlikely to coordinate alkenic substrates. 172 However, these systems may involve a H atom transfer process (see below). Hydrogenation of the substrate without coordination has also been invoked in certain dihydride catalyst systems (Section 51.1.3.2,ii, equation 86). The reversibility of steps (a) and (c) provides a ready explanation for the isomerization of simple alkenes in the absence of H 2 by metal hydride addition-elimination (7 —*- 9, equation 60) and for isotope exchange between the metal hydride and the hydrogens on the alkene. Both these reactions are common for monohydride catalysts. It should be mentioned that alkene isomerization can also occur via 7r-allyl hydrides (equation 61), and possibly via a-hydrogen abstractions involving carbene intermediates (equation 62), although the mechanism of equation (60) is strongly preferred for the metal monohydride systems.86 A catalyzed deuterium exchange with the ortho-phcnyl C—H bonds of phenylphosphine ligands is also observed in some monohydride systems. 178 ' 185 For example, [RuHCl(PPh 3 ) 3 ] in solution under D 2 slowly yields [RuDCl((2,6D 2 C6H 3 ) 3 P|] via the orthometallation process exemplified in equation (55); such a process coupled with reversible loss of H 2 (D 2 ) via a [RuH 2 Cl(PPh 3 )(o-C6H 4 PPh 2 )] intermediate accounts for the exchange. A stoichiometric hydrogenation of alkenes involving this orthometallation reaction is described in Section 51.1.3.3.

Addition ofH2 and HCN to C=C and C=C Bonds RCH2CH2CH2 M

^

RCH2CH==CH2 MH

(6)

^

RCH2CH—Me M

(7) RCH2CH=fCH2

RCHjCH—Me MH

(8) ^

RCH

A RCH2CH=FCH2 M

^=*=

307

^=^

CH fsCH2 MH

RCH2C—Me jjf

(60)

(9) =^

RCH=?CHMe

(61)

M = F ^ RCHjCHMe M

(62)

For the monohydride systems generally, formation of the saturated product from the alkyl (4) under catalytic conditions has been discussed in terms of one of the steps (d), (e) or (f). Step (d), the net direct hydrogenolysis using H2, may involve oxidation-addition of H2 to give the dihydridoalkyl intermediate (5), which then reductively eliminates the alkyl hydride (the saturated product) in step (g), with regeneration of the monohydride catalyst MH. Steps (d) and (g) comprise activation of H2 by net heterolytic splitting which was discussed in detail in Section 51.1.3.1 together with the related cleavage by H2 of metal-carbon bonds to synthesize monohydride catalysts. Catalysts that appear to operate by the (a, c, d, g) cycle include the well-characterized monohydrides [RuHCl(PPh 3 ) 3 ], 93 ' 119 [CoH(PPh 3 ) 3 ], 171 ' 172 [RhH(CO)(PPh 3 ) 3 ] 99 ' 187 and [IrH(CO)(PPh 3 ) 3 ]. 157 ' 172 It should be noted, however, that dihydridoalkyls such as (5) have not been detected; the evidence for step (d) is by analogy (sometimes including kinetic parameters) with known oxidative addition of H2 to d6 and ds systems. The kinetic data for such monohydride catalyst systems, neglecting complications due to ligand dissociation, are frequently of the form shown in equation (63), which is consistent with the (a, c, d, g) cycle, K being the overall equilibrium constant for alkyl formation (steps a and c) and k being the rate-determining oxidative addition of H 2 . The hydride transfer (step g) involving a three-centre transition state such as (10) is considered relatively fast. An important study using a dihydride catalyst (Section 51.1.3.2,ii, equation 84) has shown that reductive elimination of saturated product from an alkyl monohydride complex occurs with retention of configuration at the metal-bonded carbon. Such retention is necessary to give the usually observed overall cis addition of H 2 to alkenic bonds, steps (c) and (g). However, it should be noted that a complex [RCo(dmgH) 2 B], where R = (i?)-l-(methoxycarbonyl)ethyl and B is (i?)-a-methylbenzylamine, is said to give on deuteration (-S)-methyl propionate-2-d (equation 64), the conclusion being that cleavage of the Co—C bond has occurred with inversion of configuration at the carbon; 186 more mechanistic details on this reaction would be valuable.

-d[H 2 ] df

feJg[metal][alkcncKH2] l+AT[alkene]

=

, „ , }

(10)

Co—CHMe CO2Me

^

MeCH(D)CO2Me

(64)

308

Addition ofH2 and HCN to C=C and C=C Bonds

Ligand dissociation, for example phosphine dissociation from [RhH(CO)(PPh3)3] to give the active bis(phosphine) species (equation 65), can lead to rate laws much more complex than that given in equation (63). 187 The first phosphine dissociation equilibrium (K\) will add a further term [PPh3]/A:i to the denominator of equation (63) and in effect, at least at lower alkene concentration, leads to a less than first-order in metal with increasing metal concentration; indeed, a limiting half-order will be attained. The same [PPh3]/A^i term also accounts for an inverse dependence of the hydrogenation rate on phosphine added to the system. At very low concentrations of monohydride, further dissociation to a catalytically active monophosphine can also occur (equation 65) and the general rate law will be the sum of two such terms shown in equation (63), together with the further phosphine denominator term. Clearly, quite complex rate laws can arise but limiting forms, which exist in some of the experimental conditions used, are relatively simple and the data can be analyzed accordingly — usually by standard inverse plots, for example (rate) ! versus [olefin]"1 for a set of conditions at fixed metal, H 2 and ligand concentrations (cf. equation 63). Some general comments concerning elucidation of mechanistic details from overall kinetic data for such multi-step pathways are given in the next section.

RhH(CO)L3

^

RhH(CO)L2

=Jf

RhH(CO)L

(65)

(L = PPh3)

The possibility of a direct hydrogenolysis of the metal alkyl (4) via a transition state such as (11) is usually considered unlikely for these catalyst systems. Attempts to substantiate such a process using deuterium tracer studies have not been successful.188

:

;H

(11)

The high selectivity of the [RuHCl(PPh 3 ) 3 ] and [RhH(CO)(PPh 3 ) 3 ] catalytic systems for hydrogenation of terminal versus internal, cyclic and substituted alk-1-enes is usually attributed to the difficulty in step (c) for non-terminal alkenes, steric interaction of the phosphine groups preventing effective hydride transfer to the coordinated alkene (see below). 119187 Indeed, in the case of the non-reducible norbornadiene substrate, the hydrido-alkene complex [RuHCl(PPh 3 ) 2 (nbd)] was isolated. Nevertheless, some non-reducible internal alkenes do undergo slow hydrogen exchange and are isomerized with the catalysts, showing that the reversible step (c) is occurring in these cases, and that other factors (electronic or steric) are probably important at later stages of the catalytic cycle. Differences in the equilibrium constants for substrate binding, step (a), also play a role; terminal alkenes bind preferentially to internal ones, especially due to the presence of bulky ligands (see also Section 51.1.3.2,ii). A further aspect of the migratory insertion reaction deserves comment, since this step (c) is essential in hydrogenation of unsaturated hydrocarbons using any catalyst system, excluding those occurring by free radical processes (Section 51.1.3.1,1). Alkene isomerization (7 -* 9, equation 60) requires abstraction from the /3-methylene group of the alkyl (8) formed by Markownikov addition of the metal hydride. With the [RhH(CO)(PPh 3 ) 3 ] system,119 which operates via a /ra«5-[RhH(CO)(PPh 3 ) 2 ] intermediate, the ready hydride transfer to terminal alkenes (and fast hydrogen exchange) is thought to occur only through anti-Markownikov addition ( 7 - ^ 6 ) , since formation of (8) in which the alkyl group is mutually cis to the trans phosphines is sterically unfavoured. Hindered Markownikov addition can thus explain the slow isomerization noted under hydrogenation conditions. Metal hydride addition to 2-alkenes in either direction would be hindered by the phosphines and accounts for the slow hydrogenation and exchange. Such rationalizations for this rhodium system based on steric arguments receive support from data at higher dilution,

Addition ofH2 and HCN to C=C and C=C Bonds

309

where a monophosphine species becomes important (equation 65) and the selectivity for terminal alkenes is less marked. More generally, such detailed discussion on the important metal hydride addition is difficult due to the complex questions of the polarity of the M—H bond (Section 51.1.3.1,i) and the direction of addition to alkenes. This has been considered in some detail for cobalt carbonyl catalysts [CoH(CO) w (PR 3 ) 4 -«] (n = 1 to4), 1 3 9 where electronic factors of the ligands and substrates were stressed. With simple non-activated alkenes, increasing acidity of the hydride usually leads to more Markownikov addition; with activated alkenes, the addition normally follows the expected hydridic route, e.g. equation (66). Both steric and electronic properties of bulky ligands such as phosphines can be important in directing the course of M—H addition. Even a change in temperature can reverse the direction of the addition, this probably reflecting the relative stability of the two metal alkyls. In the case of a [CoH(dmgH)2py] system,189 the expected [Co(III)—H] addition is seen with an activated olefin ( C H 2 = C H X ) in neutral conditions, while in basic conditions the observed alkyl is [XCH 2 CH2Co(dmgH) 2 py], but this is apparently formed by a Michael addition with nucleophilic cobalt(I), Co(III)H ^ Co(I) + H + (equation 67). —CH=CH—CO—

+

MH

->

— CH2-CH—CO

(66)

M 1

Co

+

CH 2 =CHX

—*

[C0-CH2-CHX]

Co m CH 2 CH 2 X

J^-

(67)

Returning to Scheme 1 and the decomposition of the metal alkyl (4) during catalytic cycles, step (e) (protonolysis) has been invoked for several systems effective in fully or partly aqueous media. The metal reenters a catalytic cycle, either through the unsaturate route [via formation of M(alkene)], or the hydride route (via formation of MH). Examples include systems using trichlorostannate(II) complexes of Pt(II) 127 and Mo(III), 118 chlororuthenate(II) 119 and [PdCU] 2 -- 190 Again, the net cis addition of H 2 to alkenes requires the protonolysis step to occur with retention of configuration at the carbon atom. Although not demonstrated directly in catalytic systems, such a result is usually considered reasonable since electrophilic displacement of carbon from metals such as mercury or tin occurs with retention.119 Related to this, protonolysis of the alkenyl-iridium bond in (12) has been shown to give the c/s-stilbene product (equation 68), although reaction of HC1 with the platinum analogue [PtCl(CPh^CHPh)(PPh 3 ) 2 ] yields /ra«s-stilbene, which implies either metal-carbon bond fission with inversion or subsequent isomerization of an initial c/s-stilbene product. 183 Deuterium tracer studies on the hydrogenation of (Z)- and (£)-cinnamate catalyzed by the Pt(II)/trichlorostannate(II) system in fact suggest a non-stereospecific protonic cleavage of the metal-alkyl bond, but the system is somewhat complicated by an accompanying isomerization reaction 180 (see also below). PhC=CPh

+

Ph.

J>h

>=<

— H

Ph

^ IrCl2(DMSO)3

J>h

(68)

X H

H

IrHCl 2 (DMSO) 3 (12) The third method for alkyl decomposition (Scheme 1) involves reaction with a further mole of metal hydride, step (0, a binuclear reductive elimination reaction which offers an alternative pathway in cases where oxidative addition of H 2 , step (d), is unfavourable. The metal reenters the catalysis by oxidative addition of H 2 to the dimer (or 2M) species. The process is best documented for [CoH(CN) 5 ] 3 " and [CoH(CO) 3 (PR 3 )] systems, 139 ' 184 although the substrates are usually conjugated dienes and polyenes and the intermediates are allyls rather than alkyls (see below, equations 70 and 71). Systems invoking step (f) within monoene hydrogenation include some catalyzed by [RuCl 2 (CH 2 =CHCN) n ], 1 1 9 [(RuHCl(PPh 3 ) 2 j 2 ], 93 [CoH(CO) 4 ], 39 cobaloximes191 and Ziegler systems (Section 51.1.4.2). In some of the cobaloxime systems, where the [Co(III) ^ Co(I) + H + ] equilibrium pertains, step (f) has been modified to include attack by the Co(I) species, subsequent protonation of the C.O.M.C. VOL. 8—K

310

Addition ofH2 and HCN to C=C and C=C Bonds

liberated anion giving the hydrogenated product. Equation (69) shows this suggested scheme for a [Co(dmgh)2py] catalyst. 172 ComH

Com(SH)

-^

[Co n ] 2

^

H+

+

SH"

\H>

|H+

2ConiH

SH2

| Co 1

+

<69>

(S = substrate) In catalyzed hydrogen transfer reactions under inert atmospheres using solvents as the hydrogen source, steps (e) or (f) are postulated frequently (Section 51.1.3.3). The pathways summarized in Scheme 1 are readily modified for hydrogenation of dienes, polyenes and alkynes. Certain monohydride catalysts exhibit high selectivity towards hydrogenation of conjugated dienes and polyenes to give monoenes. Some such catalysts are given in Table 7. Addition of the metal hydride now yields o- or 7r-allyl intermediates, whose formation and interconversion may be dependent on the concentration of an associated ligand L (equation 70); lower ligand concentration gives a more unsaturated metal environment that allows formation of x-allyls (14), the syn form being favoured. Both a- and x-allyl complexes from some of the hydrides listed in Table 7 have been isolated or detected and used as catalysts, particularly with cobalt carbonyl phosphine systems.139 —CH=CH—CH=CH—

^ ^

-CH2—CH-CH=CH-

^

(13)

-CH 2

g ^C^T^C" H | H MLfl-! (14)

^ +L

—CH2—CH=CH-CHI MLn (15)

, x (70)

Product selectivity (for example, butadiene can give but-1-ene or but-2-enes) will depend on the relative concentrations of (13), (14) and (15), and their mode of decomposition; all three npssibilMes^leD£JU)Mo^usJoJ(U, ($)/wAJf)p£-ScheraeJ.,h^w_heeajavoked_/u3^_rb£^et3i1j?d_ nature of these is important in rationalizing product distributions. A few examples based on butadiene substrate will suffice. Thus 1-butene could be formed by protonation of (15) at the 7-carbon to give a carbonium ion that then undergoes 1,2-elimination (equation 71), or via metal hydride attack at the 7-position of (15) (equation 71), at the a-position of (13) or on the x-allyl (14). Either cis- or trans-2-butene could result from 7-protonation of (13) or attack of metal hydride on (15). 2-Butene formation via metal hydride attack on (14) would strongly favour the trans isomer. All these possibilities have been considered for catalysis using aqueous/methanolic [CoH(CN) 5 ] 3 ~ systems, where 1-butene or trans-2-butenQ formation are favoured at high and low cyanide/cobalt ratios, respectively.184 Variation of butene product ratios with concentration of the hydride at high cyanide concentration also implies direct isomerization of (13) ^ (15). ML n (CH 2 CH=CHMe)

-^

MLn(CH2CHCH2Me)

—•• MLW +

CH 2 =CHEt

(71)

|MHLW

2MLn

+

CH 2 =CHEt

Most of the other catalysts of Table 7 operate in non-aqueous media, and decomposition of the allyls via H 2 [cf. steps (d) and (g) in Scheme 1], as well by the metal hydride route (step f), has been demonstrated especially for the cobalt(I) carbonylphosphine systems.139 Reaction of

Addition ofH2 and HCN to C=C and C^C Bonds

311

Table 7 Catalysts for Selective Reduction of Dienes and Polyenes to Monoenes; * Signifies Effective for Conjugated Systems Only Hydride catalyst, or precursor (ref.) Monohydride Systems *[{Cr(CO)3Cp2}2] (1); [Cr(CO)3(phenylpolysiloxane)] (2); [RuCl2(PPh3)3] (3-5); [RuCl2(CO)2(PPh3)2] (6, 7); [RuHCi(PPh3)(i?-C6Me6)] (8); [Ru(O2CMe)2(PPh3)2] (9); Chlororuthenate(II) species (10); *[CoH(CN) 5 3 -] (11,12); *[Co(CN) 3 L]" a (13, 14); [CoCl 2 (H 2 NC 6 H 4 R) 2 ] b (15); [Co(dmgh)2py«]c(16); [CoH(CO)4] (17); [CoH(CO) w (PR 3 ) 4 -«] d (18-20); [CoH3(PPh3)3], [CoH(diphos)2]e (12, 21); RhCl 3 /L f (22, 23); {Rh(dmgh)2L}2]g (24, 25); [RhHL 4 p (6, 26); [{Rh(CO)2(PPh3)2}2] (9, 27); Ni 2 (CN) 6 ] 4 " * (28, 29); MCl 2 /L f , M = Ni, Pd (22, 23); [NiX2L2]J (29-31); MClL 2 (C 3 H 5 )] k , M = Ni, Pd (32, 33); [NiCl(r73-C3H5)2] (34); *PdCl 2 /DMF (35, 36); PdX 2 L 2 ]' (35, 37, 38); [PdCl(PPh3)L]m (37, 39); [PtCl 6 ] 4 -/SnCl 2 (40, 41); PtX 2 L 2 ] n (40, 42); some Ziegler catalysts (Table 13, Section 51.1.4.2). Dihydride Systems *[Cr(CO) 6 ], *[Cr(CO)3(arene)] (1, 43-45); [MoH2Cp2] (46); [Fe(CO)5], [Fe(CO)3 (diene)] (47 and Section 51.1.3.2,ii); [RhCl(PPh3)3] (3, 48 and Section 51.1.4.1); [ML 2 S 2 ] + °, M = Rh, Ir (6, 12, 25, 49 and Section 51.1.3.2,ii); M(diene) 2 ] + , [M(diene)L2]+ P, M = Rh, Ir (50); [Rh(O2CMe)(PPh3)3] (9); [IrCl(CO)(PPh3)2] (51); [IrH2(PPh3)2(M-Cl)2Ir(cod)] (52). Other Systems [VCp2] (53); [ReOX3L2]i (54); [{RuCl2(A^-phenylalanine)}rt] (55); [Ru(cod)(7?6-C7H8)] (56); [CoX(PPh 3 ) 3 r (57); [Co(bipy)(PR3)2]+ (58); [Co(bipy)2]+ (59); [RhCl2(BH4)(DMF)py2] (6); [RhCl(DMSO)(PPh3)2]/BH4 (60); [IrCl 6 p-/SnCl 2 (61); [Ir(catecholato)(NO)(PPh3)] (62); NiCb/BHj/DMF (Section 51.1.4.1); PdCh/BH^/polyvinylpyrrolidone (63); [Pd2(dppm)3] (38). a L = en, phen, bipy. b R = o-Me, o- and p-OMe. c n = 1,2; dmgh2 = dimethylglyoxime. d n = 1-3; R = alkyl, aryl. e AlClEt2 added as cocatalyst. f L = polyethylenimine, tyrosine, phen. &L =

H 2 O, PR3.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

L = PPh3, {Cj\

J Q ) • ' BH4

isthe

reductant. J X = halide; L = PR 3 or L2 = diphos

Ph L = tertiary-phosphine, -arsine or-phosphite. ' X = halide, CN"; L = PPh3, AsPh3, PhCN or L2 = dppm, diphos; SnCl2 sometimes added as cocatalyst. m L = r?3-C3H5, 773-CgHi3. n X = halide, CN~; L = tertiary-phosphine, -arsine, -phosphite; SnCl2 usually added as cocatalyst. ° L = monodentate tertiary phosphine; S = solvent or L. P L = nitrile. * X = halide, L2 = diphos or (PR3)2. r X = halide; BF3 • OEt2 or AgClO4 added as cocatalyst. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter VII, Section A; Chapter XIV, Section D. R. A. Awl, E. N. Frankel, J. P. Friedrich and C. L. Swanson, J. Polym. Sci., Polym. Chem. Ed., 1980,18, 2663. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIA. Ref. 1, Chapter IX, Section B. L. K. Freidlin, E. F. Litvin and K. G. Karimov, Zh. Obshch. Khim., 1974, 44, 2531. Ref. 3, Section IIB. D. R. Fahey, in 'Proceedings of the 5th Conference on Catalysis in Organic Synthesis', 1976, 287 (Chem. Abstr.,1911, 86, 29 194). M. A. Bennett, T-N. Huang, A. K. Smith and T. W. Turney, J. Chem. Soc, Chem. Commun., 1978, 582. A. Spencer, J. Organomet. Chem., 1975, 93, 389. K. L. Freidlin, E. F. Litvin and K. G. Karimov, Izv. Akad. Nauk SSSR, Ser. Khim., 1974, 821. Ref. 1, Chapter X, Section B. Ref. 3, Section IIC. L. K. Freidlin, L. I. Gvinter and L. N. Suvorova, Zh. Org. Khim., 1979,15,1818. T. Funabiki, S. Kasaoka, M. Matsumoto and K. Tarama, /. Chem. Soc, Dalton Trans., 1974, 2043. L. K. Freidlin, L. I. Gvinter, A. V. Ablov, M. O. Broitman and L. N. Suvorova, Izv. Akad. Nauk SSSR, Ser. Khim., 1977,2112. R. Miyagawa, M. Yamazaki and T. Yamaguchi, Nippon Kagaku Kaishi, 1978,1305 {Chem. Abstr., 1979, 90, 5950). Ref. 1, Chapter X, Section E. Ref. 1, Chapter X, Section F. G. F. Ferrari, A. Andreetta, G. F. Pregaglia and R. Ugo, J. Organomet. Chem., 1972, 43, 213. T. E. Zhesko, D. V. Mushenko, N. S. Barinov, A. G. Nikitina, E. G. Novikova, A. P. Khvorov and S. V. Shapkin, Kinet. Katal., 1976,17,1219. Ref. 1, Chapter X, Section G. V. M. Frolov, O. P. Parenago and L. P. Shuikina, Kinet. Katal., 1978,19,1608. V. N. Perchenko, I. S. Mirskova and N. S. Nametkin, Dokl. Akad. Nauk SSSR, 1980, .251,1437. M. V. Klyuev, B. G. Rogachev and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., 1978, 2620. k

1.

h

312 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

Addition ofH2 and HCN to C=C and C=C Bonds

Ref. 1, Chapter XI, Section G. S. Siegel and G. Perot, J. Chem. Soc, Chem. Commun., 1978, 114. Ref. 1, Chapter XI, Section D. L. K. Freidlin, L. I. Gvinter, L. N. Suvorova and S. S. Danielova, Izv. Akad. Nauk SSSR, Ser. Khim., 1973, 2260. Ref. 1, Chapter XIII, Section A. T. E. Zhesko, Y. N. Kukushkin, A. G. Nikitina, V. P. Kotel'nikov and N. S. Barinov, Zh. Obshch. Khim., 1979,49,2254. T. E. Zhesko, Y. N. Kukushkin, N. S. Barinov, V. P. Kotel'nikov, A. G. Nikitina and V. P. Spevak, Koord. Khim., 1979, 5, 423. M. Kanai and A. Miyake, Jpn. Pat. 71 29 127 (1971) (Chem. Abstr., 1971, 75,150 891). G. Strukul and G. Carturan, Inorg. Chim. Ada, 1979, 35,99. V. M. Frolov, O. P. Parenago and B. A. Dolgoplosk, Kinet. KataL, 1973,15,125. Ref. 1, Chapter XIII, Section B. A. Sisak, I. Jablonski and F. Ungvary, Ada Chim. Acad. Sci. Hung., 1980,103, 33. Y. Fujii and J. C. Bailar, J. Catal, 1978, 55,146. E. W. Stern and P. K. Maples, / . Catal, 1972, 27,120. G. Carturan and G. Strukul, / . Organomet. Chem., 1978,157,475. Ref. 1, Chapter XIII, Section C. W. Strohmeier and L. Weigelt, Z. Naturforsch., TeilB, 1977, 32B, 109. J. C. Bailar, Jr., Adv. Chem. Ser., 1979,173,1. Ref. 3, Section IID. P. Le Maux, J. Y. Saillard, D. Grandjean and G. Jaouen, J. Org. Chem., 1980, 45,4524. M. Cais, Chim. Ind. (Milan), 1979, 61, 395. A. Nakamura and S. Otsuka, Tetrahedron Lett., 1973, 4529. Ref. 1, Chapter IX, Section A; Chapter XIV, Section B. Ref. 1, Chapter XI, Section B. Ref. 1, Chapter XII, Section B. Ref. 1, Chapter XI, Section H; Chapter XII, Section C. Ref. 3, Section VIII. M. Gargano, P. Giannoccaro and M. Rossi, J. Organomet. Chem., 1977,129, 239. A. Kanai and A. Miyake, Jpn. Pat. 69 12 125 (1969) (Chem. Abstr., 1969, 71,102 460). T. E. Zhesko, K. V. Kotegov, A. G. Nikitina, N. K. Kobilov and D. V. Mushenko, Zh. Fiz. Khim., 1980,54, 202. E. F. Litvin, L. K. Freidlin, K. G. Karimov, M. L. Khidekel and V. A. Avilov, Izv. Akad. Nauk SSSR, Ser. Khim., 1971, 1539. M. Airoldi, G. Deganello, G. Dia and G. Gennaro, J. Organomet. Chem., 1980,187, 391. K. Kawakami, T. Mizoroki and A. Ozaki, / . Mol. Catal., 1979, 5,175. A. Camus, C. Cocevar and G. Mestroni, J. Organomet. Chem., 1972, 39, 355. N. Yamamoto, H. Kanai and K. Tarama, Chem. Lett., 1977, 1377. L. K. Freidlin, Y. A. Kopyttsev, E. F. Litvin and N. M. Nazarova, Zh. Org. Khim., 1974,10,430. I. Jardine, R. W. Howsam and F. J. McQuillin, / . Chem. Soc. (C), 1969, 260. B. Giovannitti, M. Ghedini, G. Dolcetti and G. Denti, / . Organomet. Chem., 1978,157,457. N. M. Nazarova, S. R. Sergienko, M. A. Annamuradov, L. K. Freidlin and V. A. Petukhov, Neftekhimiya, 1979,19, 366 (Chem. Abstr., 1979, 91,107 251).

H2 with (13) and (15) (for butadiene) would give 1-butene and 2-butenes, respectively. Oxidative addition of H2 to the 7r-allyl (14) is usually thought to proceed via the 7r-alkenyl dihydrides, e.g. equation (72), and to give less selective pathways. In general, the selectivity for hydrogenation of dienes in the presence of monoalkenes is attributed to the favoured formation of the 7r-allyls, particularly when the diene or alkene has to compete for a coordination site with ligands present in the system. Table 7 lists separately monohydride and dihydride systems together with those considered to operate via monohydrides or dihydrides, respectively, and some labelled 'other systems' in which the nature of the intermediates is unknown or more equivocal. Many of the systems, particularly monohydrides, that effect selective reduction of non-conjugated systems probably achieve this via a prior double-bond migration to give the conjugated substrate (cf. equation 60; see also Section 51.1.4.1).

N

C tT*T* s C X H

L^

CH=CH- ^

-CH^

—+ - C H 2 - C H = C H M H

+

MHL«-i

(72)

L

< *> -I

In hydrogenations of a,/3-unsaturated carbonyl compounds with monohydride (and dihydride) catalysts, 7r-oxapropenyl (or pseudo 7r-allyl) intermediates have sometimes been invoked39

Addition ofH2 and HCN to C=C and C^C Bonds

313

(equation 73; cf. equation 70), but there is no direct evidence of such species. For example, on adding [IrHCl2(DMSO)3] to benzylideneacetophenone (chalcone), the chelated complex (16) was isolated.183

-C==C—C=O

—** —CH—C^T^O

^

—CH—CH—C=O

+

2M

(73)

1V1

MH

PhC

II

CHPh

I

O—>IrCl2(DMSO)2 (16) Hydrogenation of alkynes by pathways analogous to those of Scheme 1 involves key vinyl intermediates rather than alkyls (see 12 in equation 68). Some reasonably active monohydride catalysts for reduction of alkynes are considered later in Section 51.1.6. The [RuHCl(PPh 3 ) 3 ] catalyst will selectively reduce 1-hexyne to 1-hexene first in a mixture with 1-octene, even though individually the alkyne substrate is reduced more slowly;119 such selectivity can be explained if, as seems likely, the binding of the alkyne is stronger than that of the alkene [step (a) in Scheme 1 ], while a rate-determining oxidative addition of H2 is slower for an alkyne substrate [step (b); see also Section 51.1.3.1,1]. There are a few instances reported of a net trans addition of H 2 to alkenic and alkynic substrates using monohydride catalysts, and in some cases the mechanistic pathways of Scheme 1 have been modified by including steps involving activation of C—H bonds of the substrate. For example, the reduction of hexamethyl-Dewar-benzene to mainly (18) using a bromide-promoted platinum-tin chloride system (equation 74) probably proceeds via the steps outlined in equation (75), once the initial migratory insertion reaction to give (19) has occurred. 127192 The scheme accounts for an observed deuterium isotope exchange within the methyl groups on carrying out deuteration of the Dewar-benzene in C2H5OD, and allows for formation of (18) via hydrogenation of the exocyclic alkene (20), and for formation of the cis addition product (17) via deuterolysis of (21) or (22). A process to generate (18) by direct protonolysis of (19) through a back-side attack (i.e.

(74)

(17)

PtH

(18)

PtD

(19)

(21)

314

Addition ofH2 and HCN to C=C and C=C Bonds

trans to the Pt—C bond) was also considered and seems likely in some systems.180 An observed trans-hydrogen addition to diphenylacetylene to give trans-stilbenQ, catalyzed by [RhCl 2 (BH 4 )(DMF)py2] (Section 51.1.4.1) has been accounted for by a related mechanism involving activation of the ortho-hydrogen of a phenyl group (Section 51.1.3.2,ii, Scheme 5). 122 It should be noted that the catalytic species in both the above examples are not well defined, and it is not known whether the rhodium catalyst involves a mono- or di-hydride. Some trans hydrogenations catalyzed by [CoH(CN)s] 3 ~ (acetylenedicarboxylic acid —* fumaric acid, diphenylmaleic acid —• (±)-diphenylsuccinic acid) 184 may involve free radical pathways (see below). A net trans addition to diphenylacetylene using an [IrH3(PPli3)2] catalyst with ethanol as the hydrogen source (Section 51.1.3.3) was attributed to isomerization of initially formed ds-stilbene.193 Some monohydride catalysts operate by free radical pathways involving transfer of hydrogen atoms. Wender had suggested such a possibility in the early 1950s with [CoH(CO) 4 ] in some aldehyde reduction and alcohol hydrogenolysis reactions observed under hydroformylation conditions, but there was no strong experimental evidence and indeed later work by other groups disproved the claim, at least for aldehyde reduction.39 However, a more recent reassessment of experimental data on hydrogenation of polycyclic aromatic hydrocarbons with this catalyst (especially deuterium exchange, product distributions and correlation of rates with radical localization energies) has led to the suggestion that free radicals, rather than organocobalt complexes, are involved.172 The suggested mechanism is outlined in equations (76)-(78). A stoichiometric hydrogenation of 1,1-diphenylethylene using [CoH(CO)4] has also been discussed in terms of free radical intermediates. 194

CoH(CO)4

+

substrate (S)

CoH(CO)4

+

2«Co(CO)4

—•

SH

^

—*-

Co2(CO)8

-€0(00)4•Co(CO)4 -^

+

+

SH

(76)

SH2

(77)

2CoH(CO)4

(78)

Direct evidence for such a mechanism comes from a stoichiometric hydrogenation of a-methylstyrene by [MnH(CO) 5 ] (equation 79); the observation of CIDNP effects in NMR spectra, especially the polarization of the doublet signal arising from the methyl protons of the isopropylbenzene, was attributed to competition within the geminate radical pair [PhCMe 2 , Mn(CO)s] to reform the reactants or to separate into the radicals with ultimate formation of products. 195

PhCMe=CH 2

+

2MnH(CO)5

—^

PhCHMe2 +

Mn2(CO)10

(79)

The H-atom transfer mechanism also seems well-established for certain hydrogenations catalyzed by [CoH(CN) 5 ] 3 ~. Detailed kinetic studies with some a,/3-unsaturated acids, esters and nitriles indicate a mechanism analogous to that outlined in equations (76)-(78). 172 ' 184 ' 196 Other activated alkenes including various styrenes may be reduced via the same mechanism. Hydrogenation of trans-\-phenyl- 1,3-butadiene is thought to proceed via the radical mechanism in glycerol-methanol solution, but in water the organometallic mechanism via an alkenyl plus Co—H step was favoured (cf. equations 70 and 71). Even when the radical mechanism is operating, organocobalt complexes may be present but they play no essential role mechanistically; their formation simply ties up available cobalt and inhibits reduction. Detailed kinetic studies on the addition of [CoH(CN) 5 ] 3 ~ to a,/3-unsaturated compounds R C H = C H X are generally consistent with the organocobalt product being formed via intermediate radical species (e.g. equation 80) rather than via the usual four-centre transition state (3a) or electrophilic attack via Co 6 "—H 6+ (cf. equation 19 for the various formal representations of metal hydrides).184 The usually observed net cis addition requires combination of the cobalt and the organic radical within a cage to occur at a rate greater than that of rotation about the ce,/3-carbon-carbon single bond. Indirect findings supporting the behaviour of the hydride as [Co n (CN) 5 ('H)] 3 ~ include an observed radical

Addition ofH2 and HCN to C=C and C=C Bonds

315

polymerization initiated by the hydride and production of dimers during some catalytic hydrogenations. 184 RCH=CH^X H—Co

—*"

|RCH 2 —CH-X} { \ I Co )

—*

RCH2-CH-X I Co

(80)

More serious consideration for H-atom transfer mechanisms should be given for other monohydride catalyst systems. A hydrogenation of alkenes (especially a,/3-unsaturated compounds) which often accompanies hydroformylation catalyzed by simple binary carbonyl complexes (Section 51.1.4.1) could result from such free radical pathways. 197 A further mechanistic pathway available with more acidic metal hydride catalysts [cf the M ^ H ) formulation in equation (19) and the postulated steps in equations (67) and (69)] involves a two-electron reduction of the substrate and the addition of two protons. The reduction of alkenes and alkynes by aqueous solutions of low-valence transition metal salts and complexes in stoichiometric reactions in the absence of H 2 is well known. Equation (81) represents the stoichiometry for Cr(II) 95 and V(II) salts.198 Other examples include as reductants Mo(V) 118 and Ti(III) salts,151 [Ni 2 (CN) 6 ] 4 -, 1 2 3 [Cr(CO) 6 ], 95 [Co(dipy) 3 ] + 199 and [Rh(dipy) 2 ] + , 44 the oxidation products being Mo(VI), Ti(IV), Ni(II), Cr(III)?, Co(II) and Rh(III), respectively. Acidic methanol solutions of [Co(CO)4]~ 20 ° and basic solutions containing [Fe(CO)5p 01 are similarly useful for stoichiometric hydrogenation of conjugated carbon-carbon double bonds (see Section 51.1.3.3). 2M"

+

substrate(S)

+

2H+

—>

2M ni

+

SH2

(81)

A variety of mechanisms have been presented for these 'reduction-protonation' processes, but they usually involve (i) complexation of the substrate followed by electron transfer from the metal and protonation of carbanion intermediates (cf equation 67), or (ii) protonation at the metal followed by hydride transfer to the substrate with oxidation at the metal (cf equation 47), and then protonolysis. The hydrogenations involve a net hydrogen transfer from water. Clearly, in the presence of a reducing agent such as Zn/HCl, [BH4]~ or carbon monoxide or by electrochemical means (Section 51.1.3.3), the hydrogenations can become catalytic in metal. Borohydride has been widely used as the consumable reductant in many catalyzed hydrogenations (see Table 8), but since this reagent can produce monohydride complexes directly (contrast with (ii) above), and also molecular H 2 by its slow hydrolysis, there is often considerable uncertainty in the mechanistic details. Thus, for example, with the cobalt(II) and cobalt(III) complexes listed in Table 8, borohydride could generate Co(I) or Co(III) hydride species and, considering the 'equivalence' of these and the documented reaction pathways available to them (Scheme 1, reductive protonation, equations (67) and (69), as well as hydrogenolysis of cobalt-carbon bonds Table 8 Hydrogenation Catalysts Using BHU" as Reductant Catalyst (ref.) Fe(CO)5] (1); [Co(CN) 5 ] 3 -, [Co(bipy)3]2+ (2); {Co(dmgh)2py}2]a, [CoX(dmgh)2L]b, Vit B 12 systems (2-5); Co(TSPP)] 3 - c (6); [RhH(NH 3 ) 5 ] 2 +, [RhHClL4]+,d [RhH(dmgh)2Cl]" a (7); [Ni 2 (CN) 6 ] 4 ", [Ni(CN)2phen] (8); [PdCl2(DMSO)2] (9) a

1. 2. 3. 4. 5. 6. 7. 8. 9.

dmgh2 = dimethylglyoxime. b X = halide, L = py, PR3, H 2 O. c TSPP = dianion of meso-tetrakis(p-sulphonatophenyl)porphyrin. d L4 = trien, (en)2, py4. A. Misono, Y. Uchida, K. Tamai and M. Hidai, Bull. Chem. Soc. Jpn., 1967, 40, 931. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter X, Section B5. Ref. 2, Chapter X, Section H. B. R. James, Adv. Organomet. Chem., 1979,17, Section IIC. M. N. Ricroch and A. Gaudemer, J. Organomet. Chem., 1974, 67,119. E. B. Fleischer and M. Krishnamurthy, /. Am. Chem. Soc, 1972, 94,1382. R. D. Gillard and G. Wilkinson, J. Chem. Soc, 1963, 3594. Ref. 2, Chapter XIII, Section A. L. K. Freidlin, Y. A. Kopyttsev and N. M. Nazarova, Izv. Akad. Nauk SSSR, Ser. Khim., 1974, 604.

316

Addition ofH2 and HCN to C=C and C=C Bonds

by borohydride itself), elucidation of the mechanism, even ignoring free radical intermediates, presents a quite bewildering problem! There are no clear-cut examples involving organic substrates where H2 reduced the oxidized metal species in reactions such as that shown in equation (81) back to lower valent species. The best examples are provided by the somewhat ambiguous Co(II)/Co(III)H/Co(I) systems (equation 82). 2CO11

+

H2

^

2Co IH H

^

2Co*

+

2H+

(82)

(//) Dihydride catalysts Scheme 2 shows reaction pathways available for hydrogenation of alkenes using dihydride catalysts. 22 ' 137 The k\ step defines the hydride route, and k2 the unsaturate route via oxidative addition of H 2 to a metal-alkene complex. Both lead to the same key dihydride alkene intermediate (23), which decomposes to the product by two successive hydrogen atom transfers, the &3 and &4 steps. The metal is released as M with appropriate ancillary ligands and reenters the catalytic cycle via coordination to H2 and/or alkene. As with monohydride systems, an overall cis addition of H 2 results from the coplanar migratory insertion, the k3 step (3a), and a reductive elimination of the product occurring with retention of configuration at the metal-bonded carbon, the k4 step.

MH2

H 2 , KH2

M (24) Scheme 2

Mechanistic pathways for dihydride catalysts

Dihydride formation (KH2) a n d alkene binding (Ks) are usually rapidly established equilibria, and only very rarely have intermediates such as (23) and (24) been detected (see below). If the hydrogen transfer steps (A:3 and £4) are relatively fast, a rate law of the form shown in equation (83) results, the k\K\\2 and k2Ks terms referring to the hydride and unsaturate route, respectively.143 Thus the kinetics alone do not distinguish between the two pathways; reaction via one (k\Ku2 or k2Ks = 0) or both pathways leads to a rate law of the same form. The unsaturate pathway is generally less efficient since it involves oxidative addition of H 2 to a preformed complex containing a 7r-acceptor alkene ligand (Section 51.1.3.1,i). However, in cases where the simple dihydride MH 2 is not detected (K\\2 is very small) and where there is evidence for formation of the alkene complex (K$ is measurable), the unsaturate pathway is usually postulated. In systems where treatment with H 2 in the absence of the unsaturated organic leads to metal production, the unsaturate pathway necessarily has to be invoked. This is particularly true for platinum metal systems that function in the absence of 7r-acceptor ligands (CO, PR 3 , [SnCl 3 ]~, etc.) known to stabilize metal hydrides. The unsaturated organic substrate is effectively playing the role of such a ligand. Table 9 lists some catalyst systems that are thought to operate solely via the unsaturate k2Ks path. Complexes that give isolable dihydrides (Table 1) are generally considered to operate by the dihydride route, although both routes have been invoked in cases where both alkene and hydrogen are known to coordinate separately, e.g. [Co(N 2 )(PPh 3 ) 3 ], 202 [CoH(N 2 )(PPh 3 ) 3 ], 172 [Ru(PPh 3 ),(solvent) n ] 2 + , 1 3 2 [RuH 2 (PPh 3 ) 4 ], 1 3 2 ' 2 0 3 [RhCl(PPh 3 ) 3 ] 9 3 and [IrH(CO)(PPh 3 ) 3 ]. 1 7 2 ' 1 8 2

Addition ofH2 and HCN to C=C and C=C Bonds Kate

_ "

-d[H 2 ] dt

_ ~

( ^ ^ H , + A:2^s)[M][H2IS] 1 + KH2[H2] + i^s[S]

317 (83)

Table 9 Catalysts that Operate via Dihydride Intermediates within Unsaturate Routes for Hydrogenation of Alkenic Substrates Catalyst (ref.) [Fe(CO)3(diene)] (1); [Co(PPh2Me)3(solvent)] + (2); Rh(I)/R 2 S a (3, 4); Chlororhodate(I) (3); Rh(I)/amines (5); [Rh(diphos)(solvent)2] + b (6, 7); [Rh(C 2 H 4 )2(CH 3 CN)2] + (8); [M(diene)2]+, [M(diene)(MeCN)2], M = Rh, Ir (9, 10); [M(7/6-C8H10)]+ c, M = Rh, Ir (11); [{IrCl(C8H14)2i2] (10, 12, 13); fAWw-[IrCl(CO)(PPh3)2] (14, 15); [IrCl(PPh3)(cod)] (16) R = Et, CH2Ph. b See the text in this section; chiral chelating diphosphines have also been used. c Ligand formed from dehydrogenation of 1,3,5-cyclooctatriene. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter IX, Section A; Chapter XIV, Section B. K. Kawakami, T. Mizoroki and A. Ozaki, /. Mol. CataL, 1979, 5, 175. Ref. 1, Chapter XI, Section H. B. R. James and F. T. T. Ng, Can. J. Chem., 1975, 53, 797. F. Pruchnik, Inorg. Nucl. Chem. Lett., 1974,10, 661. B. R. James, Adv. Organomet. Chem., 1979,17, Sections IIB, IIIA. A. S. C. Chan, J. J. Pluth and J. Halpern, J. Am. Chem. Soc, 1980,102, 5952. F. Maspero, E. Perrotti and F. Simonetti, Ger. Pat. 2 263 882 (1973) (Chem. Abstr., 1973, 79,92 386). Ref. 1, Chapter XI, Section H. Ref. 1, Chapter XII, Section C. P. T. Draggett, M. Green and F. W. S. Lowrie, J. Organomet. Chem.,1911,135, C60. H. Van Gaal, H. G. A. M. Cuppers and A. van der Ent, J. Chem. Soc, Chem. Commun., 1970, 1694. C. Y. Chan and B. R. James, Inorg. Nucl. Chem. Lett., 1973, 9, 135. Ref. 1, Chapter XII, Section A. M. Burnett, R. J. Morrison and C. J. Strugnell, /. Chem. Soc, Dalton Trans., 1973, 701. R. N. Haszeldine, R. J. Lunt and R. V. Parish, J. Chem. Soc. (A), 1971, 3711. a

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

As written, rate law (83) ignores any ligand dissociation reactions, processes that are usually critical for effective catalysis and are important in some or all of the A:H2, ^ s , &i and k2 steps. As discussed for the monohydride catalysts, rate laws are usually complicated by further denominator terms in ligand concentration. Again, analysis of the kinetics is usually accomplished by the use of inverse plots (Section 51.1.3.2,i). The hydrogen transfer steps create vacant sites at the metal centre and such reactions could well be promoted by ligand addition reactions. The solvent may also play an important role as a coordinating ligand, and complex rate laws containing terms in solvent concentration have appeared. 204 A cationic [Rh(diphos)(r?-C 6 H 6 )] + species containing coordinated benzene, which has been characterized crystallographically, is a precursor for catalytic hydrogenation via an unsaturate pathway in which alkenic substrate replaces the benzene. 132 ' 205 Solvent benzene is thus acting as a competitive inhibitor ligand. Some alkene and arene binding constants to a solvated [Rh(diphos)] + species in methanol at 25 °C [1-hexene (21 mol" 1 ), methyl acrylate (3), benzene (18), styrene (20), toluene (97), xylenes (-500), methyl (Z)-a-acetamidocinnamate (5.3 X 103)] show this quantitatively for this particular system; the common aromatic solvents bind much more strongly than the simple terminal alkenes. The data for styrene correspondingly indicate binding via the phenyl ring rather than the alkenic group.132 Similar cationic monomeric species containing coordinated methanol, acetone and acetonitrile have been characterized spectroscopically in solution (equation 43); 132 however, in the presence of base including triethylamine (a common additive in many catalytic hydrogenation systems), methanolic solutions of [Rh(diphos)] + , for example, contain [Rh 3 (diphos)3(OMe) 2 + with triply bridged methoxides. Further, the species isolated from a solution containing monomeric [Rh(diphos)(MeOH) 2 ] + was a dimer held together by x-arene bridges involving phenyl groups of the phosphine ligand (25). 206 Coordinatively unsaturated species, so essential for catalysis, will clearly undergo quite novel chemistry with available ancillary ligands or solvent. Such chemistry necessarily complicates an understanding of the hydrogenation reaction mechanisms. These cationic systems are considered in more detail later in this section. CO.M.C. VOL. 8—K*

318

Addition ofH2 and HCN to C=C and C=C Bonds

(25)

Observed rate laws which incorporate all the variables of a multistep process are of little value in gaining information about an individual step. Consider the simple case for the unsaturate path only with strong alkene binding (#s[S] » 1) and where no MH 2 is formed (Ku2 ~ 0); rate law (83) reduces simply to k2[M] [H 2 ]. This rate law, together with the chemical evidence, certainly indicates an unsaturate route, but the rate determining step (Scheme 2) could be the k2 step itself or even one of the hydrogen transfer steps if intermediates (23) or (24) were formed relatively rapidly and were present in undetectable amounts. If the kinetic and thermodynamic parameters for each individual step in a catalytic cycle can be evaluated, and these can be combined to calculate the experimental measured rate of the overall reaction, then the mechanism can be considered to be fully elucidated. Such a stage has almost been reached with the widely used [RhCl(PPh3)3] catalyst, and it is illuminating to consider this system and some related cationic ones to see the depth of understanding obtainable and some remaining limitations. Since its discovery (Section 51.1.2), at least 40 papers have been published dealing solely with mechanistic aspects. The historical development can be traced through the more recent references.93'207"212 An early complication arose due to molecular weight determination in solution, which indicated a high degree of phosphine ligand dissociation; this was subsequently shown to be due to the presence of trace oxygen (Section 51.1.3.1,iii). Such a problem is also encountered in the versatile [RuHCl(PPh 3 ) 3 ] catalyst93 and is probably a general one for catalysts containing tertiary phosphine type ligands. Scheme 3, based mainly on studies by Halpern's group and a group at DuPont, 93 shows mechanistic details for the hydrogenation of cyclohexene catalyzed by [RhCl(PPh 3 ) 3 ] in benzene at 25 °C. By studying separately the reaction of H 2 with [RhCl(PPh 3 ) 3 ], and the reaction of cyclohexene with the dihydride (28), kinetic and thermodynamic data for nearly all the steps important for the catalytic cycle have been estimated. The major catalytic pathway shown in the rectangle apparently involves entirely non-detectable species, including the precursor 14-electron solvated species RhClP 2 — a caveat indeed for those postulating mechanisms based on detectable or isolable species without accompanying kinetic data! For this system, only species shown outside the box, as well as a [RhClP2(alkene)] complex (not shown), have been observed. A less effective catalytic cycle based on this alkene complex, as well as one based on the chloride-bridged dimer (32) and the dihydride (33) (Section 51.1.3.1,i), also exist for this system. The boxed cycle corresponds to the hydride route of Scheme 2. The data for Scheme 3 show that the solvated dihydride (29) is formed more rapidly via nondetectable complex (26) than via the isolable dihydride (28), and appear to show that the rate-determining step is k& alkene insertion to give the alkyl hydride. At 1 atm H 2 , where the dihydrides (28) and (29) are fully formed, the catalytic hydrogenation rate is given by A^sIRh] t o tai[alkene]/(^[alkene] + P), which becomes simply &6[Rh] total a t a sufficiently high (and readily obtainable) alkene concentration.213 Under such conditions, the alkenic dihydride species (30) should be detectable unless most of the rhodium is tied up outside the catalytic cycle (e.g. as [RhClP 2 (alkene)]). The stereochemistry of complex (28) is known and since the phosphine trans to the hydride is labile, the intermediate (30) is expected to have the stereochemistry shown. The subsequent migratory insertion reaction must lead to a species with trans -disposed hydride and alkyl groups, and thus even the single k^ path as written will involve an isomerization (presumably fast) to give the required cis-disposed moieties for reductive elimination of product. An isotope effect for the rate determining k^ path (k\\2lk\y2 = 1.15) is considered consistent with the breaking of an Rh-—H bond.214 It should be noted that other groups measuring overall kinetics for the same cyclohexene system (including attempts to computer-fit extensive data to an assumed mechanism) have reached quite different conclusions regarding the mechanism, with alkene coordination to dihydride species usually being considered rate determining.93 The hazards of using initial rates only for speculation about mechanisms are self evident, considering rate laws of the form shown in (83) pertain.

Addition ofH2 and HCN to C=C and C=C Bonds

/

R h /

CY—•

-* (~H2)

HP

319

Cl' p

(27)

(28) :-3, (P)|*3,

H

Pv—I [RhClP 2 ] 2

/ R h / _C1Z

'P

("H2)

/H

Cl'

K,(S)

(32) (26)

h H 2 [RhClP 2 ] 2

fast

(33)

H k6 rate-determining

(31)

Cl

(30)

Scheme 3 Mechanistic scheme for hydrogenation of cyclohexene (S) catalyzed by [RhCl(PPh3) 3 ] in benzene; the major pathway is shown in the rectangle

Comparison of rate data for the same catalyst, e.g. [RhCl(PPh 3 ) 3 ], with a variety of alkenic substrates also has limitations in terms of mechanistic implications: there is evidence that styrene hydrogenation involves a further path requiring coordination of two styrene molecules,215 and the unsaturate route via [RhCl(PPh3)2(alkene)] appears to operate in some systems.143 It is worth noting here that a bis(styrene) species is also implicated in a catalytic hydrogenation using [Co(N 2 )(PPh 3 )3]. 172 ' 216 Differences in solvent and changes in ligands within [RhXL3] species (X = anion, usually halide; L = tertiary phosphine, phosphite, etc.) can modify the various dissociation equilibria (Scheme 3) and change the hydrogenation pathways and selectivity patterns. The factors affecting selectivity, as with monohydride catalysts, are generally complex, but with catalysts containing bulky phosphine ligands they are usually discussed in terms of abilities of the organic substrates to coordinate to the metal centre.22'143 Very often within alkenic substrates there is a general reactivity trend that decreases with increasing substitution at the alkenic link. It should be noted once again that individual rates of hydrogenation of substrates can be the wrong criteria for predicting which substrates will hydrogenate the most rapidly in a mixture. For example, like the [RuHCl(PPh3)3] catalyst,119 the [RhCl(PPh3)3] complex catalyzes hydrogenation of alkenes more rapidly than alkynes, but with a mixture of such substrates it is again possible (in acidic alcohols) to reduce the alkyne to the corresponding alkene with a high degree of selectivity prior to reduction of any alkenic substrate.143 Favoured binding of the alkyne over the alkene (commonly found within organometallic chemistry) at any stage of the catalytic cycle (Scheme 2) again readily accounts for the observed selectivity pattern. In contrast to monohydride catalysts, dihydride systems with readily reducible substrates generally show little alkene isomerization or hydrogen isotope scrambling with any component

320

Addition ofH2 and HCN to C=C and C=C Bonds

of the catalyst system. Within Scheme 2, this implies that k4 » A: 3 and is consistent with the lack of observance of hydridoalkyl intermediates. Indeed, early work on the [RhCl(PPli3)3] system had suggested that both hydrogen atoms might be transferred simultaneously in a concerted step. Data from later studies, however, on a wide variety of alkenic substrates, including accompanying isomerization and exchange reactions, require the alkene dihydride to alkyl hydride step to be an equilibrium, (23) ^ (24) 143 (cf. the alkyl-hydridoalkene requirement of Scheme 1 for the monohydride systems, equation 60). The first direct evidence for intermediates (23) and (24) has been obtained recently in some cationic iridium12 and rhodium systems.59 These [MH2L2S2] + catalysts (L = monodentate tertiary phosphine, S = solvent, Table 1) are themselves formed by hydrogenation of diene precursors (equation 43), which must involve the unsaturate mechanism of Scheme 2 with a modification for diene rather than alkene substrate. In the case of an iridium precursor system,12 a cis,ciscomplex [IrH2(cod)(PMePri2)2]+ has been detected at low temperature (—40 °C); on warming under H2, cyclooctane (presumably via cyclooctene) and the dihydride are formed according to the stoichiometry of equation (43) and the mechanisms of Scheme 2. In the absence of a hydrogen atmosphere, warming simply reverses the H 2 binding to regenerate [Ir(cod)(PMePh2)2].+ Interestingly, an isolated complex, cis,trans-[IrH2(cod)(PMePh2)2]+, which does not possess the coplanar arrangement (3a) necessary for the migratory insertion reaction, hydrogenates the coordinated diene some 50 times slower than the d^cw-analogue which has the correct geometry. The [IrH2(cod) 2 ] + species has been similarly detected at low temperature. 217 The [ML 2 S 2 ] + systems, which hydrogenate monoalkenes effectively via the hydride route, can reduce dienes specifically to monoalkenes via the unsaturate route since the diene coordinates preferentially over the monoene. 132 Because monohydrides can result also in basic conditions (equation 32), at least with the rhodium systems, the cationic systems offer considerable flexibility in available reaction pathways, and conditions have been found for effective isomerization of alkenes, as well as selective reduction of alkynes (e.g. 2-hexyne —• c/s-2-hexene; 1-hexyne —*• 1-hexene).218 An alkyl hydride intermediate (24) has been characterized, again at low temperature ( - 7 8 °C), in the hydrogenation of an activated alkene, methyl (Z)-a-acetamidocinnamate, catalyzed by [RhL 2 S 2 ] + (L 2 = diphos, S = MeOH). 59 A combination of kinetic, crystallographic and NMR techniques (*H, 31 P, 13C and 15 N) has led to a detailed understanding of this system that operates via an unsaturate route (Scheme 4). Unlike the bis-monodentate phosphine systems, the monochelating diphosphine systems do not form a dihydride; 132 ' 206 ' 219 ' 220 for example, the complex [Rh(diphos)(nbd)] + absorbs only two moles of H2 per rhodium with formation of norbornane and solvated [Rh(diphos)] + species, whereas the bis(triphenylphosphine) analogue absorbs three moles of H 2 according to equation (43). The difference has been attributed to the fact that only the latter can form a c/s-hydride in which neither hydride is in an unfavoured position trans to a phosphine (see 29 of Scheme 3), 206 although it should be noted that several bis-chelated phosphine complexes of the type [Rh{Ph2P(CH2)wPPh2)2]+ form dihydrides in which both hydrogen atoms are necessasrily trans to phosphines.221 In Scheme 4, 59 &2 is found to be rate determining over the temperature range 0-50 °C; however, at - 7 8 °C the k4 reductive elimination step is completely stopped and species (35) is detected. The NMR data confirm the expected Markownikov Rh—H addition across the alkenic link. Kinetic data were obtained for k4 (AT/* = 71 kJ mol" 1 , A S * = 25 J mol" 1 K" 1 ), which becomes comparable with k2 (AT/ = 27 kJ mol" 1 , A S * = — 118 J mol~ l K" 1 , Table 5) at about —40 °C. This system is of particular interest because related catalysts containing chiral derivatives of diphos are highly effective for the asymmetric catalytic hydrogenation of this prochiral alkene and related precursor substrates to give optically active amino acids. The chelation of the substrate via the alkenic link and the amide oxygen (Scheme 4; 34) leads to a relatively large alkene binding constant compared with values for non-chelated TT-bonded-only substrates (see above). Use of a chiral chelating diphosphine ligand, e.g. (R,R)-bappQ (formerly dipamp) (36) or (S,S)-chiraphos (37), in place of diphos has led via the mechanism outlined in Scheme 4 to hydrogenation products with extremely high enantiomeric excess.59 In such systems, intermediate (34) will now exist as two possible diastereomers in which either one face of the alkene (e.g., Ca-re, Cp-si) or the opposite face (Ca-si, C$-re) coordinates to the rhodium. The preferential cis addition of H 2 to one alkenic face gives rise to optically active saturated product. A preferred mode of binding of the prochiral alkenic substrate to the catalyst (the step corresponding to (a) in Scheme 4), brought about by a slightly more favoured conformation of the rhodium-chelate ring in which the four phenyl groups approximate to an edge-face arrangement, has usually been considered to give rise to the asymmetric hydrogenation (a preferred conformation by only 7 kJ mol" 1 is sufficient to give 95% e.e. at 25 °C). 16 ' 17 ' 222 Recent, more quantitative, data have shown that the enantioselectivity, at least in bappe and chiraphos systems,

Addition ofH2 and HCN to C=C and C=C Bonds

321

results from differences in rates of reaction of the diastereomeric catalyst-substrate complex towards H 2 (i.e step (b) of Scheme 4). It has been found that the TV-acetylphenylalanine ester product results from H2 reacting preferentially with the minor diastereomer of structure (34) [with (S,S)-chiraphos] present in solution.59 Further details on asymmetric hydrogenation are given in Chapter 53.

Ph

? [Rh(diphos)S 2 ] +

H

Ph

V-

NHCMe

+

Ph

H

V/

v?

CO 2 Me

O

Ph

Ph

II

CH 2 Ph

•

,C—CO2Me

NHCMe PhCH 2 —CH CO 2 Me

Me (35)

Scheme 4 Mechanistic scheme for hydrogenation of methyl-(Z)-a-acetamidocinnamate catalyzed by [Rh(diphos)(MeOH) 2 ] +

(36)

(37)

That reductive elimination of saturated product from an alkyl-hydride complex (A: 4 steps, Scheme 2 and Scheme 4) occurs with retention of configuration at the metal-bonded carbon has been demonstrated for a stoichiometric reaction using molybdenum-Cp species (equation 84). 223 The cis migratory insertion, reaction (a), using fumarate (R = CC^Me) yields the expected threo isomer (38), which then undergoes the reductive elimination, step (b), with retention to give racemic 2,3-dideuterosuccinate. Excluding catalytic systems, platinum(II) hydridoalkyl complexes provide accessible species convenient for studying reactions involving reductive elimination of product. 224 ' 225

MoD2Cp2

+ H

>, C = Q

'MoCp 2 ' D MoDCp 2 (38)

(84)

322

Addition ofH2 and HCN to C=C and C^=C Bonds

Reaction schemes for hydrogenation of dienes using dihydride catalysts usually follow the basic hydride or unsaturate pathways outlined in Scheme 2. However, species (24) is now a hydrido a-alkenyl or a hydrido 7r-allyl intermediate (cf. equation (70) for the monohydride systems). Such pathways have been invoked for the widely studied [Fe(CO) 5 ], [(diene)Fe(CO) 3 ] and [(triene)Fe(CO)3] catalyst systems in which the active transient species is thought to be the metal tricarbonyl. 97 ' 226 The dihydride [FeH 2 (CO) 3 ] has been detected, but unsaturate as well as hydride routes have been invoked. The reductions are not very selective due to accompanying isomerization via the 7r-allyl hydride intermediate (cf. equation 70). A final step involving molecular hydrogen to give monoene and regeneration of a dihydride, e.g. equation (85), has sometimes been invoked rather than transfer of the second hydrogen.97

FeH(CO)3(7r-allyl)

+

H2 —* FeH2(CO)3

+

monoene

(85)

Various chromium tricarbonyl complexes, particularly [Cr(CO) 3 (arene)], catalyze the hyJifyi/tf^^iji%ateAdifuif^ ,VidrtiiirtJii?s 227 22812/ctf cerium xx&raiigc studies indicated a [CrH 2 (CO) 3 ] intermediate, and a direct H 2 addition without coordination of the substrate has been invoked, as well as the more usual hydride and unsaturate routes, which could also give a net 1,4-addition of H 2 (cf. equation 70). Thus, methyl sorbate (tr-ans-2,trans•4-hexadienoate) with D 2 gives exclusively methyl 2,5-dideuterio-cw-3-hexenoate (equation 86), 95 while the tetraene shown in equation (86a) is similarly reduced, mainly to the diene shown, by addition to the top (oxygen side) face of each cyclohexadiene ring.229 Unconjugated dienes appear to isomerize to a conjugated form prior to hydrogenation (equation 87). 95 A [Co(bipy)(PR3)2] + complex and an in situ system formed by zinc reduction of Co(II)/bipyridyl solutions also catalyze a 1,4-addition of H 2 in reducing butadiene to c/s-2-butene.230'231 Some known dihydride catalysts, or systems thought to operate by dihydride routes, that are useful for selective reduction of dienes to monoenes are given in Table 7 (Section 51.1.3.2,i).

'Cr(CO)3'

(86)

(86a)

(87)

The equivalence of Scheme 2 written for the reduction of alkynes to alkenes requires that species (24) be a hydridovinyl intermediate; these have not been detected within mononuclear catalytic systems, but examples with bridging vinyl groups formed from a dihydride and alkynes within cluster systems are known (Section 51.1.6). Some dihydride catalysts that have been used for reduction of alkynes are considered in Section 51.1.6; the hydrogen addition via these catalysts to give alkene products is invariably cis. However, stoichiometric reactions of dihydrides with monosubstituted alkynes have been reported sometimes to give trans-alksnyl derivatives, e.g. equation (88). 232 In essence this reaction involves a net trans addition of a metal monohydride across the triple bond, which is difficult to formulate within a discrete metal complex; other mechanisms in which the phenyl group acts as a primary hydrogen (or metal) acceptor in a cis addition may be operative. Scheme 5 (cf. equation 75) illustrates such suggested pathways for both a mono- or di-deuteride catalytic system generating /ra«s-stilbene from diphenylacetylene;122 with the dideuteride, the initial cis- 1,4-addition of D 2 is followed by a 1,3-hydride shift, while with the monodeuteride the migration is followed by deuterium cleavage of the metal-carbon bond to generate an ort/zo-deuterated product. The [RhCl 2 (BH 4 )(DMF)py 2 ] complex (Section 51.1.4.1) carries out this catalytic reduction with incorporation of deuterium at the ortho-position. 122

Addition ofH2 and HCN to C=C and C=C Bonds

323

Cp2Zr

ZrH2Cp

(88) ^

MD 2

:—Ph D-Rh MD

Scheme 5

Possible pathways for trans -addition of H2 to phenylalkynes

Reaction (88) contrasts with that of an analogous molybdenum system (equation 89), which gives the expected c/5-stilbene product. 118 The dimeric species [Mo2(CO)4Cp2] catalyzes hydrogenation of alkynes to ds-alkenes via an unsaturate route involving reaction of H2 with the intermediate [Mo 2 (CO) 4 Cp 2 (M-RC=CR)] (cf. equation 89). 233 Ph

MoH2Cp

2PhC=CPh

+

PhCH=CHPh

(89)

51.1.3.3 Hydrogen transfer from ligands and solvents A considerable number of metal-catalyzed homogeneous hydrogenation systems not involving molecular H2 have been reported.234'235 The use of borohydride was discussed in Section 51.1.3.2,i (Table 8), where the hydrogenations can involve either hydride transfer (possibly via borohydride complexes) or electron transfer/protonation processes. Since ligands can be a source of hydride (Table 6), it is also possible to effect a stoichiometric hydrogenation of a substrate in the absence of H 2 , although for a catalytic process the hydrogen would have to be refurnished, probably by the use of molecular H 2 ; equation (90) illustrates such a stoichiometric reaction for a ruthenium system.93 Following initial formation of an alkyl, the second required hydrogen comes from the ortho position of a phenyl ring of a PPh 3 ligand. Since the chloro-bridged or^o-metalated dimer (39) does react with H2 in the presence of the mole of PPI13 already present to regenerate [RuHCl(PPh3)3], catalytic hydrogenation via such pathways is feasible, although in this particular system at least, a catalytic path under H2 is more favourably accomplished via H 2 addition to the alkyl (Scheme 1, step d). RuHCl(PPh3)3

+

alkene

- ^

i[RuCl(PPh3)(o-C6H4PPh2)]2

+

alkane

(90)

(39) Aromatic ligands are sometimes hydrogenated during exchange reactions between ferrocene and arenes that are catalyzed by AICI3 in hydrocarbon solvents;236 thus reactions with anthracene have yielded cationic species [FeCp(?76-Ci4Hi2)]+ containing 9,10-dihydroanthracene. The reactions involve free radical intermediates and the source of hydrogen is thought to be the solvent and/or the anthracene.

324

Addition ofH2 and HCN to C=C and C=C Bonds

The well-known 'ageing reaction' of [Co(CN)5p~ in water237 (equation 91) leads to formation of hydride that can reduce, for example, diene substrates in a stoichiometric reaction 184 by pathways outlined in Scheme 1 (steps a, c and f).

2Co(CN) 5 3 -

+

H2O

CoH(CN) 5 3 -

—•

+

Co(CN) 5 OH 3 -

(91)

The general use of solvents as a hydride source was mentioned in the section on formation of metal hydrides (Section 51.1.3.1,iv), and some very practical catalytic hydrogenation systems have been developed using water and alcohols as hydrogen donor solvents. Other systems employing glycols, aldehydes, amides, carboxylic acids, ethers, cyclic amines and even aromatic amines and phenols, and hydrocarbons such as alkylbenzenes and indane, have also been found effective.234 Complexes used include the following (L is a tertiary phosphine, usually PPI13): trans-[Mo(N 2 ) 2 (diphos) 2 ], [MoH 4 (diphos) 2 ], [FeCl 2 L 3 ], 238 [RuCl 2 L 3 ], [RuH 2 L 4 ], [RuHCl(L)(iyC 6 Me 6 )], [RhClL 3 ], [RI1HL4], [M(bipy)(cod)]+ (M = Rh, Ir),239>240 [IrHCl 2 (DMSO) 3 ], IrH 3 (PPh 3 ) 3 , 193 [IrCl 6 ] 3 -/P(OMe) 3 , [MCl 2 L 2 ]/SnCl 2 (M = Pt, Pd), 241 MC12L2 (M = Fe, Co, Ni, pd), 193 ' 242 ' 243 acac complexes of first-row transition metals, especially Ni(acac) 2 , 244 and simple salts of Rh(III), Pd(II) and Pt(II) (where references are not given, the systems can be traced through ref. 234). Water can become an effective H 2 donor in the presence of metal carbonyls via what is effectively a catalyzed water-gas shift reaction68'245'246 (equation 4). Several cluster carbonyls, including [Ru 3 (CO) 12 ], [Os 3 (CO), 2 ], [Ir 4 (CO) 12 ], [Rh 6 (CO) 16 ], [Pt 3 (CO) 6 ] 5 2 -, [Ru 4 H 4 (CO) 12 ], [Os 3 H 2 (CO)i 0 ] and [Os 4 H 4 (CO)i 0 ] catalyze the shift reaction in basic solution,247 although the mononuclear systems [Fe(CO) 5 ] 78 and [RhL(nbd)]"1" (L = the water soluble diphosphine (39a)) 248 in base, m-[Rh(CO) 2 I 2 ]- 249 and [PtCl 4 ] 2 -/SnCl 2 mixtures250 in acid and the A-frame complex [Rh2(At-H)(jU-CO)(CO)2(jU-dppm)2]+ in neutral aqueous propanol solution251 are also effective. The water oxidation of CO to CO 2 via a formate intermediate is well documented, and pathways for the shift reaction at a single metal site are illustrated in equation (92) for an initially zero-valent complex. The CO/H 2 O system was used to catalyze hydrogenation of alkenes in the early Reppe hydroformylation process using [Fe(CO)5], 252 and more recent examples for alkene hydrogenation include the use of [Rh 6 (CO) ] 6 ] 2 5 3 and the [PtCl 4 ] 2 "/SnCl 2 system.254 Stoichiometric hydrogenations using moist solutions of [Co(CO) 4 ]~ and [Fe(CO) 5 ] were noted in Section 51.1.3.2,1. In effect, each CO ligand provides one mole of H 2 (equation 92), and each two-equivalent reducing power of the low-valent metal provides a further mole of H 2 (cf. equation 81). Thus, for example, conversion of one mole of the [Fe(CO)s] complex to a final Fe(II) product would generate six moles of available H 2 . 201 The mechanisms could involve hydride intermediates formed according to pathways shown in equation (92), or reduction of the substrate by electron transfer from low-valency species followed by proton addition (Section 51.1.3.2,i).

(39a)

M°H -co 2 / / M°

^

M(CO) -^§?*

M(CO2H)

\H+ -H+

-H+\

M-"

M"H 2

—*

M°

+

H2

(92)

Addition ofH2 and HCN to C=C and C=C Bonds

325

Alcohols have been used most widely as hydrogen donor solvents, with platinum metal complexes usually acting as catalysts. Isopropanol generally provides the most active alcohol system and during the hydrogenations, commonly carried out under refluxing conditions, it is converted to acetone. The exact nature of the catalyst is not always known; thus carbonyl derivatives can be formed by abstraction reactions under such conditions (cf. equation 49). During the catalyzed H2 reduction of alkenes using Rh(I) phosphine in primary alcohols, some hydrogen transfer from the alcohol to the alkenes also occurs, and in this case the aldehyde product is decarbonylated which leads to deactivation of the catalyst in the form of a carbonyl.255 More generally, hydrogen transfer systems have been used to reduce alkenes but more commonly the substrates contain carbonyl functions. The [IrHCl 2 (DMSO) 3 ] and [IrCl 6 ] 3 -/P(OMe) 3 systems are particularly useful for stereoselective reduction of cyclic ketones, including the production of axial alcohols from steroid 3ketones.234 The ruthenium arene complex256 has been used for hydrogenation of benzene (Section 51.1.5). Interesting selective hydrogenations using Pd(II) or Pt(II) salts with formic acid as donor are seen in the catalyzed hydrodimerization of 1,3-butadiene to l,7-octadiene 257a or 1,6-octadie n e 257b depending on conditions. The mechanisms of the hydrogen transfer processes are exemplified by the alcohol donor system summarized in equation (93). 234 This shows an initial coordination of the substrate, followed by coordination of the alcohol as alkoxide. ^-Hydrogen transfer from the alkoxide to give a metal hydride followed by the usual migratory insertion reaction, and release of product via protonolysis together with the ketone, completes the catalytic cycle. Details of the various steps depend on the substrates and donors involved, and are poorly understood. Prior coordination of the donor followed by that of the substrate (equivalent to a hydride route) has also been invoked.234 Dihydride intermediates have been implicated,234 especially when using donor solvents such as ethylbenzene and indane (which yield styrene and indene, respectively, as the dehydrogenated products), but also with alcohols (via oxidative addition to give a hydrido-alkoxide, and then /5-hydrogen transfer). Once the intermediate mono- or di-hydrides have been formed, all the hydrogenation pathways outlined in Schemes 2 and 3, except those incorporating H2, are available and all have been invoked. A ^-hydrogen transfer step has usually been considered rate determining in the alcohol systems.

M

+

substrate(S)

^^

RIRa

M(S)

H)

fff° >

M(S) OCHIOR2

(93)

I RiR2 C = = O

+

SH2

+

M

^

MjSH O^CRiR 2

The [RuCl2(PPh3)3] complex with an optically active donor alcohol (a glucofuranose derivative) has been used for asymmetric hydrogenation of prochiral ce,/3-unsaturated ketones to the saturated ketones, while the use of analogous ruthenium or rhodium complexes containing optically active phosphines with racemic alcohols such as 1-phenylethanol leads to enantioselective dehydrogenation of the alcohols, the optical purity of the remaining alcohol increasing with conversion to acetophenone (equation 93; R1 = Ph, R 2 = Me). 234 Asymmetric hydrogenation of prochiral alkenes has been accomplished using chiral phosphine complexes with alcohol donors.258 Catalytic dehydrogenation of the donor solvent in the absence of an acceptor substrate can occur with certain alcohols and with formic acid. 234 The mechanisms involve hydride transfer to the metal from the coordinated alkoxide or formate, followed by protonolysis to give H2 and regeneration of catalyst (cf equation 93). The lability of the carboxylate ligand (including bidentate to monodentate equilibria) plays a key role in catalytic dehydrogenation of alcohols using some platinum metal carboxylato(phosphine) complexes.259 Finally in this section, mention should be made of hydrogen transfer from saturated alkanes to alkenes catalyzed by AICI3.260 The Friedel-Crafts conditions result in protonation of the alkene to give a carbonium ion that subsequently abstracts hydride from the alkane. Related superacid systems have been used similarly to hydrogenate aromatics (see Section 51.1.5).

326

Addition ofH2 and HCN to C=C and C=C Bonds

51.1.3.4 Transition metal clusters, dimers and hydrogenases Expanding interest in the use of clusters as models for surfaces in chemisorption and heterogeneous catalysis was described in Section 51.1.2. The subject has developed concurrently with the expertise and ingenuity of synthetic chemists in the cluster area. 67 ' 261 " 263 The clusters are in general soluble and hence possess the experimental advantages of the mononuclear catalyst — its stereochemical features and solid state and solution chemistry can be tackled with the use of standard physical techniques. The mode of binding (e.g. terminal, bridging) of ligands such as hydrogen, carbon monoxide and alkynes in carbonyl and other clusters has analogies in binding at surfaces, and the mobility of ligands on clusters and of chemisorbed species on surfaces has also been established. Studies, particularly involving [M3(CO)i2](M = Ru, Os), have shown the ability of clusters to cleave one or two C—H bonds, equations (94) and (95)261>263 respectively, the former being considered analogous to ethylene chemisorption on metals. The intermediate decacarbonyl contains a vinyl group bridging two metal atoms (40), while in the nonacarbonyl the vinylidene group bridges three metal atoms (41). The product of equation (95) contains a bridged o-phenylene group bonded to two osmium atoms. Cleavage of C—H bonds at both unsaturated and saturated carbon atoms is of course well documented for mononuclear complexes, and seems likely for dinuclear systems.261 However, a reaction common at metal surfaces, the cleavage of carbon-carbon bonds, has not been demonstrated at mononuclear centres, except for some cyclopropane derivatives, e.g. equation (96). 264 Carbon-carbon bond cleavage at a cluster centre is documented, although examples are few; equation (97) illustrates one in which the cyclohexa-l,3-diene ring is cleaved to give an acylic allylruthenium complex.261 Despite some obvious analogies between clusters and surfaces, there are serious, well recognized, limitations especially in a consideration of the metal-metal bond energies, which are much weaker in the clusters.263'265

Os3(CO)i2

+

C2H4

z2£

^

HOs3(CO)i0(CHCH2)

z

^

H2Os3(CO)9(C=CH2)

(40) Os3(CO)i2

+

benzene

(41) z

Cl(PPh3)3
Ru3(CO)12

+

(94)

^

hx,

cyclohexa-l,3-diene

H2Os3(CO)9(C6H4)

//

—-

(95)

[Rh(CO)2Cl]2)

I ^Mj

(97)

M = Ru(CO)3

s(CO)3

In terms of hydrogenation of carbon-carbon unsaturated bonds, solutions of [Os3H2(CO)io] reduce ethylene stoichiometrically at a few atmospheres pressure, and the resulting unsaturated

Addition ofH2 and HCN to C=C and C=C Bonds

327

intermediate [Os 3 (CO)i 0 ] can oxidatively add H 2 to reform the dihydride, or further ethylene to give the hydridoalkenyl cluster (40) (equation 94). Other activated alkenes, e.g. maleate or fumarate, react with [Os3H 2 (CO)i 0 ] to give hydridoalkyl clusters, and these react with H 2 to yield saturated products with regeneration of [Os3H 2 (CO)io]. These [Os3H2(CO)io] systems under appropriate conditions can thus become catalytic, and a hydride route via an [Os3H 2 (CO)io(alkene)] intermediate was favoured.100'266 Formation of the monohydridovinyl intermediate (40) by reaction of the [Os3H2(CO)i0] complex with acetylene267 provides evidence for such species in catalytic hydrogenation of alkynes via dihydride systems (Section 51.1.3.2,ii). The tetrahedral nickel isocyanide clusters [Ni4(CNR)7], R = cyclohexyl or Bul, catalyze hydrogenation of dialkyl- and diaryl-alkynes to the c/s-alkenes at 20 °C and 3 atm pressure. 100 ' 268 The clusters contain three bridging but mobile isocyanide ligands and a terminal one at each nickel atom (42); a detected ligand dissociation and the isolated complexes [Ni 4 (CNR) 6 (R / C=CR / )] and [Ni4(CNR)4(R'C=CR')3] suggest catalysis via unsaturate routes with such species (see also Section 51.1.6). In the latter complex, the alkynes bridge three nickel centres, and increase of the alkynic carbon-carbon bond distance is thought to enhance reduction by H 2 . The [Ni 4 (CNBu t )7] cluster also catalyzes the hydrogenation of the isocyanide to /-butylmethylamine, but in the presence of the excess isocyanide some dissociation to mononuclear species occurs, and catalysis via [Ni(CNR) n ] (n = 2,3) intermediates remains a possibility.100

(42) X = — C =

The series of reactions shown in equation (98) for some isolated iron carbonyl clusters, initially containing acetonitrile, provides an excellent model for the sequential reduction of triple bonds (in this case C = N ) that depends critically on a demonstrated H-atom mobility on the cluster surface. 100 ' 269 In equation (98), Fe represents [Fe(CO) 3 ], the three metal-metal bonds not being shown, and © represents a hydrogen atom held on a cluster face. Corresponding studies with isocyanides have been carried out starting with the osmium cluster [Os3H2(CO)io].270 It should be noted that an [Rh2H2{P(C6Hii)3J4(ju-N2)] dimer readily catalyzes hydrogen reduction of nitriles, but this may involve a monomeric catalyst since the nitrogen bridge could be cleaved easily and the related monomer [RhH(PPr'3)3] is also an effective catalyst.271 The tetrameric complex [fFe(A*3-CO)Cp)4],272 and the dimeric [Mo 2 (CO) 4 Cp 2 ] 233 and [Ni 2 (cod) 2 (M-RC=CR)] 273 complexes appear to remain as such during catalytic hydrogenation of alkynes to cismonoenes. Me I

F

4j

Me

F

ik

Me,

w

H

LNFe

^

H V

>N

F / pFe

^

Et

x

(98)

Fe^Fe

Fe The [Ru 3 (CO)i 2 ], [Os 3 (CO)i 2 ] and [Ir 4 (CO) ]2 ] clusters are not very effective for alkene hydrogenations, although the nitrosyl-substituted cluster [Ru3(CO)iq(NO)2] effects some catalytic hydrogenation of terminal alkenes, in addition to a more dominant isomerization reaction, under mild conditions via unknown species.100 Hydrogenation of 1-pentyne, alkenes and some ketonic substrates catalyzed by [Ru 4 H 4 (CO)i 2 ] and its phosphine-substituted derivatives is thought to

328

Addition ofH2 and HCN to C=C and C=C Bonds

involve cluster catalysts in which the substrate has replaced carbonyls. 274 ' 275 Kinetic studies on hydrogenation of alkenes using [Co 2 (CO) 8 ] and [Rh 4 (CO)i 2 ] indicate that the catalysts are monomeric hydridocarbonyls (Table 3), 100 although the activity in a polymer-supported [Rh 6 (CO)i 6 ]/CO/H 2 O system (Section 51.1.3.3, and see below) has been attributed to the cluster. The evidence for this cluster activity is indirect, however, and rests on a reported lower activity of the homogeneous system in which dimerization to a Rhi 2 species is postulated.253 The activity of the mixed metal clusters [Co 2 Rh 2 (CO)i 2 ] and [Co 3 Rh(CO)i 2 ] for hydrogenation of styrene at low hydrogen pressure is in marked contrast to the inactivity of [Co 4 (CO)i 2 ]. 276 The use of some clusters for catalysis of the water-gas shift reaction, and subsequent utilization of the H 2 for reduction of alkenes, were noted in Section 51.1.3.3. Detailed kinetic studies can be helpful in establishing that a multicentre complex is or is not dissociating to an active monomeric catalyst (equation 99); catalysis via the monomeric species in systems with spectroscopically immeasurably small Kd values leads to rate laws that show an n~l order in metal. A dimeric system that gives rise to half-order kinetics for hydrogenation of alkenes is exemplied by Ru(I) chloride species in iV,iV-dimethylacetamide.96 Catalysis using [RuCl 2 (MBMSO) 2 ] 3 (MBMSO = sulphur-bonded 2-methylbutyl methyl sulphoxide) reveals a one-third order kinetic dependence on metal. 277 A first order dependence implies that either the cluster itself is the active catalyst or else that K& is large and complete dissociation to the monomer has occurred; however, in such cases spectroscopic investigations should be able to identify the species in solution.

Mn

^

«M

(99)

There are, of course, many dimer precursor complexes that give rise to monomeric monohydride catalysts (Table 3), and many dinuclear and polynuclear species that dissolve in solvents, sometimes of good donor ability, and in the absence or presence of coordinating unsaturated organic substrates, that are thought to give catalytically active monomers, sometimes solvated. Examples include [(Zr(C 4 H 6 ) 2 (dmpe)} 2 (M-dmpe)], 278 ' 279 [iCo(CO) 2 PR 3 } 3 ], 139 [(RuCl2(r?-C6H6)}2],132 [{RuHCl(PPh 3 ) 2 ) 2 ], 93 [!PdCl2(PPh3)}2],124 [!PtCl2(C2H4)}2],127 [tMCl(r?-C5Me5)!2(iu-H)(iuC1)](M = Rh, Ir; Table 4) and [IrH 2 (PPh 3 ) 2 (ju-Cl) 2 Ir(cod)] (Table 7). In a related context, halide-bridged dimers, especially of Rh(I) and Ir(I), e.g. [MC1(CO)2] and [MCl(C8Hi 4 ) 2 ] 2 , have been used as precursors to a very wide range of monomeric catalysts formed simply by addition of any donor ligand that cleaves the bridges; the latter complexes are particularly useful in that the cyclooctene ligands are also very labile. Tertiary phosphine(arsine)-type ligands including amino and silyl derivatives have been used extensively, as well as nitrogen and sulphur donor Hgands. 93 ' 122 ' 131 ' 135 ' 143 ' 157 ' 222 ' 280 - 282 Note Should also be made that dimeric intermediates were commonly invoked in the considerable literature describing the hydrogenating ability of transition metal stearates. 283 However, these systems are now known to have no homogeneous catalytic activity.284 A kinetic study on the hydrogenation of ethylene catalyzed by [Pt(SnCl3)5]3~ shows an unusual rate dependence on catalyst concentration in which two maxima are observed, and together with spectroscopic data, this has been interpreted in terms of active monomeric and Pt6 cluster species;285 in this regard it should be noted that an anionic cluster species [Pt 3 Sn 8 Cl 20 ] 4 ~ and a derived diene complex [Pt 3 (SnCl 3 ) 2 (cod) 3 ] have been isolated, and the latter is active for hydrogenation of alkenes and alkynes.127 Nonlabile ylide ligands in complexes such as (43) have been used in attempts to design dimers with unusual catalytic behaviour but with little success;286 a related monomeric system [Rh(CH 2 PMe 2 CH 2 )(cod)] is noted in Table 12 (Section 51.1.4.1).

(CO)2 Me CH 2 —Rh-CH 2 \< Me

X

CH 2 —Rh—CH 2 (CO) 2 (43)

\<

Me Me

Addition ofH2 and HCN to C=C and C=C Bonds

329

An unambiguous way of proving catalysis at a cluster site would be demonstration of asymmetric hydrogenation catalyzed by a resolved chiral cluster that possesses asymmetry solely by virtue of the arrangement of metal atoms. Dissociation in solution would lead to loss of chirality and no asymmetric induction in the catalysis. This goal has not been achieved but it is of interest to note that the first tetrahedral cluster with four different metal atoms has recently been reported.287 Even when an active polynuclear system is confirmed, it is probably impossible to demonstrate unequivocally that catalysis does not occur at a single site within the cluster! In the hydrogenation of alkenes catalyzed by the dimeric Rh(II) acetate, only one metal within the dimer is thought to be involved (equation 100), 138 the postulated hydride route being favoured by an initial heterolytic splitting of H2 (Section 51.1.3.1,ii); the ju3-oxotriruthenium acetate clusters utilize similar H2 activation at just one metal centre. 120 Oxidative addition of H2 to an iridium(I) dimer to yield a dimeric product with one hydrogen bound to each metal has been discussed previously, together with examples of H2 addition to some clusters (Section 51.1.3.1 ,i, equation 12).

r»i

T

Rh 2 L 4

H 2 (-H + , -L)^

—

TT r»i

T n\

HRhL3R.l1

alkene

,

•

H+, L

x

(alkyl)RhL 3 Rh

——* Rh 2 L 4

+

product

(100)

Hydrogenation activity has been attributed to dimeric species in some systems involving, for example, chlororhodate(I) complexes in DMA, 122 [PtCl 2 (PPh 3 ) 2 ]/SnCl 2 species127 and RhClL 3 (L = p-dimethylaminophenylphosphines),288 but the evidence is rather scant. A dimeric Mo(III) complex containing bridging dithiolate ligands, [{MoCp(ju-SC2H4S)J2], undergoes a remarkable reaction with alkynes (or alkenes) which are found to displace the hydrocarbon portion of the bridged dithiolate ligand.289 Since H2 can readd to the alkyne moiety, the system can constitute a catalytic hydrogenation of acetylene to ethylene (equation 100a, written for a monomer). This novel system constitutes a modified unsaturate route in which the substrate is bound via ligands; analogies with enzyme systems are apparent, particularly nitrogenases (Section 51.1.3.4,0 which also reduce acetylene to ethylene. The system clearly involves a dinuclear site but the mode of H2 activation remains to be established.

Mo(SC 2 H 2 S)Cp

(100a)

An added problem in cluster systems which are necessarily ligand deficient is to establish that the catalysis is homogeneous. Unless the cluster is maintained in solution, treatment with H2 alone may readily yield metal in the absence of stabilizing ligands. There is a great deal of current interest in maintaining catalytic cluster complexes on support materials (Chapter 55). Systems that have been reported active for hydrogenation of alkenes and aromatic substrates include [Ni 3 (CO) 2 Cp 3 ] dispersed on silica,290 [RhH 6 (Co 2 B)i 0 ] and [RhHi5(Ni2B)i0] supported on glass wool,291 metallic Rh 6 clusters formed by photolysis of polynuclear rhodium carbonyls on phosphinated supports292 and several other polynuclear carbonyls and phosphine(carbonyls) bonded to inorganic supports such as silica, alumina and zeolites.66'293 Attempts to prepare highly dispersed metal crystallites of controlled size are promising in terms of producing highly selective catalysts, 294 and, for example, pyrolysis of [Pt3(CO)6]fl~ on silica gives (Pt 3 ) n aggregates which absorb H 2 . 295 Production of naked clusters of single or mixed metal systems of known composition by photoaggregation of metal atom species at low temperatures offers an alternative approach. 296 The potential of naked cluster ions such as Bis"1" f° r catalysis has been noted recently,263 but hydrogenations have not yet been reported. Of related interest, aggregates of Ag(I) ions were postulated as catalysts for hydrogenation of inorganic substrates such as Cr(VI) from kinetic data about 25 years ago. 112 A somewhat contrasting property of a support material to that noted above is the prevention

330

Addition ofH2 and HCN to C=C and C=C Bonds

of deactivation of a homogeneous mononuclear catalyst because of dimerization or polymerization. This concept has been used, for example, to maintain the hydrogenating activity of titanocene intermediates297 which normally dimerize to inactive fulvalene complexes (1), and [Rh(CO)2Cp] which forms [jRhCp)w] in solution.298 Some other dimer/cluster complexes, not discussed in this Section or noted elsewhere in this Chapter, that are active for hydrogenation of alkenes and/or alkynes are listed in Table 10. The nature of the active catalyst is generally unknown. Systems that are 'binuclear' in the sense of containing coordinated [ S n C y - (or related Group IV ligands) are not included since there appears to be no evidence for hydrogen or substrate activation at the tin centre; the [SnC^] ~ simply acts as an ancillary 7r-acceptor ligand. (/) Hydrogenases and model systems Some enzyme systems that activate hydrogen, the so-called hydrogenases, almost certainly involve polynuclear species. One such enzyme preparation isolated from Clostridium pasteurianum is thought to contain just three [Fe4S*4(S-Cys)4] cluster sites, where S* represents inorganic sulphide and (S-Cys) represent cysteinate.299 This cluster with overall charges of — 1, —2 or —3 is one recognized type of active site in iron-sulphur redox proteins, the others being [Fe(SCys)4]"-2~ and [Fe2S*2(S-Cys)]2-'3"-. The enzymes are able to activate H 2 for exchange with water, para-ortho conversion and reduction reactions when coupled to an electron carrier E such as NAD + , cytochrome C3 or ferredoxins; reaction (101) can be catalyzed in either direction. Forward reactions include hydrogenation of fumarate and dyes such as methylene blue (44) and methyl viologen (45), as well as hydrogen reduction of inorganic substrates (O2, NO3- and 2 300 4

so -).

H2

Tk r

TWT"

^*CX^

"'V:*'

+

"N>^

E ox

^

2H+

'^KT^^-

+

l~

E red

(101)

MeN^\-Y~^NMe \

/

(44)

W

.7"

I ^^

2

(45)

The net hydrogenations are probably of the 'reduction-protonation' type (Section 51.1.3.2,1) via the reduced electron carrier, and the initial H 2 activation by most enzyme preparations is probably via heterolytic cleavage (Section 51.1.3.1,ii). Data on H 2 /D 2 O exchange and an H 2 reduction of Ru(IV), both catalyzed by chlororuthenate(III) species (equations 102-104, cf. equation 35), has allowed for speculation on the mechanism of analogous reactions (exchange and the H 2 reduction of a ferredoxin electron carrier) known to be catalyzed by hydrogenases.299 In the suggested reactions (equations 105-108), t is a presumed Fe4 site, subscript 'a' refers to the active catalyst site and subscript 'c' refers to an electron-transfer centre coupled to an endogenous 8-Fe ferredoxin electron carrier (Fd). Equations (105) and (106), corresponding to equations (102) and (104) respectively, yield the net equation (107) which could occur in the enzyme and result in H 2 uptake or evolution, catalyzed (or autocatalyzed as in the H 2 reduction of Ru(IV)) by / a 3 ~. Equation (108) refers to the coupling reaction which with equation (107) gives net reactions such as that shown in equation (101). The oxidation level of ta remains to be established, and the model is amenable to testing by the use of synthetic analogues of the active sites of iron-sulphur proteins. Earlier studies tended to favour heterolytic H 2 activation at a single Fe(II) centre.300

RuCl 6 3 " R11HCI53-

+

H2 +

D+

R11HCI53-

^ +

Cl-

^

+

H+

+

Cl-

(102)

RuCl 6 3 -

+

HD

(103)

Addition ofH2 and HCN to C=C and C=C Bonds R11HCI53-

+

2RuCl 6 2 -

^

3RuCl 6 3 -

+

H+

331 (104)

Table 10 Some Dimer/Cluster Complexes, not discussed in the text, that Catalyze Hydrogenation of Alkenes and/or Alkynes; * Signifies Effective for Alkynic Substrates Complex (ref.) [{Cr(CO)3}2(M-dinuclear arene)]a (1, 2), *[Cr(CO)4(M-PMe2)2Mo(CO)4], *[M(CO)40u-PPh2)2Fe(CO)3], M = Cr, Mo, W (1); Mo 2 4+ /PPh 3 (3); {Mo(CO)2(C3H5)}2(At-Cl)3] (4); [MCp2(M-SMe)2M'X2]b, M = Mo, W (5); Fe 2 H(CO) 8 ]-, [Fe 3 H(CO) n ]- (6); [{M(CO)2Cp}2], M = Fe, Ru (7); ;{RuH(PPh3)3}2(M-O2C(CH2)/JCO2)]<= (8); [{Ru(PPh3)3}2(At-Cl)3] + (9); RuCl(PPh3)2(M-Cl)3RhCl(PPh3)2] (10); [{RuCl(PPh3)2(CH2=CHCN)j2] (11); ;{Os(PMePh2)3}2(M-Cl)3]+ (12); [Co(PPh 3 ) x ( M -H)CoH(N 2 )(PPh 3 ),]- (13); *[Co3(CO)9(At3-benzylidyne)],*[Co4(CO)8(M2-CO)2(M4-PPh2)2](14); [Co2(CO)8]/amines (15); [{RhCl(arene)U, [{RhCl(SnCl3)2|2]4- (16); [{RhClL)2]d (17); [{Rh(C2B9Hn)(PPh3)}2] (18); [{Rh(CO)(dppm)}2(M-S)] (19); *[{Rh(CO)(dppm)}2(M-Cl)]+ (20, 21); [Rh2{P(OMe)3}4(M-SBut)2] (22); [{Rh(cod)Gu-imidazolate)}3] (23); [{RhH2(P(OR)3)2|2], [Rh3H5{P(OMe)3|6] (24); [{MH(P(OR)3)2}Je, M = Rh, Ir (25); [{Ir(CO)(dppm)}2(M-Cl)(At-CO)]+ (20, 21); *[{Pd(M-PPh2)M}2(M-Cl)2]f (26); [{PdCl2(polyoxime)}n] (27); *[{PdX2(PPh3)}2]g (28); *[Pd5(PPh3)2] (29); [Pd2(dppm)3] (30); [{PdCl(PBu2(CH2SiMe3))|2(M-Cl)] (31); *[PdCl(PR3)(M-X)2PtCl(PR3)]h, *[Pd(M-O2CCH3)4Cu] (26); *[Pt2L3(HL)Cl]i (32).

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

a Dinuclear arene = deoxybenzoin, benzylmesitylene. b M' = Rh, Ni, Pd, Pt; X = Cl or Cp. c n = 2-4. d L = 1,2,5-triphenylphosphole oxide. e n = 2,3. f M = Cr(CO) 5 , Fe(CO)4. sX = halide. h R = Bu, Pr; X = halide, PPh2. ' HL = o-hydroxyazonaphthalenes. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter VII. H. Reinke and G. Oehme, /. Prakt. Chem., 1976, 318, 959. P. Legzdins, G. L. Rempel and G. Wilkinson, /. Chem. Soc, Chem. Commun., 1969, 825. J. C. Bailar, Jr., J. Am. Oil Chem. Soc, 1970, 47, 475. A. Pereira da Luz, Jpn. Pat. 79 70 254 (1979) (Chem. Abstr., 1979, 91,175 082). A. Misono, Y. Uchida, K. Tamai and M. Hidai, Bull. Chem. Soc. Jpn., 1967, 40,931. E. Cesarotti, A. Fusi, R. Ugo and G. M. Zanderighi, J. Mol. Catal., 1978, 4, 205. G. Sbrana, G. Braca and E. Giannetti, /. Chem. Soc, Dalton Trans., 1976, 1847. P. Abley and F. J. McQuillin, Discuss. Faraday Soc., 1968, 46, 31. R. A. Head and J. F. Nixon, /. Chem. Soc, Dalton Trans., 1978, 913. Ref. 1, Chapter IX, Section B2d. K. C. Dewhirst, U.S. Pat. 3 454 644 (Chem. Abstr., 1969, 71, 91 647). S. Tyrlik, /. Organomet. Chem., 1973, 50, C46. R. C. Ryan, C. U. Pittman, Jr. and J. P. O'Connor, /. Am. Chem. Soc, 1977, 99, 1986. K. Kunio and J. Kumanotani, Bull. Chem. Soc Jpn., 1973, 46, 3562. Ref. 1, Chapter XI, Section H. D. G. Holah, A. N. Hughes and B. C. Hui, Qan. J. Chem., 1972, 50, 2442. R. T. Baker, R. E. King III, C. Knobler, C. A. O'Con and M. F. Hawthorne, J. Am. Chem. Soc, 1978,100, 8266. C. P. Kubiak and R. Eisenberg, Inorg. Chem., 1980,19, 2726. M. Cowie and S. K. Dwight, Inorg. Chem., 1980,19, 2508. M. M. Olmstead, C. H. Lindsay, L. S. Benner and A. L. Balch, J. Organomet. Chem., 1979,179, 289. P. Kalk, R. Poilblanc and A. Gaset, Belg. Pat. 871 814 (1979) (Chem. Abstr., 1979, 91,129 602). N. Kihara, K. Saeki and Y. Toda, Jpn. Pat. 76 141 866 (1975) (Chem. Abstr., 1977, 86, 171 604). E. L. Muetterties, A. J. Sivak, R. K. Brown, J. W. Williams, M. F. Fredrich and V. W. Day, in 'Fundamental Research in Homogeneous Catalysis,' ed. M. Tsutsui, Plenum, New York, 1979, vol. 3, p. 487. V. W. Day, M. F. Fredri, G. S. Reddy, A. J. Sivak, W. R. Pretzer and E. L. Muetterties, /. Am. Chem. Soc, 1977,99,8091. Ref. 1, Chapter XIII, Section B. S. J. Kim and T. Takizawa, Makromol. Chem., 1975,176, 1217. A. S. Berenblyum, A. P. Aseeva, L. I. Lakhman and 1.1. Moiseev, Izv. Akad. Nauk SSSR, Ser. Khim., 1977, 2163. A. S. Berenblyum, A. G. Knizhnik, S. L. Mund and 1.1. Moiseev, Izv. Akad. Nauk SSSR, Ser. Khim., 1979, 2157. E. W. Stern and P. K. Maples, /. Catal., 1972, 27,120. G. Chandra, Br. Pat., 1 412 257 (1972) (Chem. Abstr., 1976,84, 50 368). A. S. Todozhokova, P. S. Chekrii and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., 1975, 1684.

332

Addition ofH2 and HCN to C=C and C=C Bonds ta*~ 3

H/a "

+

+

Hr a 3 ~

H2 ^ 2

2tc ~

^

t*~

2tc2-

+

H2

^=

Fd(oxidized)

+

2/ c 3 "

^

+

2tc3~

H+

+ 3

2tc ~ +

(105) +

H+

2H+

Fd(reduced)

+

(106) (107)

2tc2~

(108)

Comparisons with hydrogenases, using model systems with elements other than the second-row ruthenium analogue, have involved complexes that activate H2 by both heterolytic and homolytic cleavage, and include one group such as cobaloximes,140 rhodoximes 131 and [Rh2A2Cl]~ (A = 7V-phenylanthranilate),122 all of which promote H2 reduction of nitrobenzene in the presence of some other heterogeneous or homogeneous hydrogenation catalyst; that is, the complexes act as H 2 carriers in a catalytic chain (cf. equations 105-108). Heterolytic splitting of H2 readily accounts for the variation of hydrogenase activity with pH which passes through a maximum at slightly alkaline conditions. A basic site in the enzyme probably stabilizes the released proton of a reaction such as that shown in equation (105). Thus, decreased activity at low pH may result from neutralization of the basic site, while at high pH the iron(II) site may be hydrolyzed.300 Such activity vs. pH profiles have been observed in some monomeric systems, for example in a Cu(II)/glycine-catalyzed hydrogenation of dichromate112 and in a hydrogenation of 1-hexene catalyzed by [Pd(salen)].301 The basic sites thought to undergo protonation are the carboxylate and phenolic oxygen atoms of the respective chelated ligands. In the palladium system, direct hydride transfer to coordinated alkene followed by protonation brings about the net hydrogenation (cf. Scheme 1), and such a pathway has sometimes been invoked for hydrogenase systems rather than a reduction-protonation mechanism (cf. equations 105 and 106). In cobaloxime systems with axial amine ligands, both the amine and glyoxime ligands have been considered as basic sites,302 although these systems are usually thought to activate H2 homolytically (Table 3). Homolytic splitting at two iron centres has also been proposed in earlier work for some hydrogenase preparations that give very low exchange rates compared with the rates for hydrogenation of substrates.300 This is not easily explained in terms of equations (105) and (106), where the H2 activation step is considered rate limiting. Subsequent transfer of hydrogen atoms from the iron centres to the isoalloxazine nucleus of a flavin generates the hydroquinone form (equation 109; R = an adenine dinucleotide residue) that is capable of coupling to one- or two-electron acceptors such as methyl viologen and methylene blue.300 The catalyzed hydrogenation of such dyes by aqueous [Co(CN)s] 3 ~ solutions (Section 51.1.3.1,1) has been considered to mimic the homolytic hydrogen activity of some hydrogenases.300

(109)

quinone (FAD)

semiquinone (FADH)

hydroquinone (FADH 2 )

Molybdenum has sometimes been suggested as an active second metal, and in this regard, [FesMo] cluster species analogous to the [Fe4S*4(S-Cys)4] clusters have been synthesized.303 The major interest of such tetranuclear sites is their role in biological nitrogen fixation within nitrogenase systems. These are complicated by the presence of hydrogenases, which results in the reduction of protons to H2 (equation 101) with consumption of ATP, a reaction that may compete with the N 2 reduction and lead to less efficient nitrogenases.299 51.1.4 Practical Homogeneous Catalysts 51.1.4.1 General: monoenes, dienes and polyenes As outlined in the Introduction, there are several texts and reviews dealing with the practical application of homogeneous hydrogenation catalysts in organic syntheses. The previous Section,

Addition ofH2 and HCN to C=C and C=C Bonds

333

in discussing activation of H 2 and mechanisms available for mono- and di-hydride systems, has referred to a very wide range of catalytically active systems; this has generally meant 'active' for hydrogenation of alkenic bonds. References listed in Tables 1, 3 and 4 and 6-10 provide access to the literature on such systems. Mention was also made of the usefulness of several practical catalysts under purely H 2 atmospheres for various selective hydrogenations (e.g. [Cr(CO) 3 (arene)], [CoH(CN) 5 ] 3 -, [RuHCl(PPh 3 ) 3 ], [RhCl(PPh 3 ) 3 ] and [M(PR 3 ) 2 S 2 ]+ (M = Rh Ir; S = solvent), as well as [CoH(CO) n (PR 3 ) 4 _ rt ] and other systems listed in Table 7 that are effective for reduction of dienes and polyenes to monoenes). This section will present further examples to illustrate experimental applications, stressing particularly the selectivity attainable with some commonly used catalysts for addition of H 2 to non-aromatic carbon-carbon double bonds. Ziegler catalysts, the hydrogenation of unsaturated fats, aromatic substrates and alkynes will be considered separately in following sections. The [RhX(PR 3 ) 3 ] complexes, usually [RhCl(PPh 3 ) 3 ], operating at ambient temperatures and 1 atm H 2 pressure, have found wide applicability, normally in benzene or benzene/ethanol solutions. 22 ' 31 ' 93 ' 143 Considerable data point generally to the reactivity sequence shown in equation (110) for the usual cis addition of H 2 , D 2 or T 2 . The readily hydrogenated terminal alkenes include nonconjugated, nonchelating dienes, while with cyclic alkenes, the rate decreases with increasing ring size. Conjugated dienes are reduced but much more slowly than terminal alkenes, firstly to monoenes and then to the saturated product. The following groups or compounds are not hydrogenated or cleaved by hydrogenolysis: arenes, ketones, carboxylic acids, esters, amides, nitriles and ethers, and azo, chloro, hydroxy and nitro compounds. The catalyst is of limited use when an aldehyde function is present due to an accompanying decarbonylation reaction (equation 111), which produces the relatively inactive //ww-[RhCl(CO)(PPh 3 ) 2 ] complex.107

R—CH=CH2

>

[^ J R

> RCHO

>

R2C=CH2

R £=C

R

>

H

H

+

RhCl(PPh3)3

H /C=C H R

>

f

ysJl J > ^ ^

—• RhCl(CO)(PPh3)2

+

RH

R 2 C=CR 2 +

PPh3

(110) (111)

Chlorinated solvents, particularly CC14 and CHC13, are best avoided as reaction media. Hydride abstraction can occur quite readily from transition metal hydrides, and indeed, formation of chloroform from carbon tetrachloride has been used to indicate the presence of such hydrides (equation 112).49 The reaction occurs via a net oxidative addition of the Cl—CC13 molecule followed by reductive elimination of the H—CC13 moiety, although radical pathways appear likely. It should be noted that complexes such as [RuCl 2 (PPh 3 ) 3 ], a precursor for the active [RuHCl(PPh 3 ) 3 ] catalyst, have been found to effect catalytic addition of CC14 to alkenic bonds by free radical pathways. 304

MH

+

CC14

—•

MCI

+

CHC1 3

(112)

Tables of data are published elsewhere for hydrogenations and deuterations catalyzed by [RhCl(PPh 3 ) 3 ]. 22 ' 31 ' 93 ' 143 Substrates include simple alkenes, dienes, cyclic monoenes and dienes, allenes, terpenes, natural products with exocyclic methylene groups, antibiotics, spirocyclic compounds, prostaglandins, steroids and a wide range of unsaturated carboxylic acids, esters, nitriles, ketones, aldehydes and nitro compounds. Equations (113)-(126) and formulae (46) and (48)-(62) provide representative examples that illustrate some of the selectivity patterns outlined above; the reaction products shown represent at least 80% (usually >95%) of the total products. The reductions of dihydro-aromatic substrates (equations 113-117) proceed without the complications often seen with familiar heterogeneous palladium and platinum catalysts — dispro-

334

Addition ofH2 and HCN to C=C and C=C Bonds

portionation to aromatic and cyclohexane derivatives. Within conjugated dienes, butadiene and a-phellandrene (5-isopropyl-2-methyl-l,3-cyclohexadiene) are not readily hydrogenated, possibly because of complexing too strongly, while thebaine (46) is unusual in showing selective hydrogenation at the trisubstituted 8,14-position. A catalytic tritiation of prostaglandin Ej (a nonconjugated diene) to tritium-labelled prostaglandin E2 (a monoene) involves a selective reduction of a 5-heptenoic acid moiety. The reduction shown in equation (121) to give the c/s-decalin is of interest in that it takes place with X = Na but not with X = H, which implies coordination of the rhodium at the oxygen as well as the alkene (cf. the chelation observed with the acetamidocinnamate substrate and cationic rhodium(I) catalysts, Scheme 4, complex 34). Alkenic bonds in sensitive spirocyclic compounds have been reduced (equations 122 and 123), compound (47) being the antibiotic griseofulvin.

335

Addition ofH2 and HCN to C=C and C=C Bonds

R

O > Q < MeO ° O^

(123)

;CH 2

MeO

CH 2 =C=CEt 2

(124)

CH3—CH=CEt2

(125)

OH PhCH=CHCOMe

PhCH2CH2COMe; 1,4-dicyanobutenes

NC(CH2)4CN

(126)

The selectivity towards less-hindered alkenes is well established by the facile reduction of (a) the isopropenyl group in carvone (48), 7-gurjunene (49), thep-menthadiene (50), eremophilone (51) and the /3-cubebene ketone derivative (52); (b) the vinyl group in linalool (53), although non-selective reduction of geraniol (54) and its cw-isomer, nerol, occurred with a concomitant decarbonylation of the primary alcohol group (cf. equation 49) to give [RhCl(CO)(PPh3)2] (nerol acetate was reduced completely without loss of CO); the exocyclic methylene groups in methylenecyclohexanes, methylenetetracyclines and several sesquiterpene derivatives such as psilostachyine (55); the less-substituted alkenic link only in sesquiterpenes such as santonin (56); the terminal alkene bond in allenes (equation 124) and a wide variety of steroids.

(46)

(51)

(49)

(48)

.OH HO

(53)

(54)

(55)

(56)

Examples in steroid substrates include the facile reduction of the disubstituted alkenic link in 1-cholestene (57) (or 2- and 3-cholestene), A1-3-keto-5a-steroids and 5/3-pregn-ll-ene3,20-dione (58); the reduction of ergosterol acetate (59) and closely related 7-dihydrostigmasterol derivatives to the 5a,6a-dihydro (or af2) derivative, in which the trisubstituted 5,6-position is reduced via the less-crowded face of the steroid in preference to the 7-ene or trans 21-ene moieties; the reduction of diene-keto steroids to the monoene-keto derivative, e.g. of androsta-l,4-diene3,17-dione (60) to the 4-enedione (\a,2a-d2), followed by slower reduction of the trisubstituted monoene bond, this being exemplified also by reduction of the 4-en-3-one grouping of testosterone (61) to the 4a,5a-dihydro product. Catalytic c/s-deuteration of various unsaturated 5a-spirostane derivatives such as (62) has yielded steroids labelled in the side chains. The trisubstituted 5-double bond in cholesterols, which are analogous to structure (59) but with reduced 7- and 22-double

Addition ofH2 and HCN to C=C and C=C Bonds

336

bonds, is not reduced, implying some assistance through conjugation for reduction of (59) (cf. the reduction of thebaine 46). Stereoselective hydrogenations of steroids using platinum metal complexes generally has been reviewed.305

=O

(59)

(58)

(57)

(62)

(61)

(60)

Use of higher pressures of H2 with the [RhCl(PPh3)3] catalyst tends to subdue competing decarbonylation reactions and, in contrast to the attempted H2-reduction of geraniol at 1 atm noted above, reduction at 100 atm proceeds selectively via citronellol to the saturated alcohol (equation 125).306 The reactions noted in equation (126) also require elevated temperatures and pressures. The [RhCl(PPh3)3] catalyst,307 as well as [RhH(CO)(PPh 3 ) 3 ] and [RuHCl(PPh 3 ) 3 ]3, 308 has also been used for hydrogenation of butadiene rubbers. Such hydrogenation is important commercially and has been studied mainly using Ziegler catalysts (Section 51.1.4.2). Finally during discussion of the [RhCl(PPh 3 ) 3 ] complex, note should be made of its use in the synthesis of organometallic complexes and modification of some nitroxide labels. For example, catalytic hydrogenation of the side chain in bis(ethyl cinnamate)chromium(O) leads to the bis(2-phenylpropionate) complex, whereas use of heterogeneous systems cleaves the bis(arene) system. 309 The radical (62a) is reduced initially at the vinyl group and then at the nitroxide. 310

CH 2 =CH (62a) The [RuHCl(PPh 3 ) 3 ] catalyst is extremely effective for terminal alkenes119 (Section 51.1.3.2,1) and is useful for selective reduction of dienes and trienes93 (Table 7). Terminal alkenes are hydrogenated more rapidly than with [RhCl(PPh 3 ) 3 ] under comparable conditions, but the greater air sensitivity of the Ru(II) complex, coupled with its ability to catalyze H atom exchange and isomerization reactions, has probably limited its applicability. The selective reduction of the androstadienedione (60) to the 4-ene, and the successive hydrogenations shown in equation (125), have been accomplished using the precursor [RuCl2{(/?-XC6H4)3P}3] complexes (X = OMe, Me, H) at elevated pressures.93 Under such conditions the ruthenium systems have also found use in hydrogenation of other functional groups (NO 2 , CO, CHO, CN). 9 3 Two other complexes of use for hydrogenation of alkenic bonds within steroids are [RhCl 2 (BH 4 )(DMF)py 2 ], which readily reduces A4-3-keto steroids (61) to the 3-one,122 and a [PtCl 2 (PPh 3 )2]/SnCl2 mixture that reduces cholest-4-en-3/3-ol (cf. 57) to 5a-cholestan-3/3-ol

Addition ofH2 and HCN to C=C and C=C Bonds

337

and dehydration products.127 These rhodium and platinum systems are quite versatile. At ambient conditions the former, readily formed in situ from [RhCl 3 py 3 ], reduces monoenes and polyenes selectively and alkynes stepwise via the alkene, as well as N = N , C H = N , NO2 and CO groups, including pyridine to piperidine and quinoline to 1,2,3,4-tetrahydroquinoline.122'132 The catalyst appears to be homogeneous, judging by the fact that use of optically active amides in place of DMF leads to effective asymmetric hydrogenation (implying the presence of a coordinated amide ligand). 222 The platinum-tin chloride systems with and without added phosphine ligands have again found wide applicability, especially for reduction of mono-, di- and tri-enes at elevated pressures and temperatures. 127 ' 180 ' 195 ' 311 Some mixed ligand systems [PtCl 2 LI/] (L = PPh 3 , L' = amines, sulphides) are more effective than [PtC^I^] or [PtC^L^], the effect being attributed to the ability of the weaker U ligand to function as a leaving group during the catalytic cycle involving an intermediate monohydride (Section 51.1.3.2,i).312 The [Co 2 (CO)8]/[CoH(CO) 4 ] catalyst system remains useful.39'172 Thus, under hydroformylation conditions the catalyst effects concomitant reduction of any conjugated double bonds present; at higher temperature conditions (>150 °C) the aldehyde product is reduced to the alcohol. This has been demonstrated with mono-, di- and tri-enes, furans, terpenes, epoxides and alkynes. Alkenic carboranes have also been reduced.313 Equation (127) shows possible routes for an alkyne substrate. At lower temperatures (MOO °C) the aldehyde group is not hydrogenated, but there is sometimes more hydrogenation than hydroformylation of the carbon-carbon double or triple bond (equation 127, lower paths). This obtains particularly for branched and activated alkenes (e.g. isobutene, 1,1-dichloroethylene, styrenes, vinyl ketones, acrylonitrile and a,/3-unsaturated aldehydes). 39 ' 314 Hydroformylation of dienes and trienes usually gives a saturated aldehyde either via hydrogenation of an initially formed unsaturated aldehyde, or via a hydrogenation-hydroformylation sequence. 39 ' 315 Equation (128) shows this for the production of 3,4dimethylpentanal from 2,3-dimethylbutadiene. Of interest, 1,5,9-cyclododecatriene (cdt) undergoes only hydrogenation to give cyclododecene. This reaction has potential since it converts a readily available butadiene trimer to a precursor of dodecanedioic acid and laurolactam, two commercial polyamide intermediates.25 Stoichiometric reductions of monoenes and dienes using [CoH(CO) 4 ] under mild conditions (<25 °C, 1 atm CO) 39 continue to attract attention but mainly from a mechanistic viewpoint (Section 51.1.3.2,1), while [Co2(CO)g] remains useful for hydrogenation of aromatic substrates (Section 51.1.5).

Oxo

RCH=CHCHO

RC^CH

-^

RCH2CH2CH2OH

fo*°

(127)

RCH=CH 2 CH 2 =C(Me)—C(Me)=CH 2

-^

|CO/H 2

CH 2 =C(Me)-CH(Me)CH 2 CHO

-2*

RCH2Me

MeCH(Me)—C(Me)=CH2 |CO/H 2

-^

(128)

MeCH(Me)CH(Me)CH2CHO

For hydrogenations in water, the [CoH(CN) 5 ] 3 ~ catalyst remains extremely useful (Section 51.1.3.2,i). 172 ' 237 The system is selective for hydrogenation of carbon-carbon double bonds that are conjugated with one another or with C = O , C = N or phenyl groups, although reduction of NO2, NO and C = N O H groups is quite common at elevated pressures, and hydrogenolysis of carbon-halogen bonds via radical processes is very facile.237 Selective reductions of carvone (48), other terpenes and a A1-3-keto steroid to the 3-one have been reported. 316 ' 317 Carbon-carbon triple bonds are reduced only when part of a conjugated system. The [CoH(CN) 5 ] 3 ~ catalyst is relatively unreactive towards unconjugated dienes such as cod.237 The chromium catalysts [Cr(CO) 6 ], [{Cr(CO)3Cpj2] and [Cr(CO) 3 (arene)] behave similarly in this respect,95'228 and differ from many other catalysts listed in Table 7 which also hydrogenate unconjugated dienes and trienes to monoenes. These more versatile catalysts are thought to isomerize the substrates to a conjugated form (Section 51.1.3.2, equation 60). The [Cr(CO) 3 (arene)] complexes, when the arene is phenanthrene, naphthalene or anthracene, are more active for diene hydrogenation than when the arene is a substituted benzene, and this is attributed to an easier displacement of

338

Addition ofH2 and HCN to C=C and C=C Bonds

the arene by the diene substrate, the phenanthrene type being asymmetrically bonded, having two longer and more readily cleaved chromium-carbon bonds.195'227'318'319 The [CoH(CO) 3 (PR 3 )] catalysts, added as the relatively stable [jCo(CO)3(PR3)2}2] dimers, show excellent selectivity to monoenes, and appear particularly useful.139 Thus 1,5,9-cdt has been fully converted to cyclododecene with 99% selectivity using the dimer with R = Bun; several other catalysts listed in Table 7, including [RuCl 2 (PPh 3 ) 3 ], [RuCl 2 (CO) 2 (PPh 3 ) 2 ], [RhCl 2 (BH 4 )(DMF)py 2 ], [NiI 2 (PPh 3 ) 2 ] and Pt(II)/SnCl 2 systems, are similarly efficient for this hydrogenation.25 The [Ru(cod)(r/6-C7H8)] complex fully converts 1,3,5-cycloheptatrienetocycloheptene with 100% selectivity at 20 °C and 1 atm H 2 . The [CoBr(PPh3)3] system listed is unusual in that 1,2-addition of H 2 occurs at the more substituted double bond of a conjugated diene. Of the catalysts listed in Table 7, [Ni 2 (CN) 6 ] 4 -, [PdCl(PPh3)(773-C3H5)] and [RhCl(PPh3)3] are reported to be effective for the hydrogenation of allenes to monoenes, a topic that seems to have been little studied. Water soluble platinum metal catalysts have been developed by using as ligands water soluble hydroxymethylphosphines,320 diphenylphosphinobenzene-m-sulphonic acid (dpm)93-321 324 and chelating phosphines of the type shown in (39a) made from anhydride acid chlorides.248 The most active system thus far appears to be [RuHX(dpm) 3 ] (X = Cl or acetate), which is effective for unsaturated carboxylic acids at conditions and rates similar to those found for the aqueous chlororuthenate(II) system. The potential of the water soluble phosphine-containing catalysts remains to be developed more fully; they could be useful in phase-transfer catalysis (Section 51.1.8). Borohydride reduction of NiCl 2 and CoCl 2 in DMF or DMA solution provides a simple way to generate very active catalysts at ambient conditions. They show remarkable versatility under H 2 in hydrogenating monoalkenes, cyclic dienes to monoenes, unsaturated fats (Section 51.1.4.3), alkynes (Section 51.1.6) and saturated aldehydes and ketones.126 The systems are considered to be generally homogeneous,325"327 although corresponding systems from the acetate complexes are described as containing 'nearly colloidal' black metal boride catalysts; the nickel catalyst has been labelled T-2 Ni'. 328 An often quoted advantage for homogeneous over heterogeneous catalysts is tolerance towards sulphur-containing compounds. The activity of the [RhCl(PPh 3 ) 3 ] catalyst (and some Ziegler systems, Section 51.1.4.2) for a range of substrates is reduced on adding thiophene or thiophenol, but the rates are still usable. 143 It is not clear whether there are new complexes formed that do not activate H 2 , or whether new inactive hydride complexes are formed. The latter has been found in attempts to use [RhCl(PPh 3 ) 3 ] in coordinating nitrile solvents.143 Addition of thiophene to a DMF solution of PdCl 2 actually increases its activity for hydrogenation of dicyclopentadiene to the monoene.125 The activity of a [(Ru(77-C6Me6)}2(M-H)2(ju-Cl)] complex for hydrogenation of arenes (Section 51.1.5) decreases on addition of thiophene or elemental sulphur but still gives effective rates. 256 Unsaturated side chains in thiophene derivatives have been reduced using [RhCl(PPh 3 ) 3 ] to give the saturated thiophene,143 while [Co 2 (CO) 8 ] has been used to reduce the thiophene ring completely.39 Added sulphides do not poison the rhodium catalyst, which is also effective for reduction of allyl phenyl sulphide to phenyl propyl sulphide.143 3-Sulfolene has been reduced to sulfolane using [PtCl 2 (H 2 O){P(C 6 F 5 ) 3 }]. 329 Of some water-gas shift catalysts, [M(C0) 6 ](M = Cr, Mo, W) and [M 3 (CO)i 2 ] (M = Ru, Os) remain active in the presence of a large excess of sulphide, while [Fe(CO) 5 ] becomes inactive due to formation of [Fe 3 (CO)9S 2 ]. 330 Hydrogenation catalysts (or precursors) containing sulphur-bonded ligands are not uncommon and include [RuCl 3 (SEt 2 ) 3 ], 331 [Co 2 (CO) 7 SR 2 ], 332 [RhCl 3 (SR 2 ) 3 ], 333 [IrBr(Ph 2 PCH 2 CH 2 SPh) 2 ], 334 [PtCl 2 (SPh 2 ) 2 ]/SnCl 2 , 335 [Ti(SPh) 2 Cp 2 ], 336 [Rh(CS)+ 337 (PPh 3 ) 3 ] , a wide range of [M(77-C5Me5)] complexes (M = Rh, Ir) with sulphur-containing ligands, 338 [Rh 2 L 2 (PR 3 ) 4 ] 2 + , [RhLCl] n (L = 1,4-dithiacyclohexane),338 [Ru(/?-cymene) (tetramethylthiophene)]2"1"338 and several complexes of Ru(III), Pd(II) and Rh(I) containing sulphur-bonded sulphoxide ligands including chiral examples for asymmetric hydrogenation.277'339 Table 11 lists a range of mononuclear catalysts, not noted elsewhere in this Chapter, that have been used for hydrogenation of monoalkenic substrates. Many of the systems are essentially derivatives of well-recognized types listed in Tables 1, 3,4,6,7 and 9. A listing in Table 11 implies that there is no reported reactivity of the complex towards H 2 via either homolytic or heterolytic splitting. More novel catalysts listed include a rare example of a system based on copper, although this operates only at high temperature and pressure with an SnCl 2 cocatalyst, the first wellcharacterized rhenium catalyst and an interesting tetraphenylborate zwitterionic molybdenum complex which also reduces alkynes, but nonselectively. The iridium-carbon dioxide complex is the first such complex reported with hydrogenating ability, although the CO 2 ligand is probably displaced by H 2 under the hydrogenation conditions, the system becoming akin to that of [Ir(diphos) 2 ] + (Table 1). The [Ni(cod) 2 ] system operating at 50 °C and 40 atm H 2 pressure for

Addition ofH2 and HCN to C=C and C^C Bonds

339

Table 11 Mononuclear Catalysts for Hydrogenation of Monoalkenes; not discussed or listed elsewhere in this chapter Complex (ref.) [CuCl(MR 3 )]/SnCl 2 a (1); Ti(III) carboxylates (2); [Mo{(776-Ph)BPh3}(r/-C7H7)} (3); ReCl 5 /SnCl 2 (4); [ReCl4(NO)py2] (5); [Ru(O2CCF3)3] (6); RuCl3/poly(ethylenimine) (7); [RuHL(PPh 3 ) 3 ] b (8); [RuCl2L4], [RUC1 2 L(T/-C 6 H 6 )], [RUC1 2 (CO) 2 L 2 ] C (9);

[Ru(CO)3(PPh3)2] (10); [RuH 2 (PPh 3 ) 3 L], [RuH2L2]d (11); [RuH(BH 4 )L 3 p (12, 13); Ru(I)/PPh 3 (14); [CoH(BH4)(PCy3)2] (15); [Co(CO)2L(arene)]f (16); [MH{PhP((CH2)«PPh2)2}]g, M = Co, Rh (17); [RhCl(Ph2PH)3] (18); [RhCl2(HL)], {RhCl2}L]h, [RhH(PF3)(PPh3)3] (19); [Rh(CH2PMe2CH2)(cod)] (20); [Rh(CO)(PPh3)3]+ (21); M(NO)(PR 3 ) 3 ], M = Rh, Ir (22, 23); [RhCl(CO)L], [Rh(CO)(CH 3 CN)L] + , [RhL2]+ * (24); Rh(acac)(PN)],[MCl(CO)(PN)]J, M = Rh, Ir (19, 25); [IrCl(PR3)w(cod)]k (26, 27); Ir{Me2P(CH2)2PMe2|2(CO2)]+ (28, 29); [Ni(cod)2] (30); [Ni{Ge(OMe)3}(PPh3)Cp] (31); PdCl2(DMSO)2] (32); [Pd{CH2(o-C6H4)NMe2}2] (33); [PdCl(diphos)(DMF)]+ (34); PdCl2(diphos)], [Pd(diphos)2] (35); [Pt(O2CCF3)2(PPh3)2] (6); [Pt(O2CMe)(PhNH2)(PPh3)] (36); [PtCl2(H2O){P(C6F5)3|] (37). a M = P, As; R = Ph, OPh, PhCH2. b L = O2CCH2OH, O2CCH2CO2H. c L = PMe3. d L = PF 3 or PF 2 (NMe 2 ). e L = PPh3, PMePh2. f L = CS, PPh3, AsPh3, SbPh3, P(OPh)3, PF 3 , CNCOPh,

CNCH 2 Ph, CNCONMe 2 . g« = 2,3.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

h

H2L is indigosulphonic acid.

j

L= PhP^ ^

^NH .

j PN = chelated aminophosphines, e.g. o-Ph2PC6H4NMe2, Ph 2 P(CH 2 ) 2 NMe 2 . k n = 1,2; R = alkyl, aryl and R3 = PCy x Ph 3 _ x . J. G. Thatcher and W. R. Deever, U.S. Pat. 3 732 329 (1973) {Chem. Abstr., 1973, 79,4982). V. V. Abalyaeva, O. N. Efimov and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., 1972,1496. D. A. Owen, A. Siegel, R. Lin, D. W. Slocum, B. Conway, M. Moronski and S. Duraj, Ann. N.Y. Acad. Sci., 1980,333,90. A. P. Khrushch and A. E. Shilov, Kinet. Catal. (Engl. Transl.), 1969,10, 389. M. A. Ryashentseva, K. M. Minachev, E. P. Belanova, E. S. Shpiro and N. A. Ovchinnikova, Izv. Akad. Nauk SSSR, Ser. Khim., 1976, 2647. M. I. Kalinkin, Z. N. Parnes, D. K. Shaapuni, N. P. Shevlyakova and D. N. Kursanov, Dokl. Akad. Nauk SSSR, 1916,22% 626. H. Hirai and T. Furuta, /. Polym. Sci., 1971, 9B, 459 and 729. G. Sbrana, G. Braca and E. Giannetti, J. Chem. Soc, Dalton Trans., 1976, 1847. H. Singer, E. Hademer, U. Oehmichen and P. Dixneuf, /. Organomet. Chem., 1979,178, C13. R. A. Sanchez-Delgado, J. S. Bradley and G. Wilkinson, /. Chem. Soc, Dalton Trans., 1976, 399. R. A. Head and J. F. Nixon, J. Chem. Soc, Dalton Trans., 1978, 913. R. H. Crabtree and A. J. Pearman, J. Organomet. Chem., 1978,157, 335. D. Holah, A. N. Hughes, B. C. Hui and C. T. Kan, J. Catal., 1977, 48, 340. B. C. Hui, Ph.D. Dissertation, University of British Columbia, 1969. M. Nakajima, H. Moriyama, A. Kobayashi, T. Saito and Y. Sasaki, J. Chem. Soc, Chem. Commun., 1975, 80. R. Dabard, G. Jaouen, G. Simonneaux, M. Cais, D. H. Kohn, A. Lapid and D. Tatarsky, J. Organomet. Chem., 1980,184,91. D. L. Dubois and D. W. Meek, Inorg. Chim. Ada, 1976,19, L29. P. Svododa, M. Capka and J. Hetflejs, Collect. Czech. Chem. Commun., 1973, 38, 1235. B. R. James, Adv. Organomet. Chem., 1979,17, Section XI. R. A. Grey and L. R. Anderson, Inorg. Chem., 1977,16, 3187. R. Uson, P. Lahuerta, D. Carmona, L. A. Oro and K. Hildenbrand, /. Organomet. Chem., 1978,157,63. G. Reichenbach, S. Santini and G. Dolcetti, J. Inorg. Nucl. Chem., 1976, 38,1572. J. P. Collman, N. W. Hofmann and D. E. Morris, /. Am. Chem. Soc, 1969, 91, 5659. C. Pradat, J. G. Riess, D. Bondoux, B. F. Mentzen, I. Tkatchenko and D. Houalla, J. Am. Chem. Soc, 1979, 101, 2234. D. M. Roundhill, R. A. Bechtold and S. G. N. Roundhill, Inorg. Chem., 1980,19, 284. R. Bonnaire, L. Horner and F. Schumacher, J. Organomet. Chem., 1978,161, C41. J. Solodar, J. Org. Chem., 1972, 37,1840. T. Herskovitz and G. W. Parshall, U.S. Pat. 3 954 821 (1976) (Chem. Abstr., 1976,85, 108 778). T. Herskovitz, J. Am. Chem. Soc, 1977, 99, 2391. S. A. Butter and J. G. Murray, U. S. Pat. 3 959 239 (1976) (Chem. Abstr., 1976, 85, 63 702). G. V. Lisichkin, F. S. Denisov and A. Y. Yuffa, Katal. Konversiya Uglevodorodov, 1975,2,170 (Chem. Abstr., 1975,83,178 166). N. M. Nazarova, L. K. Freidlin, Y. A. Kopyttsev and T. I. Varava, Izv. Akad. Nauk SSSR, Ser. Khim., 1972, 1422. G. Longoni, P. Chini and F. Canziani, Ger. Pat. 2 148 925 (1972) (Chem. Abstr., 1972, 78,43 718). J. A. Davies, F. R. Hartley and S. G. Murray, Inorg. Chem., 1980,19, 2299. E. W. Stern and P. K. Maples, /. Catal., 1972, 27, 120 and 134. A. S. Berenblyum, T. P. Goranskaya, S. L. Mund and 1.1. Moiseev, Izv. Akad. Nauk SSSR, Ser. Khim., 1979, 1367. V. F. Odyakov and K. I. Matveev, Kinet. Katal., 1973,14,1441.

340

Addition ofH2 and HCN to C=C and C=C Bonds

butadiene-styrene copolymers might be expected to be heterogeneous but the styrene (and toluene solvent) were not reduced.

51.1.4.2 Ziegler systems Transition metal salts in the presence of alkyl derivatives of aluminium, alkaline earths and alkali metals constitute the Ziegler catalyst systems. The majority known for dimerization and polymerization of alkenes are heterogeneous,340 but homogeneous ones have been developed and many are effective, especially for reduction of monoenes, dienes and aromatic substrates.341 A Russian text has appeared on Ziegler-Natta type catalysts in hydrogenation reactions.342 Unless referenced, information presented in this Section is based on material covered in ref. 341. The systems are complicated, poorly understood, and in some cases may not be entirely homogeneous, 341 ' 343 ' 344 but they appear practical in many cases and have been the subject of hundreds of patents, mainly by petrochemical companies concerned with hydrogenation of various unsaturated polymers. Homogeneous hydrogenation of such polymers, in which the catalyst migrates to the site of unsaturation, can lead to controlled regular unit products as opposed to heterogeneous systems where the polymer has to unfold to present itself to the surface catalytic site. Equation (129) shows a homogeneously catalyzed formation of an ordered copolymer of ethylene and propene by hydrogenation of polyisoprene.25 +CH 2 C(Me)=CHCH 2 ^

^

f CH2CH(Me)CH2CH2-K

(129)

Some representative catalyst systems are listed in Table 12. Pressures of a few atmospheres of H 2 and temperatures of up to 100 °C are usually employed to hydrogenate the alkenes, while somewhat more severe conditions are required for the aromatic substrates. Most first- and second-row transition metals have been used with optimum metal alkyl: transition metal ratios varying from < 1.0 to ~20. Aluminium alkyls are the most common cocatalysts, with the acac and carboxylate complexes of cobalt and nickel generally showing highest activity. A wide range of monoenes are reduced, but conditions can be found for selective reduction of dienes to monoenes; such dienes include butadiene, isoprene, 4-vinylcyclohexene and cyclopentadiene. Unsaturated polymers have been reduced using mainly the catalyst systems of Table 12 labelled with superscripts e, f, h and j . 3 4 1 ' 3 4 5 Substrates include polyethylene, polystyrene, polybutadiene, poly(norbornenes) 346 and various copolymers of styrene with butadiene and isoprene, and butadiene/1,3-cyclohexadiene347 copolymers. Since double bonds are more readily reduced than aromatic nuclei, selective hydrogenations are carried out readily, for example with styrene-isoprene rubbers. Reduction of 1,5,9-cdt with a nickel or cobalt system (Table 12, footnotes e and f) gives only cyclododecane. 341 ' 348 Aromatic substrates (benzene, xylenes, phenol, aniline, phthalate esters, naphthalene and diphenyl) are generally reduced to fully saturated products, although the second ring in naphthalene is harder to hydrogenate than the first.341 Anthracene has been reduced to the 1,2,3,4-tetrahydro derivative. 349 The hydrogenations of the xylenes and phthalates are predominantly cis (Section 51.1.5). Nitrobenzene gives only aniline. The Co(2-ethylhexanoate)2/AlEt3 catalyst effects selective reduction of benzene in a mixture with o-xylene, and preferential reduction of naphthalene in a mixture with benzene. Generally, ketones, aldehydes, esters, nitriles and nitro and azo compounds are not readily reduced. Hydrogenolysis of benzaldehyde and chlorobenzene to toluene and benzene, respectively, noted with some of the systems under elevated conditions (150 °C, 100 atm H2) may be due to the presence of free metal. Liquefaction and/or gasification of coal by hydrogen treatment is improved markedly in the presence of Ziegler catalysts. The patent literature indicates that systems based on nickel are the most effective.350 Reduction of alkynes is often complicated by competing cyclization and polymerization reactions, although selective reduction to monoenes has been noted for longer-chain substrates such as 1-heptyne using some metal acac or stearate/AlEt3 catalysts. First row metals, particularly cobalt and nickel, and palladium have been used principally.341.351,352 Mechanisms presented for alkene reduction usually suggest an initial alkylation of the transition metal (equation 130) with subsequent monohydride formation via hydrogenolysis (equation 41) or /3-hydride abstraction (equation 52). The standard monohydride catalytic cycles of Scheme R3AI

+

MXrt

—* R2A1X

+

RMX«_,

(130)

Addition ofH2 and HCN to C=C and C=C Bonds

341

Table 12 Ziegler Catalyst Systems Catalyst components (ref.Y For monoene and diene substratesb [TKOPr^] or [TiCl4]-AlR3; [MCl2Cp2]-R,,AlX3-,,c; [MCl2Cp2]-BuLi or PhMgBrd; VO(OEt) 3 ]-AlR 3 ; [M(acac)xl-R«A1X3-W or R3Al2Cl3e; [MoH(O 2 CCF 3 )(CO) 2 (THF) 2 ]-AlR 3 (4); WCl 6 ]-EtAlCl 2 (5); [ML2]-AlEt3, Et2AlCl (6) or LiRf; [MY2L2]-AlR3g; PdCl2(PBu5)2]-AlBu3; [M(CO)3Cp](M = Mo, W), [FeCl(CO)2Cp], [Co(CO)2Cp], Ni(C 3 H 7 )Cp]-AlEt 3 or PhMgBr. For aromatic substrates [M(acac) x ]-AlEt 3 h or Et2AlFi; [TiCl2Cp2]-AlEt3; [MLx]-AlEt3J; [Co(acac)2]/PBu5-AlEt3 (14); [Co(acac)3]-MgBu2 (15).

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

a Data based on Ref. 1 unless noted otherwise. b R = Et, Bu1; X = Cl, OEt; n = 0-3. c M = Ti, V2, Zr. d M = Ti, V, Zr. e M = VO, Cr, Mn, Fe, Co, Ni, Pd, MoO2, Ru; x = 2,3; phosphines have been added in some systems.3 f M = Co and/or Ni; L = 2-ethylhexanoate, octanoate, stearate, acetates, phenolates, naphthenates, dimethylgloximate, hydroxammates,7'8 lactams,9 halide. s M = Co and/or Ni, occasionally Fe; Y = Cl, NO 3 , SCN; L = py,10 PPh3, PBu3, OPPh3. h M = VO, Cr, Mn, Fe, Co, Ni, Cu; x = 2 or 3. ' M = Pt, Ir.1 • J M = Cr, Fe, Co, Ni, Cu; L = 2-ethylhexanoate, acetate, octanoate; x = 2 or 3. 1 ' 12 ' 13 B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter XIV, Section E, and Chapter XV. F. K. Shmidt, V. V. Saraev, S. M. Krasnopolskaya and V. G. Lipovich, Kinet. Katal., 1973,14, 617. Y. Sakakibara, S. Yagi, M. Sakai and N. Uchino, Nippon Kagaku Kaishi, 1980, 240. E. Goldenberg, F. Da wans, J. P. Durand and G. Martino, Ger Pat. 2 232 769 (1973) (Chem. Abstr, 1973, 78,148 787). D. Wewerka and K. Hummel, Colloid Polym. Sci., 1976, 254, 116. M. Tzinmann, D. Cuzin and F. Coussemant, /. Mol. CataL, 1978, 4,191. A. F. Halasa, U. S. Pat. 3 988 504 (1976) (Chem. Abstr., 1977, 86, 74 166). D. V. Sokol'skii, N. F. Noskova, N. I. Marusich and T. A. Petrova, Kinet. Katal., 1976, 17, 1325. A. F. Halasa, U.S. Pat. 3 868 354 (1975) (Chem. Abstr, 1975, 83,194 865). A. F. Halasa and R. Gutierrez, Ger. Pat. 2 450 089 (1975) (Chem. Abstr., 1975, 83, 80 904). J. Cosyns, G. Martino and J. F. Le Page, Ger. Pat. 2 438 366 (1975) (Chem. Abstr, 1975, 83, 96 680). S. W. Eachus and C. W. Dence, Holzforschung, 1975, 29, 41. G. Hillion, G. Martino, C. Lassau and L. Sajus, Ger. Pat., 2 325 549 (1973) (Chem. Abstr, 1974, 80, 59 789). F. K. Shmidt, Y. S. Levkovskii, N. M. Ryutina, O. L. Kosinskii and T. I. Bakunina, React. Kinet. Catal. Lett., 1977,7,445. K. Madeja, K. Luehder and D. Nehls, East Ger. Pat. 137 329 (1979) (Chem. Abstr., 1980, 92, 153 733).

1 (steps a, c, d and f) are then invoked.341 However, some stoichiometric hydrogenations of monoenes using Co(II) stearate/RMgBr systems under inert atmospheres, which become catalytic under H2, are thought to involve dihydride intermediates.341 Of interest with respect to the Group IV Ziegler catalysts (Table 12; c and d), the titanium and zirconium alkyls [M(CH2Ph)4], studied primarily for use as polymerization catalysts, become active under H 2 for hydrogenation of alkenes at ambient conditions. 151 ' 353 The Zr(IV) complex with H 2 produced toluene {cf. equation 41) and, although the zirconium product was not characterized, it was also reported active at 50 °C and 1 atm H 2 for reduction of aromatic substrates. The dialkyl complex [TiMe2Cp2] is cleaved by H 2 to give initially methane and what was originally thought to be titanocene [{TiCp2}2],151 although this is now known to be the fulvalene complex (1) (see also Section 51.1.3.1,iii). The alkylation reaction, equation (130), hinges on the Lewis acid property of the alkyl component of the Ziegler-type catalyst and involves abstraction of the ligand X by the aluminium moiety. Attack by the aluminium at a coordinated chloride or at the oxygen atom(s) of acac and related ligands is well documented,341 as are carbonyl oxygen-aluminium interactions (M—C—O—Al), 354 and in this regard the enhanced activity of carbonyl- and phosphine-containing catalysts such as [CoH(CO)(PPh 3 ) 3 ], 355 [CoH(diphos) 2 ] 356 and [FeH 4 (PEtPh 2 ) 3 ] 357 by aluminium alkyls has been explained by ligand abstraction to give coordinatively unsaturated species. In this connection, addition of aluminium salts and alkyls to the complexes [RuCl2(PPh3)3] and [RhCl(PPh 3 ) 3 ] in attempts to remove a phosphine ligand gives somewhat unexpected results. 358 ' 359 Reactivity of the former complex is enhanced but via a scavenging of HC1 (equation 131) that promotes formation of the active hydride catalyst (cf equation 21); ruthenium alkyls were also formed via chlorine-alkyl interchange. Enhancement of the activity of the rhodium complex by AlBr 3 results simply from formation of the more active bromo complex, while enhancement by addition of AlBu'3 is attributed to formation of [RhH(PPh 3 ) 3 ]. c.o.M.c. V O L . 8 — L

342

Addition of H2 and HCN to C=C and C=C Bonds HC1

+

A1R3

—• A1R2C1

+

RH

(131)

The reducing properties of aluminium alkyls must also play a role in many of the Ziegler catalysts, and complexes of zero-valent states, e.g. Cr(0), Co(0), Ni(O), appear to be the active catalysts in some systems.341'360 ESR signals in a nickel system have been attributed to a Ni(I) catalyst [(Bu3P)2Ni(M-Cl)2AlEt2],361 while Mossbauer studies on iron systems are consistent generally with the presence of Fe(II). 362 Bridging hydrides between aluminium and the transition metal have frequently been invoked. Stabilization of the lower-valence metal states by 7r-acceptor aluminium moieties has also been suggested.341 A detailed understanding of the Ziegler hydrogenation catalysts will be difficult to achieve. Doubt still remains as to the nature of species present in hydrocarbon solutions of the well-documented polymerization systems based on [TiCl2Cp2]/AlEt3 mixtures, even in the absence of j ! 2 340 The interesting complex (63) formed from [ZrCl2Cp2]/AlHBu2 mixtures plays a key role in reduction of CO (by the aluminium hydride) to alcohols;363 more generally, the M—C—O—Al interaction has been shown to increase the electrophilicity of the carbon atom for its subsequent attack by hydridic reagents. 364 The [TiCl2Cp2] complex with BuLi, BuMgBr (with or without added phosphines) or [UAIH4] gives systems that are all active for the H2 hydrogenation of monoenes. More recent data, especially ESR, indicate species with bridged hydrides between titanium and the other metal. 278 ' 341 ' 365 H—AxlR2 Cp 2 Zr x -H Cl H—A1R2 (63)

Closely related to the Ziegler catalysts are the systems composed of halides of Fe(III), Co(II) and Ni(II) with magnesium Grignard reagents or [LiAlH 4 ]. 341 ' 366 ' 367 The dark slurries which operate under mild conditions with H 2 or N 2 for a wide range of monoenes, including terpenes, may have some homogeneous activity. A vanadium tetrachloride/magnesium system has been used to hydrogenate cyclohexene.368 Similarly, reduction of transition metal complexes, including simple halides, with alkoxyalanates (e.g. LiAlH n (OR) 4 _ /7 ]) 341 or other aluminium hydride derivatives 369 produces extremely effective, stable, and reportedly homogeneous catalysts for hydrogenation of monoenes, dienes, 1-alkynes and aromatic substrates.341'370'371 The systems appear akin to some produced by corresponding reductions using borohydride (Section 51.1.4.1). 51.1.4.3 Hydrogenation of unsaturated fats Soybean, linseed and cotton seed oils consist of mixtures of long-chain unsaturated compounds such as monoenes, dienes and trienes. Soybean oil occurs as glycerine esters of linolenic (64), linoleic (65), oleic (66), stearic (67) and palmitic acids (68) in which any olefinic links are all cis. Commercial use is found in oleomargarine, salad dressings and other foods, but unfortunately the linolenate constituent (9,12,15-octadecatrienoates) has an undesirable flavour, and a catalyst has long been sought for its selective reduction to linoleate (ds-9,c/s-12-octadecadienoate). Other unsaturated fats studied include dehydrated castor oil, camelia, safflower and tung oils, as well as pure linolenate, linoleate and oleate substrates; sorbate (/ra«s-2,fra«s-4-hexadienoate) has served as a simple model substrate. The subject is reviewed extensively every year from the viewpoint of both heterogeneous and homogeneous catalysts. 311 ' 372 " 379 MeCH2CH=CHCH2CH==CHCH2CH=CH(CH2)7CO2R (64) Me(CH2)4CH==CHCH2CH=CH(CH2)7CO2R (65) Me(CH2)7CH=CH(CH2)7CO2R

Me(CH2)16CO2R

Me(CH2)14CO2R

(66)

(67)

(68)

Addition ofH2 and HCN to C=C and C=C Bonds

343

A wide range of homogeneous catalysts have been tested under H2, usually at elevated temperatures and pressures, 311 ' 380 and include some that are known to be effective usually only on conjugated systems {e.g. [CoH(CN) 5 ] 3 -, 3 8 1 [Co 2 (CO) 8 ], 3 8 2 [Mn 2 (CO)i 0 ] 3 8 2 and [Cr(CO)3(arene)]) 228 and some that induce migration of alkenic bonds to conjugation prior to reduction ([Fe(CO) 5 ], 97 [Fe(CO) 3 (diene)], 97 [Co(CO) 3 (PR 3 )] 2 353 and phosphine complexes of divalent Pt, Pd and Ni with and without SnCl 2 or other Group IV halides, particularly [PtCl 2 (PPh 3 ) 2 ]/SnCl 2 ). 241 ' 311 Other catalysts that usually operate at ambient conditions and show high selectivity within monoene or diene substrates have also been tested. These include [RI1CIL3], [RhH(CO)L 3 ], [MH 2 L 2 ] + (M = Rh, Ir) and [IrCl(cod)L] (L = a tertiary phosphine). 383 ' 384 The true catalysts in some molybdenum and tungsten systems using [MCl2(CO) 3 (PPh 3 )2]/SnCl2 mixtures may be decomposition species formed at the high temperatures utilized.385 Molybdenum and tungsten complexes of the type [M(CO) 3 (arene)] effect only nonselective hydrogenation of sorbate.118 Hydrocarbon solutions of some Ziegler catalysts, especially [Ni(acac) 2 ] and [Co(acac) 3 ] in conjunction with AlEt 3 , 341 ' 386 " 389 some systems of Table 12 (footnote f) 341,386,390,391 a n ( j ^ m e t a j acetylacetonates alone in methanol solution (with or without H 2 : see Section 51.1.3.3) have also been used. Whether these acac systems are all purely homogeneous is somewhat equivocal since accompanying black precipitates, said to be catalytically inactive, are sometimes formed. 244 ' 392 The catalysts formed by borohydride reduction of amide solutions of NiCl2 and CoCl2 (Section 51.1.4.1) also effect hydrogenation of unsaturated fats and linoleate to monoenes, and with the linoleate substrate reveal little positional or geometric isomerization which contrasts with the behaviour of, for example, heterogeneous nickel catalysts. 325327 Use of [RhCl 2 (BH 4 )(DM F)py 2 ] gives considerable translocation of the alkenic bond.326 The use of [CoH(CN) 5 ] 3 ~ is thought to be limited because it is not fat soluble;381 however, use of phase-transfer catalysis (Section 51.1.8) might usefully circumvent this restriction. Iron carbonyls readily effect reduction to monoenes with considerable accompanying geometric and positional isomerization, while the major monoene products show a double bond distribution that indicates formation from only conjugated dienes. Selectivity is related to the reactivity and stability of triene, diene and monoene carbonyl complexes.97'393 Thus, a catalytically active [Fe(CO)3(7r-diene)] complex (69) (diene = conjugated methyl octadecadienoates), isolated as a mixture of isomers during reduction of linoleate, could be hydrogenated to monoenes (and further to the fully saturated stearate), while selective binding to the iron leads to preferred reduction of diene over monoene. Nevertheless, direct reduction to stearate also occurs without going via a monoene stage. Labelling studies ( 14 C) on linoleate and diene complexes allow for ligand exchange studies, and quite detailed mechanisms, invoking both unsaturate and hydride pathways, have been presented for the iron carbonyl systems (Section 51.1.3.2,ii).97

HChrCH Me(CH 2 V- HC^ \H—(CH 2 )x—CO 2 Me fe(CO) 3 x, y = 4, 8; 5, 7; 6, 6; 7, 5; 8, 4; 9, 3; 10, 2 (69) The [Co2(CO)s] and [Mn2(CO)io] catalysts are selective for reduction of linoleate and conjugated octadecadienoates 382 ' 394 to monoenes, but selectivity for reduction of linolenate to monoenes is low, and high trans unsaturation is observed in the monoene and diene products.382'395 No intermediate complexes have been isolated. There seems to be disagreement over whether conjugation is required prior to hydrogenation, although this seems a usual requirement for [Co 2 (CO) 8 ]-catalyzed systems (Section 51.1.4.1). The [{Co(CO)3PR3}2] systems effect H 2 reduction of soybean oil to monoenoic fatty acids but the products have mainly trans geometries. 396 The ability of [Cr(CO) 3 (arene)] to reduce conjugated dienes selectively to c/s-monoenes was noted in Section 51.1.3.2,ii (equation 86). The most favourable stereochemistry for the addition is with a trans,trans-diene since, in the cisjrans- and cw,cw-dienes, one and two substituents interfere with the concerted addition. Thus, within a mixture derived from dehydrated methyl ricinoleate, c/s-Me(CH 2 ) 5 CH(OH)CH 2 CH=CH(CH 2 ) 7 CO 2 Me, the conjugated dienes (mainly cis-9jrans-\ 1-octadecadienoate) are completely reduced to monoene while the nonconjugated dienes are unaffected.228 The unconjugated linolenate and linoleate in fats have been hydrogenated

344

Addition ofH2 and HCN to C=C and C=C Bonds

to mainly c/s-monoenes (a useful property of the [Cr(CO)3(arene)] catalysts) by use of higher temperatures, which induces a rate-determining isomerization of the substrates to the conjugated 9,11- and 10,12-dienes; thus, a thermally more stable arene complex such as the methyl benzoate derivative gives optimum activity. 228 ' 319 Chromium hexacarbonyl appears less useful than the arene derivatives because of easier decomposition.397 The Ziegler and acetylacetonate systems give mainly cis products via 1,2-addition to alkenic links in linolenate and linoleate with no movement of double bonds. 244 The properties of the platinum-tin chloride catalysts (usually [PtCl4]2~/SnCl2 mixtures) without phosphines are markedly solvent dependent, but with unsaturated fats the systems generally give nonconjugated dienes as well as monoenes, with conjugation prior to hydrogenation and extensive isomerization to /ra«s-monoenes being quite common.241'311 Thus, for example, double bond migration via & Pt(II) hydride catalyst (equation 60) leads to conjugated trienes, or conjugated diene groups within trienes. The conjugated diene part of these trienes then forms complexes with the hydridoplatinum(II) species (equation 70), and subsequent hydrogenolysis of the alkyl or allyl with H2 or the hydride (cf. Scheme 1) results in the unreactive nonconjugated diene products. Conditions have been reported for the selective reduction in methanol of linolenate to diene after conjugation, while with soybean esters in acetic acid a remarkable preference has been noted for hydrogenation of diene (linoleate) to monoene (particularly oleate) prior to any reduction of triene (linolenate). Addition of tertiary phosphines (or arsines) to these platinum systems does not affect the overall catalytic properties, but it appears to improve the selectivity patterns within product distributions.241 Corresponding palladium systems are generally more active but are less stable. The [MX2(PPri3)2] complexes (M = Pt, Pd; X = anion) are generally much less reactive without the addition of tin(II) chloride, although analogous nickel systems are unaffected by the presence of the cocatalyst.241 These catalysts operate less effectively in the absence of H2 via hydrogen transfer from solvents (Section 51.1.3.3). The [RhCl(PPh 3 ) 3 ] complex catalyzes deuteration (hydrogenation) of linoleate and oleate under mild conditions to stearate-9,10,12,13-^4 and stearate-9,10-^2, respectively.241 This complex is unlikely to effect conjugation of the triene constituents of fats and, in any case, does not usually catalyze reduction of simple conjugated dienes because of strong complexing (Section 51.1.4.1). Nonselective hydrogenation of linolenate is predicted. This rhodium system is of greater utility in catalytic hydroformylation of fats. 398 ' 399 Use of the [RhCl(PPh3)3] catalyst in modulating membrane fluidity by hydrogenation of fatty acid residues is noted in Section 51.1.8. The [RhH 2 (PR3)2] + and related iridium catalysts show a range of selectivity patterns depending on the phosphine used; conjugation of linoleate prior to reduction is generally found, and 1,4addition of H 2 to 10,12-dienes has been accomplished (cf equation 86). 383 ' 384 Schiff base complexes of the first-row transition metals, particularly Cu(II) and Fe(II), are also noted to effect homogeneous hydrogenation of soybean oil to mainly c/s-monoenes.400 The ability of platinum metal complexes to catalyze hydrogen transfer from various solvents (Section 51.1.3.3) has led to the testing of such systems for hydrogenation of fats.401 ~403 Complexes used include [RuCl 2 L 3 ], [R11H2L4], [RhHL 4 ], RhCl 3 , [PdCU] 2 " and [PbCl 4 ] 2 " (L = PPh 3 ) with donor solvents, e.g. alcohols (including glucose and steroids) and hydrocarbons (including indoline and terpenes such as limonene). Systems based on [RuCl 2 (PPh 3 ) 3 ]/isopropanol, [RhH(PPh 3 )4]/cholesterol or [PdCU] 2 ~/ascorbic acid or indoline appear particularly useful for reducing nonconjugated polyenes such as linoleate with 100% selectivity to monoenes with no trans isomers; hydrogenation follows conjugation of the substrates. In summary, the aim of reducing the C-15 double bond only has not yet been achieved, although linolenate has been reduced to unreactive dienes with unconjugatable double bonds, for example by the platinum-tin systems. Two direct approaches, both formidable tasks, involve selective complexing and reduction at the C-15 double bond or formation of a stable complex with the C-9 and C-12 double bonds, followed by reduction of the free C-15 double bond and decomposition of the complex to give linoleate. The principles of the latter method are exemplified in the characterization of a [Fe(CO) 3 (triene)] complex (70), formed from [Fe(CO)s] and linolenate after

RHC

I CH(CH 2 ) 2 CH=CHR' Fe(CO)3 (70)

Addition ofH2 and HCN to C=C and C=C Bonds

345

double bond migration {cf. equations 60 or 61). Chemical reduction of the uncoordinated alkene was effected by hydrazine. 97

51.1.5 Hydrogenation of Aromatic Hydrocarbons Attention to systems effective for the hydrogenation of aromatic substrates has been rekindled by the renewed interest in coal utilization, anthracene usually being considered a simple model substrate. Section 51.1.4.2 describes the use of Ziegler catalysts and related systems under fairly severe conditions, and that of [Zr(CH2Ph)4] as a precursor for a catalyst active under mild conditions for the hydrogenation of aromatic substrates. The systems are poorly characterized, as are those involving some O2-oxidized [RhCl(PPli3)3] complexes and some Rh(I) 7r-complexes with phenyl carboxylates, which are reported active at ambient conditions.150 The latter complexes, formulated as H[Rh2A 2 Cl], where A is typically phenylacetate, tyrosinate or 7V-phenylanthranilate, hydrogenate benzene to cyclohexane, and polyaromatic substrates with some selectivity {e.g. anthracene to the 1,2,3,4-tetrahydro product). 122 ' 404 Attempts to hydroformylate aromatic compounds containing 'reactive double bonds' using [Co2(CO)g] led to the discovery in the 1950s that the system was effective under the high temperature and pressure conditions for reduction of some of the substrates, 39405 probably via radical pathways (Section 51.1.3.1,i). Isolated benzene rings and phenanthrenoid systems are generally resistant to reduction, and highly condensed systems tend to form phenanthrene derivatives. Thus naphthalenes give tetralins, anthracene the 9,10-dihydro derivative, indoles the 2,3-dihydro compounds and chrysene the 5,6-dihydro compound (equation 132), while perylene and pyrene give phenanthrene derivatives (reactions in equation 133). Acenaphthene and fluoranthene give tetrahydro compounds (reactions in equation 134). The conversion of benzanthrone to 1,10-trimethylenephenanthrene (equation 135) involves reduction of an aromatic ring as well as the ketone group, with subsequent dehydration. Thiophenes are reduced completely to the corresponding thiolanes. Both H 2 and CO can be added to coal under hydroformylation conditions using the [Co 2 (CO) 8 ] catalyst at relatively low temperatures (~200 °C). 406 ' 407

(132)

(133)

(134)

(135)

More recently, some allylcobalt(I) complexes

[COL 3 (T7 3 -C 3 H5)]

(L = tertiary phosphine or

Addition ofH2 and HCN to C=C and C^C Bonds

346

phosphite) have been found to catalyze hydrogenation of benzenes to cyclohexanes at ambient conditions, but catalyst lifetimes are limited and turnover numbers are very low (~15 in 24 h). 150 With L = P(OMe)3, C6D6 yielded solely c/s-C6D6H6 with no competing hydrogen exchange, the catalysts show a slight selectivity for hydrogenation of arene when using benzene/cyclohexene or benzene/hexene mixtures as substrates, and because neither cyclohexadiene nor cyclohexene was detected during hydrogenation of benzene, a mechanism was presented in which the C6 moiety remains attached at the cobalt until a cyclohexyl species is formed (equation 136, Scheme 6). 150 The scheme is essentially that of the hydride route of Scheme 2, modified for a conjugated triene (diene) substrate(s) involving intermediate 7r-allyls. The undetected intermediate [CoH2L(r;1C3H5)(774-C6H6)] (72) is thought to be formed from the detected dihydride (71) following dissociation of the phosphite L. In the absence of arene the dihydride (71) yields propene and [C0HL3] (cf. equations 42 and 72), although under H 2 the trihydride [C0H3L3] exists (Table 1), this being a catalyst for hydrogenation of alkenes but not aromatic substrates. Benzenes with substituent groups R, OR, CO 2 R, CHO, COR, C H = C H R , C = C R and NR 2 are also reduced by the allyl catalysts, sometimes with concomitant hydrogenation of the substituent group; furan, naphthalene and anthracene can also be fully hydrogenated, although various coals are not hydrogenated. 150 ' 408 Some rhodium analogues, e.g. [Rh{P(OPr i )3) 2 (^ 3 -C 3 H 5 )], are also catalysts for arene hydrogenation, but they decompose rapidly under H 2 to give monohydrides (cf. equation 42), which catalyze only hydrogenation of alkenes.150

CoL3073-C3H5)

^

CoL^-CaHg)

^

CoHzLaW-CaHg)

(136)

(71)

(72)

Co

^ L

VC3H5C0L3 + C6H12

C66H Hn

Scheme 6 Mechanistic scheme for hydrogenation of benzene to cyclohexane catalyzed by allylcobalt(I) complexes Within r76-arene complexes, [Ru6C(CO)i4(?7-PhMe)] is converted stoichiometrically by H 2 at 150 °C to methylcyclohexane, while a [Ru(r;-C6Me6)(^4-C6Me6)] complex (cf. complex (72) is reported to be a long-lived homogeneous catalyst for arene hydrogenation but, in contrast to the cobalt.system, extensive H-D exchange occurs in the aromatic ring and in methyl groups of xylene substrates. The hydrides [RuHCl(PPh3)(7/-C6Me6)] and [(77-C6Me6)Ru(ju-H)2(juCl)Ru(77-C 6 Me 6 )] + (Table 4) are effective at 50 °C and 50 atm H 2 in isopropanol for reduction of arenes (C 6 H 5 R) to the cyclohexanes, when R = H, OH, OMe, CO 2 Me, COMe, COPh and

Addition ofH2 and HCN to C=C and C=C Bonds

347

NMe2; styrene gives ethylcyclohexane and nitrobenzene gives aniline, while hydrogenolysis of the functional group occurs with halogenobenzenes and diphenyl ether. 256 ' 409 The reactions are thought to be mainly homogeneous, although a minor heterogeneous component is said to be present. A ready deprotonation of coordinated hexamethylbenzene in some derivatives of a precursor [{RuCl2(r7-C6Me6))2] complex suggests a possible role for the arene methyl group in the catalysis (equation 137). Hydrogenation of C 6 D 6 gave exclusively C6H6D6 as a cis/trans mixture, and retention of the arene moiety throughout the hydrogenation was again invoked (see Scheme 6). The monohydride complex also catalyzed hydrogen transfer from 1-phenylethanol to benzene, but in poor yield. The cobalt and nickel complexes [M(C6F5)2(r?-C6H6)] obtained by metal atom syntheses also catalyze arene hydrogenation but are short-lived.410 The reluctance of the arene ligand in several metal-arene and allylmetal-arene complexes of the type [RuCl(773-C3H5)(77C6H 6 )] and [{MoCl(r73-C3H5)(ry6-PhMe)}2] to undergo hydrogenation and the lack of intramolecular hydrogen transfer within [MoH2(PPh3)2(j?6-PhMe)] suggest that intermediates with 7j4-arene binding are a requisite for catalytic hydrogenation of aromatic substrates (cf. Scheme 6). 411

(137)

The cyclopentadienyl complex [{RhCl2(r7-C5Me5)}2] (Table 4), at 50 atm H 2 in the presence of base, effects stereoselective hydrogenation of benzenes, although substrates with unprotected OH or CO2H groups are not effectively hydrogenated. 412 Aryl ethers, esters, ketones and 7V,7V-dimethylaniline are reduced, sometimes accompanied by hydrogenolysis of the functional group. 150 A patent has described the use of a zero-valent rhodium catalyst, formed from [RhCl(cod)] 2 /BuLi mixtures, that hydrogenates aromatic substrates, 413 and the nickel cluster [Nis(CO)2Cp3] dispersed on silica effects hydrogenation of benzene at ambient conditions.290 The [jMn(CO)4(PBu?)}2] dimer catalytically hydrogenates anthracene to the 9,10-dihydro compound, 414 while the anionic ruthenium(II) orthometallated dihydride [RuH 2 (o-C6H4PPh 2 )(PPh3)2]" effects the less common reduction to the 1,2,3,4-tetrahydro derivative in essentially quantitative yield.415 In general, permethyl arenes and Cp ligands are very resistant to hydrogenation and seem to be optimal ligands for catalytic hydrogenation of aromatic substrates. The less well-characterized catalysts described at the beginning of this section may all possibly contain 7r-bonded phenyl groups. The use of [RhCl2(BH4)(DMF)py2] for the reduction of pyridine and quinoline was noted in Section 51.1.4.1, and the rhenium nitrosyl listed in Table 11 effects hydrogenation of pyridine. Some bis(salicylaldehydo) and Schiff base complexes of Co(II), Ni(II) and Cu(II), with or without [LiAlH4], catalyze hydrogenation of benzene and alkylbenzenes at 200 °C, but the systems appear to be heterogeneous. 416 Several transition metal systems, including [FeCp2], [Co(acac) 2 ]/LiAlH 4 , C0CI2/BH4-, [Co(CN) 5 ] 3 ", [RhCl(PPh3)3], [Rh 2 A 2 Cl]- (A = 7V-phenylanthranilate), /ra«5-[IrCl(CO)(PPh 3 ) 2 ] and MC13 (M = Rh, Ir), are not effective under mild conditions (5 atm H 2 ,50 °C) for hydrogenation (liquefaction) of coal because of rapid deactivation of the catalysts. 417 Patents have described the use of metal halides such as TaFs in a mixture with anhydrous protonic acids, such as HF in isopentane solution, for the hydrogenation of aromatic compounds;418 the mechanism presumably involves hydride transfer from the alkane to a carbonium ion formed by proton addition to the aromatic substrate in the superacid medium (see Section 51.1.3.3).

51.1.6 Hydrogenation of Alkynes The wealth of information on the catalytic hydrogenation of alkenes, stemming from initial testings of complexes for activity towards such substrates, is contrasted by the rather limited data on alkynic substrates. Most catalysts found effective for alkenic substrates are probably tested with alkynes, and initial rate data are sometimes recorded with selected substrates, typically

348

Addition ofH2 and HCN to C=C and C=C Bonds

acetylene, 1- and 2-hexyne, phenyl- and diphenyl-acetylene, alkynic alcohols or acetylenedicarboxylic acid. However, there are remarkably few detailed kinetic studies on alkyne substrates, using either mono- or di-hydride catalysts, even for the 'overall kinetic dependences' of the system, let alone individual steps within the catalytic cycle (cf. Scheme 3); reaction pathways for hydrogenation to the alkene stage are assumed by analogy to correspond with those established for converting the alkene to saturated product (Schemes 1 and 2) after substituting the intermediate alkyls by vinyl species (see Section 51.1.3.2). The question of the usual net cis addition to alkynes has been addressed earlier (Section 51.1.3.2), and use of the [RuHCl(PPh 3 ) 3 ] and [RhCl(PPh 3 ) 3 ] complexes for selective reduction of terminal alkynes to alkenes in an alkyne/alkene substrate mixture was also discussed in the same Section. The stronger binding (7r-fashion) of the alkyne probably explains the selectivity, but also accounts for the relatively low activity of both catalysts towards alkyne substrates alone, at least under ambient conditions. 119 ' 143 Too strong a metal-substrate bond is likely to hinder the subsequent migratory insertion reaction (Schemes 1 and 2); similar reasoning holds for the lack of activity of these two catalysts for most conjugated diene systems. Some 'intermediate' stability range for substrate binding is clearly required for successful hydrogenation generally with any catalyst system. Alkenes seem to be accommodated fairly readily in this optimum range, while alkyne complexes are apparently somewhat on the too-stable side. Once bound, the more electron-deficient alkynes, as well as strongly ?r-acid alkenic substrates (e.g. tetrafluoro- and tetracyano-ethylene), will also increase the promotional energy required for subsequent dihydride formation via oxidative addition (Section 51.1.3.1,i) and thus catalytic hydrogenation via such an unsaturate route (Scheme 2) is not favoured. A further complication with alkynic substrates is their tendency to oligomerize and polymerize in the presence of a transition metal complex, particularly at higher temperatures. Examples include systems involving [RhCl(PPh 3 ) 3 ], 143 [Ni4(CNR) 7 ] 100 and Ziegler catalysts.341 Nevertheless, conditions for effective reduction of alkynes to monoenes have been found with the nickel cluster and some Ziegler systems (see Sections 51.1.3.4 and 51.1.4.2, respectively). Several other cluster and dimeric species that catalyze the reduction of alkenes to monoenes were noted in Section 51.1.3.4 and in Table 10. Most, if not all, hydrogenation catalysts that involve monomeric species and are found to reduce alkynes will also hydrogenate alkenic substrates. The A-frame, dimeric complex [Ir2Ou-S)(CO) 2 (dppm) 2 ] (Table 1) also reduces acetylene 'directly' to the alkane. 94 Nevertheless there are systems that appear practical at ambient conditions for the selective reduction of a range of alkynes to the monoenes. As noted above, this is probably related to the preferred binding of the alkyne, and it is usually necessary to monitor the reaction to detect the onset of any subsequent hydrogenation of the alkene product. The [Ni 4 (CNR)v] cluster mentioned above has no tendency to hydrogenate alkenes, which argues indirectly for catalysis at a cluster site involving bridged alkyne substrates. 100 Systems based on cationic and neutral rhodium precursors of the type [Rh(diene)L 2 ] + , [Rh(diene)LL/] + and Rh(diene)LX (L = tertiary phosphine, U = py, X = benzoate) also seem particularly effective for terminal and internal alkynes; base is sometimes required, suggesting that in these cases monohydrides may be involved (see equations 43 and 32).132 Protonation of [Rh 2 (CO 2 Me) 4 ] in solution followed by addition of phosphines gives species that are similarly active;98 hydrogen reduction to Rh(I) could lead to the same catalysts generated by the diene precursors. Palladium chloride in DMF 4 2 0 and DMSO, 339 and the readily formed [RhCl 2 (BH 4 )(DMF)py 2 ] catalyst 122 (Section 51.1.4.1), have been used for selective reduction of triple bonds, while the versatile NiCl 2 /BH 4 -/DMF system (Section 51.1.4.1) with 328 or without421 added ethylenediamine under H 2 converts internal alkynes to the ds-alkenes with 100% selectivity. A photoassisted [Fe(CO)s] system is noted in Section 51.1.7. The first complexes of/-orbital elements to catalyze homogeneous hydrogenation were reported in 1979.422,423 Cocondensation of lanthanoid metal atoms with internal alkynes at low temperature yields species not yet completely identified that effect hydrogenation of the alkynes to the cisalkenes at room temperature and 1 atm H 2 ; with 3-hexyne, Sm and Yb form [MQHio] species, while Er gives ErCgHis isomers and a dimeric [Er 2 Ci 8 H 30 ] complex (some p-aminosalicylate complexes of trivalent lanthanides have been used for hydrogenation of nitrobenzene).424 Hydrogen transfer systems based on IrCl 3 /DMSO/isopropanol solutions are also effective for reducing activated alkynic bonds, e.g. in phenyl- and diphenyl-acetylene.183'425 Under more severe conditions at elevated temperatures and pressures, [M(CO) 2 Cp 2 ] (M = Ti, Zr, Hf), 151 ' 426 [{Fe(M3-CO)Cp)4],272 [Mo 2 (CO) 4 Cp 2 ] 233 and cobalt complexes such as [CoH(CO)(PR 3 ) 3 ] 139 and [CoCljP(OEt)3j3]171 have been used for hydrogenation of terminal alkynes to the alkenes, while [Pd(PPh 3 ) 4 ], 125 [Pt(PPh 3 ) 2 (C 2 H 2 )] 127 and cationic iridium species

Addition ofH2 and HCN to C=C and C=C Bonds

349

such as [Ir(CO)(dppm) 2 ] + and [Ir(diphos) 2 ] + , 157 as well as a range of mixed metal dimers (Table 10), have been used for the selective reduction of alkynes in alkene/alkyne mixtures. Of interest, the [Ti(CO)Cp(PhC=CPh)] complex synthesized from [Ti(CO) 2 Cp 2 ] effects reduction of alkynes and alkenes at ambient conditions following hydrogenation of the coordinated diphenylacetylene.427 Reduction of alkynes has sometimes been noted under hydroformylation conditions but with little selectivity; thus with [Co2(CO)g], diphenylacetylene gives only the saturated product 1,2-diphenylethane.39 A trans addition of H 2 to acetylenedicarboxylic acid catalyzed by [CoH(CN) 5 ] 3 ~ at high pressure was noted in Section 51.1.3.2,i. The [RhCl(PPh 3 ) 3 complex under its normal operating conditions (Section 51.1.4.1) generally reduces alkynes slowly to the saturated product, but sometimes monoenes are also detected.143 The complex [RuHCl(PPh 3 ) 3 ] is generally inefficient, although conditions have been reported for reduction of diphenylacetylene, 2,5-dimethylhex-3-yne-2,5-diol and stearolic acid to the corresponding alkene. 93 ' 119 The [MC1(CO)(PR 3 ) 2 ] and [MH(CO)(PR 3 ) 3 ] compounds (M = Rh, Ir) tend to yield isolable complexes with alkynes, and are thus not very efficient catalysts;157'187 acetylides may also be formed with the monohydrides, and since treatment of the acetylide with H 2 at severe conditions (50 atm, 70 °C) yields alkane with regeneration of hydride, a somewhat different but impractical catalytic cycle for the reduction of alkynes to alkanes can be envisaged, e.g. equation (138). 428 A photoassisted [IrCl(CO)(PPh 3 ) 2 ] system is reported to reduce 2-butyne-l,4-diol to the butene derivative (Section 51.1.7). 429 RhH(COXPPh3)3

t

^ 2 > Rh(0=CRXCOXPPh3)3

3H2

(138)

I

-EtR

Some other catalysts reported to reduce alkynes under mild conditions (usually only acetylene to ethylene is noted) are given in Table 13. The Co(III) porphyrin system utilizes borohydride Table 13 Catalysts not discussed in Section 51.1.6 that effect Hydrogenation of Alkynes to Monoenes Complex (ref.) CuPc.]-/[Cr(acac) 3 ] a (1); [Mn(CO)2(PR3)2(T73-C3H5)]b (2); [FeH2(PPh2Et)3] (3, 4); RuH2(PPh3)4] (5, 6); MHCl(CO)(PPh3)3, M = Ru, Os (5, 7); [Co(TSPP)] 3 " c (8); Rh 2 L 2 Cl]- d (9); [Rh(TPP)]" e (10); [RhH2Cl(PBu3)2] (11); [Rh(SnX3)(PPh3)3]f (12); Rh(OCOPh)(PPh3)(cod)], [Rh(PPh3)(py)(cod)]+ (13); [IrH3(PPh3)3]s (14); PdCl 2 (PPh 3 ) 2 ] h (15); [PdHCl{P(OEt)2(OH)}2] (16); [Pd{P(OPh)3}4] (17); PtCl 6 ] 4 -/SnCl 2 (18-20); PtHCl(PPh 3 ) 2 /SnCl 2 (18).

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

a Pc- = phthalocyanine anion radical. b R = OMe, OCHMe2, Et. c TSPP = dianion of meso-tetrakis(p-sulphonatophenyl)porphyrin. d L = phenylacetate, TV-phenylanthranilate, tyrosinate. e TPP = dianion of mmKetraphenylporphyrin. f X = Cl, Br. 8 Using H 2 or Pr'OH as the hydrogen source. h Activated with N 2 H 4 . T. Shimura, R. Iwata, I. Ogata and Y. Arai, Tokyo Kogyo Shihensho Hokoku, 1978,73,505 (Chem. Abstr., 1980,92,128 332). L. S. Stuhl and E. L. Muetterties, Inorg. Chem., 1978,17, 2148. W. E. Newton, J. L. Corbin, P. W. Schneider and W. A. Bulen, J. Am. Chem. Soc, 1971, 93, 268. B. R. James, Adv. Organomet. Chem., 1979,17, Section XI. B. R. James, 'Homogeneous Hydrogenation', Wiley, New York, 1973, Chapter IX, Section B. Ref. 4, Section IIB. Ref. 5, Chapter IX, Section C. E. B. Fleischer and M. Krishnamurthy, J. Am. Chem. Soc, 1972, 94,1382. Ref. 5, Chapter XI, Section H. B. R. James and D. V. Stynes, J. Am. Chem. Soc, 1972, 94, 6225. C. Masters, W. S. McDonald, G. Raper and B. L. Shaw, Chem. Commun., 1971, 210. N. V. Borunova, L. K. Freidlin, P. G. Antonov, Y. N. Kukushkin, Y. N. Trink, V. M. Ignatov and A. R. Ganeeva, Izv. Akad. Nauk SSSR, Ser. Khim., 1977, 2045. R. H. Crabtree, A. Gautier, G. Giordano and T. Khan, J. Organomet. Chem., 1977,141, 113. Ref. 5, Chapter XII, Section Bl. T. M. Beloslyudova and C. A. Il'ina, Kinet. Katal., 1975,16, 788. T. S. Kukhareva, I. D. Rozhdestvenskaya and E. E. Nifant'ev, Koord. Khim., 1977, 3, 241. G. K. Shestakov, A. M. Vasil'ev, L. M. Tishchenko, O. N. Temkin and R. M. Flid, Kinet. Katal., 1974,15, 1070. Ref. 5, Chapter XIII, Section C. Y. A. Dorfman and L. V. Lyashenko, Eleketrokhimiya, 1977,13, 535. N. V. Borunova, L. K. Freidlin, Y. G. Mukhtarov and N. I. Malofeev, Neftekhimiya, 1974,14, 572.

C.O.M.C. VOL. 8—L*

350

Addition ofH2 and HCN to C=C and C=C Bonds

in aqueous solution as reductant and not H2; Co(I) intermediates probably effect hydrogenation via a reduction/protonation mechanism (Section 51.1.3.2,1). Stoichiometric hydrogenation of alkynes to alkenes has been accomplished by hydrogen transfer from water using, for example, Cr 2+ , 95 [Ni 2 (CN) 6 ] 4 - 1 2 3 and [Fe(CO) 5 ] solutions;226 the iron system could presumably be made catalytic in the presence a CO atmosphere (Section 51.1.3.3).

51.1.7 Photocatalysis Examples in the literature of photoassisted catalytic hydrogenation reactions are becoming increasingly common;429 they invariably involve ghotoinduced dissociation of a ligand and generation of coordinatively unsaturated species which then become thermally active via the usual hydride or unsaturate routes (Section 51.1.3.2). The systems appear generally to be true cases of photocatalysis where a catalytic reaction is triggered by light and the number of molecules ultimately reacting does not depend on the number of photons absorbed. With near-UV irradiation, [Fe(CO) 5 ] becomes effective at ambient conditions for hydrogenation and isomerization of alkenes and alkynes, in contrast to the more severe, purely thermal conditions required (150 °C, 10 atm H2). The active species are tricarbonyl species (equation 139), which then appear to operate via an unsaturate route through complex (73), 430 although both unsaturate and hydride routes (via [FeH 2 (CO) 3 ]) have been invoked for the thermal systems (Section 51.1.3.2,ii).226 Isomerization is effected through [FeH(CO)3(7r-allyl)] formed from (73) (cf. equation 61). Loss of H2 from complex (74) was also considered to be photoinduced430 and several hydrides, including neutral and cationic dihydride derivatives of Ir(III) and Ru(II) phosphine complexes, as well as [WH2(7?-Cp)2], have been shown to undergo such reductive elimination of H2. 429 Photoassisted oxidative addition of H2 is seen in equation (139), 430 and is exemplified further in equation (140) . 43 l

Fe(CO)5

-^

Fe(CO)4

Fe(CO) 3 ( = = \)

—

(73) -H2|H2

H2Fe(CO)4

^

<13»

H2Fe(CO) 3 ( = \) (74)

H3Ir(PPh3)3

Zp^T

[H3Ir(PPh3)2]

^

H5Ir(PPh3)2

(140)

The thermally catalyzed 1,4-hydrogen addition to conjugated dienes to give mainly cismonoenes requires elevated temperatures and pressures when using [Cr(CO)3(arene)] complexes (Section 51.1.3.2,ii, equation 86). 95 The same reaction, however, is accomplished at ambient conditions using UV-irradiated solutions of [Cr(CO) 3 (MeCN) 3 ], [Cr(CO) 4 (nbd)] or even [Cr(CO) 6 ]. 429 ' 432 - 434 Corresponding photoassisted systems using [Mo(CO) 6 ] and [W(CO) 6 ] are accompanied by isomerization of the dienes and monoenes.429 A mechanism presented for the [Cr(CO) 4 (nbd)] system is similar to that outlined in equation (139), but involving a [CrH2(CO)4(771-diene)] intermediate, the photolysis here causing the originally bidentate diene ligand to become monodentate. 434 An isolated /ra«.s,fra«.s-2,4-hexadiene tetracarbonylchromium(0) complex was shown to be an intermediate in the hydrogenation of the diene using [Cr(CO) 6 ]. 435 Other photoassisted hydrogenations induced by loss of carbonyl ligand(s) include studies with [Zr(CO)2(T7-C5Me5)2],436 [Ti(CO)2(r?-Cp)2],437 [Fe 3 H 2 (NSiMe 3 )(CO) 9 ], 438 [Ru 4 H 4 (CO) 12 ] 439

Addition ofH2 and HCN to C=C and C=C Bonds

351

and a phosphinated polymer-supported [Rh6(CO)is] system292 (Section 51.1.3.4), while an alternative source of the [RuHCl(PPli3)3] catalyst (Table 4) is via UV irradiation of [RuHCl(CO)(PPri3) 3 ]. 440 A report 441 notes photoactivation of a range of mono-, di- and trinuclear carbonyl complexes, including [Mo(CO) w L5_ n ] (M = Fe, Ru; L = PPI13; n = 3-5), [M 3 (CO) 12 ] (M = Fe, Ru, Os), [Ru 3 (CO)9(PPh 3 ) 3 ] and [Co 2 (CO) 8 L 2 ] (L = PBuS, P(OPh) 3 ), for hydrogenation of 1 -pentene. The reason for markedly enhanced activity on irradiation (by a factor of 40) of [IrCl(CO)(PR3)2] catalysts for alkene hydrogenation was not delineated, but the intermediate formed, presumably by loss of carbonyl or phosphine, was also active for selective hydrogenation of cyclic dienes and an alkyne (2-butyne-l,4-diol) to the monoenes.429'442 The activity of [RhH(CO)(PPh3)3], which decreases with time because of formation of the inactive dimer [{Rh(CO)(PPh3)2)2], can be regenerated by UV irradiation. 429 The activity of [RhCl(PPh3)3] for cyclooctene hydrogenation is increased by about a factor of three on UV irradiation.429 Photochemical loss of N 2 from [FeH 2 (N 2 )(PEtPh 2 ) 3 ] (Table 1) initially aids binding of alkenic substrates in catalytic systems that operate via hydride routes. 429

51.1.8 Supported Catalysts, Membrane Systems, Phase-transfer Catalysis and Molten Salt Systems A major advance in overcoming the problem of separating the homogeneous catalyst from the product has been the development of heterogenized homogeneous catalysts formed by bonding a transition metal complex to various supports. Mention of a few such catalysts was made in Section 51.1.3.4, but the topic will not be discussed in further detail here because it is fully covered in Chapter 55 of this volume. There have been other approaches to catalyst recovery and reuse. One involves the use of membrane systems, 443 ' 444 permeable to reactants and products but not to the catalyst, or vice versa. Thus, in an attempt to overcome the problem of inaccessibility of some catalytic sites in supported polymers, some soluble Rh(I), Pt(II) and Pd(II) complexes with noncross-linked phosphinated polystyrene have been used to hydrogenate alkenes; the catalysts were then recovered quantitatively by membrane filtration (or precipitation with hexane). 445 In this particular case, however, the soluble polymer-supported systems were no more active than some insoluble analogues formed from cross-linked phosphinated polymers. Membranes have also been used for selective removal of an involatile ligand, e.g. free phosphine in an equilibrium such as equation (141), in order to increase the concentration of the active catalyst species, in this case the tris(phosphine) species.446

RuH2(PPh3)4

^

RuH2(PPh3)3

+

PPh3

(141)

The use of biphasic systems looks promising. Sulphonated triphenylphosphine (Section 51.1.4.1) allows formation of water-soluble in situ rhodium phosphine complexes which can, for example, catalyze hydrogenation of cyclohexene, present as a separate organic phase. Removal of the hydrogenated organic layer leaves the aqueous catalyst solution ready for further use. 324 Hydrogenation rates have been markedly enhanced by adding small amounts of cosolvents (alcohols, amides), which presumably increase effective concentrations of the catalyst and substrate at the interface. The same principle has been used in hydrogenating aqueous solutions of water-soluble alkenes by shaking with benzene solutions of [RhCl(PPh 3 ) 3 ] under H 2 . 324 The addition of micelle-forming surfactants to aqueous solutions of [CoH(CN)5p~ leads to stabilization of the catalyst (the ageing reaction is not encountered; see equation 91) and a solubilizing of, for example, diene substrates. 447 Using a neutral polyoxyethylene reagent, Ci 2 H 2 5(OCH 2 CH 2 ) 2 3 OH, conditions were found for selective hydrogenation of 2-methylbutadiene to 2-methyl-2-butene (a 1,4-addition of H 2 ), and of 2,3-dimethylbutadiene to 2,3-dimethyl-1-butene (a 1,2-addition). The use of such surfactant reagents is really an example of phase transfer catalysis, where usually a quaternary ammonium salt or a polyether is added to a twophase organic/aqueous system to assist the bringing together of two reagents (in this case, hydrogenation catalyst and an unsaturated organic substrate) that have incompatible solubility properties. The [CoH(CN) 5 ] 3 " complex has been used directly in this way using benzene/aqueous alkaline mixtures containing [NEt 3 (CH 2 Ph)] + Cl~ as the phase transfer catalyst.448 The more general principles of phase transfer catalysis can be found in reviews and monographs.449"451

352

Addition ofH2 and HCN to C=C and C=C Bonds

An interesting development resulting from hydrogenation within a bilayer is a method for the modulation of membrane fluidity. The fatty acids associated with the phospholipids of cell membranes, and particularly their degree of unsaturation, are important in determining the fluidity of their structure and hence their biological properties. Use of the [RhCl(PPh 3 ) 3 ] catalyst and a sulphonated derivative (Section 51.1.4) with membranes, and model structures in the form of lipid bilayer dispersions in water, gives selective hydrogenation of the polyunsaturated residues and marked changes in fluidity.452"455 The use of molten salts as solvent allows easy separation of organic products by distillation,456 and in this way PtCb with tetraalkylammonium salts of [SnCl 3 ]~ and [GeCl 3 ]~ has been used to hydrogenate 1,5,9-cyclododecatriene selectively to cyclododecene, the salts acting as both solvent and ligand.457 51.1.9 Concluding Remarks It is clear that intense interest continues in hydrogenation reactions catalyzed by transition metal complexes, with the general aim of developing catalysts for selective processes under mild conditions. Hydrogenations of unsaturated hydrocarbons are important commercially within petrochemical, pharmaceutical and food industries, and selectivity is critical to the success of industrially based processes. Greater product selectivity has an important impact on energy and resource utilization in terms of reduced process energy requirements for product separation and purification, and in terms of low-value by-products. The development of chiral catalysts for commercial production of optically active amino acids is an outstanding example of the application of well-understood principles of H2 activation and its subsequent addition to alkenic bonds. Such advances tend to stem initially from organic chemists searching for selective hydrogenations, often of 'sensitive' substrates that are converted unsatisfactorily with the more familiar heterogeneous catalysts. It is exciting and satisfying indeed to see a new catalyst, discovered and delineated by inorganic chemists, be subsequently exploited by their organic colleagues. The development of applications of the [RhCl(PPh 3 ) 3 ] complex by academic and industrial chemists is a classic example. Catalysts for selective reductions of monoenes, polyenes to monoenes, and alkynes to monoenes, are abundant. The subject is reaching the stage where one can almost use the metal of one's choice! If necessary, the use of high temperatures and pressures, irradiation (thus far, UV/VIS) or cocatalysts {cf. Ziegler systems) allows for some sort of hydrogenating activity for a wide range of transition metal complexes. Indeed, cynics might say that any transition metal complex will probably effect some catalytic hydrogenation under appropriate conditions, even if this necessitates generation of metal atoms, colloidal or deposited! Nevertheless, the successful selective systems will require, in most cases, ancillary ligands (including solvent) with appropriate steric and electronic properties, even though their role may not be fully understood. Interest in the hydrogenation of aromatic substrates and in metal cluster and dimer systems is likely to continue mainly as a result of renewed attention in the utilization of coal. Within aromatic compounds, selective hydrogenation of benzene to cyclohexene (a precursor to adipic acid and the synthetic fibre industry) would be one worthwhile goal. The very unusual selectivity for preferred hvdrogenation of an aromatic compound in the presence of an alkene has been noted already (see Section 51.1.5). The topical subject of hydrogenation of carbon monoxide and other triply bonded molecules (RCN, N2) will ensure continued activity in the cluster (dimer) area. Although not considered in detail in this chapter, asymmetric hydrogenation continues to be given major emphasis by many workers interested in catalytic hydrogenation. Interest is growing in chiral catalysts based on less expensive metals {e.g. cobalt) and a wider range of chiral ligands, including naturally occurring ones (see Chapter 53). Improved models for hydrogenases should accompany a better understanding now being developed of the enzyme systems themselves. More generally, in the area of catalyzed H2 addition to unsaturated moieties, catalysts that effect mild hydrogenation of the carbonyl group are becoming more common 458 ' 459 (see also Chapter 50), and increasing knowledge in this area coupled with the extensive data on C = C and C = C systems should lead to further catalysts that preferentially reduce the carbonyl group in a,/3-unsaturated aldehydes and ketones, as well as in ketone (or aldehyde)/alkene mixtures. Hydrogenation of groups such as C = O , C = N , NO2 and C = N , especially in the presence of other sensitive functionalities, provides an area in which a great deal still needs to be done. Further developments in catalyzed H2 addition to carbon-carbon double and triple bonds are now considered generally to be at a 'fine tuning' stage.

Addition ofH2 and HCN to C=C and C=C Bonds

353

Once the potential of a homogeneous catalyst, including chiral examples, has been realized, procedures for the separation of product and catalyst recovery, often critical for the expensive platinum metal systems, have to be considered. The development of heterogenized homogeneous catalysts (see also Chapter 55) will undoubtedly continue; the selectivity patterns of the supported complex, however, are often different to those of the homogeneous analogue (which can be used to advantage), and it remains a difficult and often empirical task to match the catalyst and support to give the required activity and selectivity. Alternative separation procedures for systems in which the catalyst remains homogeneous, e.g. the use of membrane systems and particularly phase transfer catalysis, are likely to attract increasing attention. Finally, concerning mechanistic aspects of hydrogenations catalyzed via monohydrides, more serious consideration should be given for free radical mechanisms via H-atom transfer (Section 51.1.3.2,i). It would also be satisfying to detect, within catalytic monohydride system, the hydridoalkene to alkyl step and the often postulated dihydridoalkyl intermediate (Scheme 1). Some ambiguities still arise when considering stereochemical aspects of certain systems, again particularly with monohydride catalysts. Further details are required on the formation of the saturated product from the metal alkyl whether by protonolysis, intramolecular reaction with metal hydride or even direct hydrogenolysis. More definitive conclusions could then be presented on the sometimes observed net trans addition of H2 across alkenic and alkynic bonds, and the induced chirality within optically active products, although this is reasonably well understood when dihydride catalyst systems are involved (Section 51.1.3.2,ii).

51.2 ACTIVATION AND ADDITION OF HYDROGEN CYANIDE 51.2.1 Introduction Addition of HCN to carbon-carbon double and triple bonds is analogous in concept to addition of H2, and in the case of catalyzed systems involving transition metal complexes is probably closely related mechanistically. In the absence of transition metal catalysts, HCN does not react with ordinary unactivated alkenes, but polyhalo alkenes and activated systems (C=C—X; X = CO 2 R, CN, COR, CHO, aryl groups etc.) undergo base-catalyzed nucleophilic addition reactions, the so-called 'conjugate hydrocyanation', to give the expected nitrile via a 1,4-addition mechanism (equation 142), although in the case of C = C — C = O , a net 1,2-addition occurs as a result of tautomerization of the initially formed enol. 460 " 462 Bases used include cyanide itself, hydroxides, carbonates and bicarbonates of the alkali metals, triethylamine and tertiary phosphines. Several patents describe such basecatalyzed addition of HCN to dimerization products of acrylonitrile;463464 such chemistry is linked to formation of adiponitrile/l,4-dicyanobutene derivatives which are precursors to amines used in the nylon industry (see below). Similar addition of HCN (sometimes present as acetone cyanohydrin, Me2C(OH)CN) to a,/3-unsaturated ketones has been used to give intermediates useful in syntheses of prostaglandins, 465 steroids, 466 ' 467 and other drugs. 468 (142)

Base-catalyzed cyanide addition to cyclic ce,/3-unsaturated ketones has been shown to involve preferential axial attack to give a mixture of cis and trans products. The same ratio of products was obtained from nucleophilic addition of cyanide to the 7r-allyl palladium derivatives which are readily formed from the ketones {e.g. equation 143); this implies reaction via the parent ketones and no role for the metal in these particular nucleophilic additions.469 COMe I PdCl)2

COMe i CN

COMe I ,CN

CMe

CMe3

CMe3

3

Trialkylaluminium compounds or alkylaluminium halides appear particularly useful as catalysts

354

Addition ofH2 and HCN to C=C and C=C Bonds

for the conjugate addition of HCN; an alternative procedure involves stoichiometric use of a dialkylaluminium cyanide. 461 ' 470472 The mechanisms of the reactions with a,/3-unsaturated ketone substrates, involving nucleophilic attack by a cyanoaluminate anion (R3A1CN~~) or attack by neutral solvated R2AICN species, have been discussed in detail 471 and are outside the scope of this section which emphasizes the homogeneous, transition-metal catalyzed systems with organometallic intermediates. Patents have reported the heterogeneous vapour-phase hydrocyanation of ethylene over supported nickel and palladium. 473475 Base-catalyzed addition of HCN to alkynes has also been reported, and conditions have been found for conversion to alkenic products. Thus, maleonitrile can be formed from cyanoacetylene,476 and acrylonitrile from acetylene itself. Acrylonitrile is important industrially,477 and several patents describe the use of charcoal- and zeolite-supported base catalysts (KCN, NaOH), 478 ~ 480 while a commercial synthesis is based on a homogeneous copper system (Section 51.2.4). Brown has reviewed the addition of HCN to alkenes and dienes catalyzed by transition metal complexes, 481 ' 482 while two recent texts briefly discuss this topic, as well as addition to alkynes. 483 ' 484 Many of the data have appeared in the patent literature and although reasonable organometallic mechanisms can be envisaged, they are not well documented.

51.2.2 Hydrocyanation of Alkenes Arthur and co-workers first used transition metal complexes as hydrocyanation catalysts in the early 1950s,482 butadiene in the presence of [Ni(CO) 4 ], particularly with added PPh3 or AsPh3, being converted at temperatures ^100 °C mainly to pentenenitriles (equation 144). Other systems, including [Fe(CO) 5 ]/diphosphine, [Co 2 (CO) 8 ], [CoH(P(OPh)3}4], [M{P(OR)3}4] (M = Ni, Pd; R = alkyl, aryl), [NijPPl^CHs^PPhih], [Ni(CO) 4 ]/diphosphine, [Ni(CN) 4 ] 4 ~ and copper(I) chloride, also effect this 1:1 addition, sometimes giving 2-methyl-3-butenenitrile also. A wide variety of other activated alkenes (substituted butadienes, styrene, a-nitrostyrene, acrylonitrile, esters of acrylic and methacrylic acids, allene, a,/3-unsaturated ketones, 1-vinylcyclohexene and cyclopentadiene) undergo similar 1:1 additions. 481 ' 482 A table summarizing typical products and yields is given in reference 482 which covers the literature up to 1974-75; about 90% of the reports appear in patent literature. The widely studied nickel systems usually require cocatalysts (see below). More recent patents further describe the use (including catalyst recovery) of [Ni{P(OR) 3 j 4 ] 485 ' 486 and copper(I) salts, 4 8 7 4 8 9 as well as of derivative systems such as [Ni(P(OR) 3 ) 3 (MeCH=CHCH 2 CN)], 490 ' 491 [Ni(CO) 4 ] substituted with a range of Group V donor ligands492 and copper(I) adiponitrile complexes,493 especially for the hydrocyanation of butadiene. The conjugate 1,4-addition (cf. equation 142) usually predominates, although 1,2-addition is favoured in a few cases (e.g equation 145), and is observed with the a,/3-unsaturated carbonyl substrates (see Section 51.2.1). Greater than 90% conversions are noted within some [Ni(CN) 4 ] 4 ~-catalyzed systems,482 although similar yields can be realized with activated alkenes using metal-free base-catalyzed systems (Section 51.2.1). CH2=CH—CH=CH2

^ >

MeCH=CHCH2CN, CH2=CH(CH2)2CN, CH2=CHCH(Me)CN

(144)

° 45) Hydrocyanation of 4-pentenenitrile (cf. equation 144) yields adiponitrile, the precursor of hexamethylenediamine used in the production of nylon 66, and a commercial process has been developed for producing the adiponitrile directly from butadiene and HCN (equations 144 and 145) 483,484,494 j n ^ e conditions used (reportedly modest temperatures, and pressures just high

JHCN MeCH(CN)(CH2)2CN

I HCN NC(CH2)4CN

(146)

Addition ofH2 and HCN to C=C and C=C Bonds

355

high enough to condense unreacted butadiene), a [Ni(P(OR)3}4] catalyst, sometimes in the presence of excess phosphite, isomerizes any initially formed 3-pentenenitrile (and 2-methyl-3butenenitrile) (equation 144) to the required 4-pentenenitrile with good selectivity, prior to addition of the second mole of HCN; Lewis acids (metal salts, boron compounds) and borohydride have been used to promote the nickel-catalyzed reactions. Other complexes effective for formation of adiponitrile from 3-pentenenitrile via the isomerization process include 482 ' 495 ' 496 [Mo(CO) 3 !P(OR) 3 } 3 ] (R = p-tolyl), [W(CO) 3 (P(OPh) 3 } 3 ], [RuCl 2 (PPh 3 ) 3 ], [CoH{P(OR)3j4] (R = Phorp-tolyl), [CoH{P(OPh) 3 } 3 (MeCN)], [RhCl(PPh 3 ) 3 ] and [Pd(P(OPh) 3 ) 4 ], while in situ systems formed by reducing Fe, Co, Ni and Pd salts in the presence of phosphite (or phosphines) have also been used. The Ru(II) and [CoH{P(OR)3)4] systems form substantial amounts of 1,3-dicyanobutane, presumably via direct addition to the 3-pentenenitrile (equation 146). A [Co 2 (CO) 8 ] catalyst gives only the 1,3-dicyanobutane. Patents on the Ni(6) and Co(I) phosphite catalysts have also described conversion of 2methyl-3-butenenitrile to 1,3-dicyanobutane482 (equation 147), and thus it is not clear in the industrial adiponitrile synthesis how the preferred alkene rearrangement to 4-pentenenitrile is controlled; details have not been published. The same catalysts convert 3-butenenitrile to the expected 1,3-dicyanopropane but smaller amounts of 1,2-dicyanopropane are also obtained,482 presumably via addition to 2-butenenitrile that is formed. The regiospecificity and direction of HCN addition to dienes, cyanoalkenes and unactivated alkenes (see below) clearly depend on the nature of the metal catalyst and conditions, but the factors are poorly understood.

CH 2 =CHCH(Me)CN

—•

NC(CH2)2CH(Me)CN

(147)

Ethylene is hydrocyanated to propionitrile in the presence of [Co2(CO)g] or [M{P(OR)3)4] (M = Ni, Pd) complexes; with unsymmetrical higher alkenes the cobalt catalyst (sometimes with added PPh 3 ) gives exclusively the product from Markownikov addition (e.g. propylene gives 2cyanopropane), while the nickel and palladium phosphite systems give product mixtures.482 The [Co2(CO)s] system hydrocyanates terminal alkenes more readily than internal ones, but this does not obtain more generally with the other catalysts. Iron(O) complexes such as [Fe{P(OMe)3}5] and [Fe{P(OMe) 3 }(Ph 2 PCH=CHPPh 2 ) 2 ] are useful for hydrocyanation of ethylene.497 A silica-supported mixed Pd/Pb catalyst has been used to synthesize acrylonitrile from ethylene and HCN, presumably via a hydrocyanation-dehydrogenation process.498 Conversion of a dehydrovalerolactone to 5-cyano-3-pentenoic acid via the hydrocyanationhydrolysis-dehydration sequence shown in equation (148) is catalyzed by bromocuprate(I) species.499

H2

~

* OssS\r^H2CN

~~*

HO2C

CH2CN

(148)

Table 14 lists data reported for hydrocyanation of cycloalkenes.482 The hydrocyanation of norbornene catalyzed by [Pd{P(OPh)3}4] gives the exo product exclusively which is consistent with binding at the metal on the less hindered side of the alkene, although it is not known whether the HCN addition is cis or trans. Similarly, 5-cyanobicyclo[2.2.1]hept-2-ene (exo or endo) (75) with the same catalyst gives products (76) in which the entering cyano group is again exo, while norbornadiene first gives (75) as an exo-endo mixture, prior to slow formation of (76) as a mixture of four products. Only the strained bond of dicyclopentadiene (77) reacts with HCN. 5-Vinylnorbornene (78) gives several products, some resulting from isomerization of the vinyl substituent, but also a tricyclic nitrile (79) whose formation from em/o-5-vinylnorbornene was rationalized via a process initially involving chelate formation through the two alkenic bonds (equation 149).

Addition ofH2 and HCN to C=C and C=C Bonds

356

Table 14 Hydrocyanation of Cycloalkenesa'b Substrate

Q

Ni(0)

0

OcN

<>CN ( A N

/"\-CN

f/^l l^jJ

Y^"\-CN

\(J

'

[Co2(CO)8]; [Co2(CO)6(PR3)2];d Ni(0); Pd(0)

l\^TCN

[( J(75)

Ni(0)

Co,Ni(0)

f^Y l\J

TA

(76)

-j-CN

| ( T )

NC-{< T )O

^ " ^

<

CX^(7?)

NC

iO (78)

Catalyst0

Major product(s)

\^CN

~CXy>

^'C^CN ^O~CN

Co(0);Pd(0) Co

Co(0)

Cu2Cl2;Ni(0)'

Co(0);Pd(0) pd(o)

a Taken from Ref. 482 unless indicated otherwise. b -]-CN indicates products formed by addition of HCN to the double bond in both directions. c M(0) = [M{P(OR)3}4] (M = Ni, Pd; R = alkyl, aryl), usually with excess phosphite; Co = in situ Co2+/reducing agent/P(OR) 3 ; Co(0) = [Co2(CO)8]/PPh3. d R3 = Ph3 or Ph(OPh)2. e R = H, OMe. 1. H. B. Swift and C.-Y. Wu, U.S. Pat. 4 215 068 (1980) (Chem. Abstr., 1980, 93, 238 927). 2. C.-Y. Wu and H. E. Swift, U.S. Pat. 4 151 194 (1979) {Chem. Abstr., 1979, 91, 56 480).

Mechanistic aspects of hydrocyanation are considered further in the next Section. Recent interest in formation of dicyano cyclic hydrocarbons results from their use as intermediates in the production of urethane.

Addition ofH2 and HCN to C=C and C=C Bonds

357

NC M.

/ ^

u™

NC

-HPdCN'

1^ '

'

'

(149^

Use of the chiral [jPd(diop))n] catalyst with norbornene yields the 2-exo-cyanonorbornane with up to 30% e.e. (see Chapter 53). 500 ' 501 Vinyl- and allylsilanes (e.g. CH 2 =CHSi(OEt) 3 and CH 2 =CHCH 2 Si(OEt) 3 ) undergo hydrocyanation via Markownikov and anti-Markownikov addition nonselectively in the presence of [M{P(OPh)3}4] (M = Ni, Pd) catalysts. 482 51.2.3 Mechanism of Alkene Hydrocyanation Hydrocyanation is considered to involve an intermediate metal hydride. Oxidative addition of HCN to give ds-hydridocyano complexes is well-documented for d* and d10 systems:482'502'503 simple addition occurs with trans-[lrC\(CO)(PP\\3)2] and [Ni(PCy 3 ) 2 ], while [Pt(PPh 3 ) w ] (n = 3,4) gives [PtH(CN)(PPh 3 )] 2 ; [RhCl(PPh 3 ) 3 ] gives [RhHCl(CN)(HCN)(PPh 3 ) 2 ] in which a second HCN molecule adds as a coordinated ligand (Rh—N=CH), and the [{IrCl(cod)j2] dimer yields [IrHCl(CN)(cod)]. The addition of HCN to a transient [Rh(CN) 4 ] 3 ~ species has been studied kinetically.504 Of relevance to catalytic hydrocyanation, a wide range of [NiL4] complexes form [NiH(CN)L 3 ] species (equation 150), where L is PR 3 , P(OR) 3 (R = alkyl, aryl) or PPh(OR) 2 (R = alkyl). 505 Addition of HCN to Pd(0) phosphite complexes, the other major class of hydrocyanation catalysts, has not been reported. Whether addition of HCN proceeds via a nonionic and/or ionic mechanism (cf the homolytic and heterolytic splitting of H 2 , Sections 51.1.3.1 ,i and 51.1.3.1 ,ii) remains to be established. Plausible pathways are exemplified in equation (150); the protonation step to give [NiHL 4 ] + is well documented.505 (cf. Table 6b). NiL4

- ^

NiL 3

^ ^

NiH(CN)L 3

H\

(150)

/CN-

NiHL4+

^

NiHL 3

+

Further support for involvement of a hydride is the observed activity of [CoH{P(OPh)3}4] and accompanying alkene isomerization during some hydrocyanations (Section 51.2.2; cf. equation 60). The isomerization of 1-pentene catalyzed by [Ni(dppb)2] in the presence of HCN 5 0 6 is attributed to a detectable five-coordinate [NiH(CN)(dppb) 2 ] species that contains bidentate and monodentate dppb ligands. 505 The formation of alkyl nitriles by the acid-catalyzed decomposition of alkylpentacyanocobaltate(III) complexes also offers insight into potential hydrocyanation mechanisms. The nitrile production occurs via the pathways outlined in equation (151) (R = pyridioalkyls);507 the hydrogen isocyanide ligand (HNC) of the protonated species inserts into the cobalt-alkyl bond, and subsequent treatment with base liberates the nitrile. Of interest, the released tetracyanocobaltate(I) product has been postulated to undergo reversible oxidative addition of HCN to give [CoH(CN)5] 3 ~. 237 The net reaction shown in equation (151) is formally a reductive elimination of RCN from [Co(CN) 5 R] 3 -. NH 3

RCo(CN) 5 RCN

2

^

RCo(CN)4CNH +

Co(CN) 4 3-

+

—• RCCo(CN)42(base)H+

base

«—'

(151)

358

Addition ofH2 and HCN to C=C and C=C Bonds

Scheme 7 illustrates some plausible pathways for catalytic hydrocyanation. Analogies to Schemes 1 and 2 for hydrogenation of alkenes via mono- and di-hydride intermediates are readily apparent. Following addition of HCN (step a), formation of the M(alkyl)CN intermediate via the migratory insertion process (steps b and c), seems realistic. The nitrile product could then be formed by direct reductive elimination (step d), by the protonation-hydrogen isocyanide insertion pathway (steps e-g) or by intermolecular reductive elimination (step h). Precedent for a reaction such as step (d) is seen in the formation of benzonitrile from Ni(II) complexes508 (e.g. equation 152), while the reverse reaction, oxidative addition of alkyl and aryl nitriles, has been reported for several d10 systems including [Ni(PEt3) 4 ]. 502 ' 509 ' 510 The protonation step (e) is written as incorporating cyanide binding, and step (g) can then regenerate MH(CN) directly; following instead the pathways of equation (151) would regenerate the catalyst as M. An intermolecular reaction (step h) with an initial Ni(0) or Pd(0) catalyst (M) could generate [HM—MCN] as shown (and Pd(I) dimers are known 51! ), or 2M (following loss of HCN), or even MH and MCN; several processes to regenerate the active MH(CN) species could be envisaged. Almost certainly, as in hydrogenation, different metal catalysts will utilize different reaction pathways. Other possibilities are considered below. Several of the [NiH(CN)L 3 ] complexes mentioned above dissociate a ligand L to give coordinatively unsaturated species that could readily accept coordination from an alkene (or diene). 123 ' 505 A limitation using [Co2(CO)8] as catalyst is that large quantities of complex are required because HCN reacts with the carbonyl to give less effective cyanocarbonyl catalysts,482 while inactive Ni(II) and Pd(II) cyanide complexes are formed when using the zero-valent tetrakis(phosphite) catalyst systems, possibly via the processes outlined in equation (153) (cf. equation 27) and/or equation (154). Excess phosphite is commonly used to suppress the deactivation processes, which will require ligand dissociation prior to oxidative addition of HCN, but this slows down the rate of hydrocyanation.482 The excess ligand may also promote step (d) via a reaction such as (152).

M

+

HCN

4^

MH(CN)

-0>) ^SXalkene

V ^

M(alkyl)CN

4^

•

(h)JMH(CN)

(eN

alkyl CN

M(alkyl)(CNH)CN ,

HM—MCN

(f

\ () V

MH(alkene)CN

\HCN

11

-

(NC)M-C-alkyl NH

y

^

MH(CN) Scheme 7 Possible pathways for catalytic hydrocyanation of alkenes

PhNi(CN)(PCy3)2 M

^ t

MH(CN)

M(CN)R

^ ^

^

P( Et)3

°

>

PhCN

MH2(CN)2 MH(CN)2R

—•

+

Ni(0) complex

—• M(CN)2 M(CN)2 +

+ RH

(152) H2

(153) (154)

The role of the Lewis acid promoters remains unclear; in contrast to a substantiated role of Lewis acids such as aluminium alkyls in some catalytic hydrogenation systems (Section 51.1.4.2), it is clearly not that of abstraction of a ligand like phosphite. The hydrocyanation of 1 -hexene catalyzed by NiL 4 (L = tri-p-tolylphosphite) in acetonitrile has been reported to increase with added ZnCl2 up to a 1:1 Zn/Ni ratio. 512 'Abstraction' of cyanide was considered to enhance the acidity of HCN and lead to more effective protonation of NiL 4 (equation 155); an alternative viewpoint is that cyanide abstraction occurs from [NiH(CN)Ls] (see above). In contrast to the pathways of Scheme 7, a mechanism involving cationic Ni(II) intermediates was postulated

Addition ofH2 and HCN to C=C and C=C Bonds

359

(equation 156, Ni = NiLs). An accompanying isomerization to 2-hexene is readily accounted for via the [NiCH(Me)R] + alkyl (equation 60), while the ratio of mainly linear to branched product depends on the direction of hydrometallation across the double bond, and the rate of what is formally nucleophilic cleavage of the nickel-carbon bond by cyanide. Use of other solvents (toluene, p-cresol) and Lewis acids (AICI3, TiCb) changed the product ratio. Catalyst deactivation was attributed to a reaction akin to equation (154). This report 512 is the only one in the open literature to present some quantitative kinetic data on catalytic hydrocyanation. NiL4 NiH +

+ +

HCN

+

RCH=CH 2

NiHL 3 +

ZnCl2 ^ =^

RCH2CH2Ni+

+

+

ZnCl2CN-

(+

L

\ZnCl2CN-

Ni

(155)

RCH(Me)Ni+

/ZnCl 2 CN-

RCH2CH2CN

+

+

ZnCl2)

(156)

RCH(Me)CN

51.2.4 Hydrocyanation of Alkynes Up to the end of the 1960s, a major industrial synthesis for acrylonitrile (CH2=CHCN) was the copper(I)-catalyzed addition of HCN to acetylene, but it has since been replaced by the Sohio olefin-based process, the cooxidation of propylene and ammonia. 477 ' 484 ' 513 The hydrocyanation is affected by passing gaseous C2H2 and HCN into an aqueous solution of copper(I) and ammonium chlorides at 80-90 °C and about 1 atm pressure. Major by-products are acetaldehyde (from addition of H 2 O) and vinylacetylene (from dimerization). Interest in the addition of HCN to acetylene continues, particularly by Russian workers. 514518 The mechanism outlined in equation (157) is typical of that for catalyzed addition of HX (X = OH, Cl, CN, OCOMe) to alkynes, involving initial 7r-complex formation which activates the triple bond toward nucleophilic attack; the resulting c-vinyl complex is then protonated to yield the vinyl product (with X = OH, the vinyl alcohol tautomerizes to give acetaldehyde).484 Copper(I) readily coordinates up to four cyanide ligands but in the chloride media, the exact nature of the catalyst is uncertain. Nevertheless, the X~(CN~) of equation (157) could thus be coordinated at the metal, in which case the intermediate vinyl complex would be of cis configuration resulting from an intramolecular metal cyanide addition across the triple bond, and generally this has been favoured in the hydrocyanation reactions. Whether the X~ is coordinated or free has led to a continuing controversy in the related oxypalladation of alkenic substrates (addition of Pd—OH).519"521 Cleavage of the Cu—C bond in the cyanovinyl complex is thought to require a strong acid like HC1. H

H

M+HII^Xc

—•

X

Si 7cx

H

-^

X

ft

+

M+

(157)

7cx

Successive hydrocyanation and hydrogenation of alkynes to yield secondary nitriles has been accomplished using [Co(CN) 5 ] 3 ~ in aqueous solution under H 2 , or [Ni(CN) 4 ] 2 " with borohydride or zinc. 522 ' 523 The conditions are typically those for hydrogenation with these catalyst systems (Section 51.1.3.2,i); HCN is not used directly. The mechanisms, outlined in equation (158) for the cobalt system, invoke hydrocyanation corresponding to steps b, c and d of Scheme 7, followed by a hydrogenation pathway corresponding to that of steps a, c and f in Scheme 1 for monohydride systems. The cobalt systems, used with alkynic alcohols (R = EtC(Me)OH, Me2C(OH), 1hydroxycyclohexyl), are stoichiometric with respect to cobalt because addition of excess alkyne ties up the cobalt as an alkyne complex leaving no [CoH(CN) 5 ] 3 ~ reagent available, and there

360

Addition ofH2 and HCN to C=C and C=C Bonds

is some accompanying hydrogenation to give the alkenes, R C H = C H 2 (equation 158). Phenylacetylene was hydrocyanated only in the presence of amine (en, dipy), which gives more active [CoH(CN)3(amine)]~ species (see Table 3), but a competing hydrogenation to give PhEt was then the major reaction. Evidence for the cobalt(I) alkenic intermediate in the reaction outlined in equation (159) 524 shows a plausible pathway for the step involving net reductive elimination of unsaturated nitrile in equation (158). The nickel systems, which involve the intermediate [NiH(CN) 3 ] 2 ~ hydride for hydrocyanation and probably [Ni 2 (CN) 6 ] 4 ~ for hydrogenation (see page 315), are catalytic in the presence of excess cyanide for several R C = C H substrates (R = Ph, Ph(CH 2 ) 2 , 1-hydroxycyclohexyl) and the internal alkyne PhC=CEt, which gives equimolar amounts of PhCH(CN)CH 2 Et and PhCH 2 CH(CN)Et. RO=CH

CoH(CN)53

">

CoP^ RCH=CH 2

Co—CR=CH2

RC(CN)=CH 2

CN

H

RCH(CN)Me rPhCH==CHCN"|3~

Co(CN)5-CH=CHPh3- - * [

—•

^J

^ -

(158)

Co-CR(CN)-Me

rN _

^

PhCH=CHCN

+

Co(CN)54-

(159)

51.2.5 Concluding Remarks Transition metal complex catalyzed hydrocyanation of carbon-carbon double and triple bonds has been used commercially for production of adiponitrile and acrylonitrile, respectively, both of which are of wide application in the synthetic fibre, rubber and plastics industries,477'525 while acrylonitrile is of importance also in the pharmaceutical and dye sectors.477 The processes are largely based on well known reactions of the nitrile group; hydrogenation to amine and hydrolysis to amide. There appears from the patent literature to be a continuing interest in hydrocyanation of alkenes and cyanoalkenes to give saturated nitriles and dinitriles, respectively, which are key industrial precursors. The recently discovered catalyzed hydrocyanation-hydrogenation reaction for alkynes offers an alternative route to saturated nitriles. The hydrocyanation reactions are generally poorly understood, especially for homogeneously catalyzed systems which are usually amenable to quite detailed studies (Section 51.1.1). This conclusion, however, is based on studies reported in the nonpatent literature of which there is remarkably little. The mechanisms postulated, at least for alkene hydrocyanation, are largely based on analogy with other well-studied homogeneously catalyzed processes, particularly hydrogenation and hydrosilation (Chapter 48). The possibilities outlined or alluded to in Scheme 7 and equations (151)-(156) probably offer greater diversity than is evident for hydrogenations using H 2 catalyzed by monohydride species. For example, a protonation reaction such as equation (155) cannot be written for H 2 , at least at a mononuclear metal site. There is considerable scope for further mechanistic studies, particularly since a less expensive Group VIII metal (nickel) is effective. The area of asymmetric hydrocyanation, followed by hydrogenation of the nitrile group, offers a potential route to optically active amines (see Chapter 53). REFERENCES 1. 'Metal Ion Activation of Dioxygen', T. G. Spiro, ed. Wiley, New York, 1980. 2. 3. 4. 5. 6. 7. 8.

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