Metal Catalysis in Organic Chemistry

Metal Catalysis in Organic Chemistry

Chapter Ί Metal Catalysis in Organic Chemistry Inorganic complexes arefindingincreasing use in organic chemistry, both as reagents and as catalysts f...

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Chapter Ί Metal Catalysis in Organic Chemistry

Inorganic complexes arefindingincreasing use in organic chemistry, both as reagents and as catalysts for carrying out a variety of syntheses.14 In a number of cases, metal catalysis is uniquely suited for effecting reactions which are not otherwise possible, including such conceptually simple transformations as: CH 2 =CH 2

+ HOAc + o 2

(i) 5 - 7

CH2=CH-OAc + H 2 0

(2) 8 · 9

CH3OH + CO

H CM

3 \jCy c"3 + ° 2 2 CH,C=CH

CH 2 =CHCH 3 + NH 3 + 0 2

" H O * c ^O^ c °: H + H^° P)1 CH,

(4)11.12 (5)13.14

■ CH,=CHCN + H , 0

(6)15,

3 HC=CH

<] + r

COjEt

2 CH 3 CH=CH 2

/

^.C02Et

(7)1

—>^y — ► CH 2 =CH 2 + CH 3 CH=CHCH 3

+ HOAc

O + 2H O

16

+

(8)18a'

(9) 19

b

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1, Metal Catalysis in Organic Chemistry

Metal catalysis is important in industrial chemistry20 since it allows for high selectivity and economic efficiency in such processes as: Hydrogenation 2H 2 + HOCH2C=CCH2OH

!— H O ^ \ ^ \ ^ O H

(10)2123

Polymerization n CH 3 CH=CH 2 - ^miU

CH3

(ii) 2 4 ' 2 5

r _ 2 -CH-i' n -LcH

Oxychlorination CH 2 =CH 2 + HC1 + 0 2 JSiüL, CH2=CHC1 + H 2 0

(12) 2 6

Hydroformylation CH 3 CH=CH 2 + CO + H2

[Rh]

> CH3CH2CH2CHO

(13) 2 7 '

28

Oxidation

+ 0 2 -£±+

(14)29

< ^ - O H + Cy=0

(15) 3 0

CH 2 =CH 2 + 0 2 - 2 5 3 - , CH3CHO Oligomerization

^ CCO

<">

Epoxidation CH 3 CH=CH 2 + PhCHCH3 OOH

J^L

CH 3 CH-CH 2 + PhCHCH3 O

<γη\?>2, 33

OH

Hydrocyanation -CN

(18)34a'b

Despite the large number and variety of important catalytic processes extant, many of the major processes are understood only in general outline, and others are hardly understood at all. This situation is a natural consequence of the difficulty of studying catalytic reactions in which the steady state concentrations of the reactive intermediates are perforce low. To promote further developments in this field, a mechanistic understanding of the chemical interactions between the metal complex and the organic substrate

1. Metal Catalysis in Organic Chemistry

3

is desirable and important. Organometals, in which there is bonding between metal and a carbon-centered ligand, play key roles as reactive intermediates in a number of these systems. However, there is surprisingly little that is quantitatively known about how these organometal intermediates are formed and how they undergo further reaction. There are two principal driving forces to consider in the reactions of inorganic complexes: ligand coordination and oxidation-reduction of the metal center. Although these factors are not necessarily mutually exclusive properties,35-37 we will consider them largely as separate. Collman38 has pointed out that a vacant coordination site is a most important property of a catalyst, for it allows the substrate to be brought close to the metal. Retardation and inhibition as well as the requirement of thermal and photochemical activation can often be traced to the necessity of expelling a ligand to generate an active catalyst.39 The optimum coordination number in a transition metal complex with dn configuration is (18 — n)/2. For example, in those metal centers with d6, d8, and d10 spin-paired configurations, full saturation in a metal complex is characterized by 6-, 5-, and 4-coordination, respectively. Coordinative unsaturation is most commonly effected in a metal complex by either loss of a ligand, e.g. 40 ^ 1 PdCl42~ : PdCl3- +CH 2 =CH 2 ;

PdCl3- + C P

(19)

PdCl3(CH2=CH2)- - ^ — CH3CHO

(20)

42 43

dissociation of a bridged dinuclear species, e.g. '

"RiOih4 = " Cl

Cl

/Ci - R l K c l + CH 2 =CH 2 ; = ;

2

(21)

(~R
h T^a \-RKa

etc.

^ ^ > - ^ ^ ^

,„v (22)

or π-σ rearrangement of polyhapto ligands, e.g.44,45 /H -N..L \\

Ni_

= ±

+CH 2 =CH,

Alternatively, oxidative addition can also lead to coordinative unsaturation, e.g.46 CpRh

CN

+ H+

►CpRh^^

(25)

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1. Metal Catalysis in Organic Chemistry

Addition of electrophiles in this manner is tantamount to an overall twoequivalent oxidation, since the metal changes formal oxidation state from Rh(I) to Rh(III). A one-equivalent oxidation may also be effected by electrophiles, e.g.47 CpMn(CO) 2 L + I 2

► CpMn(CO) 2 L + + I · + I"

(26)

in which the metal changes formal oxidation state from Mn(I) to Mn(II). Starting with diamagnetic precursors, one-equivalent oxidation or reduction leads to paramagnetic intermediates which are often metastable. One-equivalent and two-equivalent changes constitute the two basic classes of mechanisms by which reaction pathways can be formulated. Heretofore, most of the mechanistic considerations of organometallic and metal-catalyzed organic reactions have neglected or underemphasized the possible role of paramagnetic species as reactive intermediates. 4853 This notion is firmly embodied in the formulation of the 16- and 18-electron rule,54 which precludes such species as viable intermediates as applied to organometals in homogeneous catalysis. To quote Tolman54 The basic premise ... is that 16- and 18-electron configurations are readily accessible to diamagnetic transition metal complexes. Species with other configurations or reactions by other paths will generally be so energetically unfavorable by comparison that they are negligible.

who further states The 16- and 18-electron rule in organometallic chemistry is consistent with such a large body of experimental evidence, including studies on reaction mechanisms, that anyone proposing an exceptional compound or reaction path must bear the burden of proof.

The contrary point of view is that catalytic reactions like other chain processes often depend on labile and unusually reactive intermediates, including paramagnetic species, for facile reaction. The principal objective of this book is to discuss how various reactive intermediates including paramagnetic species, both organic (alkyl) and organometallic free radicals, are involved in a variety of metal-catalyzed organic reactions as well as in the reactions of organometals. A focus on paramagnetic species stems in part from a consideration of the kinetic chain character of both metal-catalyzed and a variety of homolytic reactions in which the course of reaction can be dissected into three distinct phases: initiation, propagation, and termination. The facility of the propagation sequence is related to the turnover number or the kinetic chain length of the process. It is largely responsible for the rates of the overall transformation, and consequently each step must occur rapidly for the cycle to be efficient. Paramagnetic species, being reactive, possess the requisite properties for such facile reactions. Indeed, Zahradnik and Beran55 have pointed out the

1. Metal Catalysis in Organic Chemistry

5

interesting comparison between metal-catalyzed reactions and the reactions of paramagnetic species and photochemically excited states, in which the changes in symmetry of the frontier orbital as well as the multiplicity of the state can lead to the facilitation of reactions. The increase in reactivity of closed shell molecules accompanying their change to ion-radicals or excited states on charge transfer interaction with the catalyst was treated by secondorder perturbation theory and related to the increase in the number of low-lying excited states. The subject of metal catalysis in organic reactions and the attendant chemistry of organometal intermediates is considered in this book from three separate but interrelated perspectives. Part One: Various oxidation-reduction processes are emphasized in which the metal undergoes discrete changes in oxidation states. Paramagnetic species, including organic and organometal radicals, are important intermediates in such transformations. Part Two: The cleavage of organometals involving both concerted (twoequivalent) and homolytic (one-equivalent) pathways are delineated. These basic transformations represent key steps in various catalytic processes available for the formation of carbon-carbon bonds. Part Three: Organometals are considered as donors in many electron transfer and charge transfer processes, resulting in the labilization of the alkyl-metal bonds in radical-ion intermediates. The variety of reactions of organometals known to occur with electron acceptors such as organic halides, carbonyl compounds, oxygen, and peroxides, and electrophiles such as molecular halogen, mercury(II), and metal ions are then considered within the context of charge transfer interactions. These three parts are not mutually exclusive, but represent different perspectives on organometallic mechanisms. This multifaceted view stems from the recognition that reaction mechanisms are merely mental constructs. As such, mechanistic schemes depend largely on the preconceived views held by the formulator. To minimize any restrictive view of this important area of chemistry, the separate focus into oxidation-reduction, organometals, and charge transfer allows the subjects to be examined more critically, and with less bias. For example, the chemistry of carbon monoxide and metal carbony Is is treated from the standpoint of redox processes in Chapters 6 and 8, and reexamined in the formation of carbon-carbon bonds in Chapter 14; then the migratory insertion is considered in Chapter 12, Section IV as a concerted process and in Chapter 18 as an electrophilic interaction. Such a format allows organometallic reactions and catalytic processes to be classified according to mechanistic type, rather than by organic functional

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/. Metal Catalysis in Organic Chemistry

group or by metal, as has been commonly done in the past. Some strange, unexpected bedfellows result, and hopefully this organization will encourage, not impede, future developments in organometallic chemistry and catalysis. REFERENCES 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.

A. P. Kozikowski and H. F. Wetter, Synthesis p. 561 (1976). J. Weill-Raynal, Synthesis p. 633 (1976). P. J. Smith, Chem. Ind. (London) p. 1025 (1976). J. Tsuji, "Organic Synthesis by Means of Transition Metal Complexes." Springer-Verlag, Berlin and New York, 1975. I. I. Moiseev, M. N. Vargaftik, and Ya. K. Syrkin, Akad. Nauk SSSR, Dokl. Chem. 133, 801 (1960). Compare J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Ruttinger, and H. Kojer, Angew. Chem. 71, 176 (1959). P. M. Henry, Adv. Organomet. Chem. 13, 363 (1975). J. F. Roth, J. H. Craddock, A. Hershman, and F. E. Paulik, Chemtech 1, 600 (1971). D. Forster, Ann. N.Y. Acad. Sei. 295, 79 (1977). See R. A. Sheldon and J. K. Kochi, Adv. Catal. 25, 272 (1976). E. Vedejs and P. D. Weeks, Tetrahedron Lett. p. 3207 (1974). K. Takagi, N. Hayama, T. Okamoto, Y. Sakakibara, and S. Oka, Bull. Chem. Soc. Jpn. 50, 2741 (1977). J. L. Callahan, R. K. Grasselli, E. C. Milberger, and H. A. Strecker, Ind. Eng. Chem., Prod. Res. Dev. 9, 134 (1970). S. P. Lankhuyzen, P. M. Florack, and H. S. van der Baan, J. Catal. 42, 20 (1976). P. M. Maitlis, Ace. Chem. Res. 9, 93 (1976). K. P. C. Vollhardt, Ace. Chem. Res. 10, 1 (1977). R. Noyori, T. Odagi, and H. Takaya, J. Am. Chem. Soc. 92, 5780 (1970). (a) R. J. Haines and G. J. Leigh, Chem. Soc. Rev. 4, 155 (1975). (b) N. Calderon, E. A. Ofstead, and W. A. Judy, Angew. Chem. Int. Ed. Engl. 15, 401 (1976). E. I. Heiba, R. M. Dessau, and P. G. Rodewald, J. Am. Chem. Soc. 96, 7977 (1974). G. W. Parshall, J. Mol Catal. 4, 243 (1978). P. N. Rylander, "Catalytic Hydrogenation Over Platinum Metals." Academic Press, New York, 1967. J. Kwiatek, in "Transition Metals in Homogeneous Catalysis" (G. N. Schrauzer, ed.), p. 13. Dekker, New York, 1971. B. R. James, " Homogeneous Hydrogenation." Wiley (Interscience), New York, 1973. J. C. W. Chien, ed., "Coordination Polymerization." Academic Press, New York, 1975. P. J. T. Tait, Chemtech 5, 688 (1975). L. Friend, L. Wender, and J. C. Yarze, Adv. Chem. Ser. 70, 168 (1968). D. Evans, G. Yagupsky, and G. Wilkinson, J. Chem. Soc. A p. 2660 (1968). R. Fowler, H. Connor, and R. A. Baehl, Chemtech. 6, 772 (1976). S. A. Miller, Chem. Process Eng. 50, 63 (1969). R. Jira, W. Blau, and D. Grimm, Hydrocarbon Process. 55, 97 (1975). G. Wilke, Angew. Chem., Int. Ed. Engl. 2, 105 (1963). N. Indictor and W. F. Brill, J. Org. Chem. 30, 2074 (1965). M. N. Sheng and J. G. Zajacek, Adv. Chem. Ser. 76, 418 (1968). (a) Y.-T. Chia and W. C. Drinkard, U.S. Patent 3,766,237 (1973); see ref. 20.

Additional Reading

35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55.

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(b) E. S. Brown, Aspects Homogeneous Catal. 2, 57 (1974). R. S. Nyholm, Proc. Chem. Soc, London p. 273 (1961). C. K. Jorgensen, Struct. Bonding (Berlin) 1, 234 (1966). C. K. Jorgensen, Struct. Bonding (Berlin) 3, 106 (1967). J. P. Collman, Ace. Chem. Res. 1, 136 (1968). J. P. Collman, Trans. N.Y. Acad. Sei. 30, 479 (1968). P. M. Henry, J. Am. Chem. Soc. 86, 3246 (1964). I. I. Moiseev, O. G. Levanda, and M. N. Vargaftik, J. Am. Chem. Soc. 96, 1003 (1974). R. Cramer, J. Am. Chem. Soc. 89, 1633 (1967). A. C. L. Su and J. W. Collette, J. Organomet. Chem. 90, 227 (1975). P. W. Jolly and G. Wilke, "Organic Chemistry of Nickel," Vol. 2, p. 18. Academic Press, New York, 1975. See also M. Tsutsui and A. Courtney, Adv. Organomet. Chem. 16, 241 (1977). L. P. Seiwell, Inorg. Chem. 15, 2560 (1976). N. G. Connelly and M. D. Kitchen, J. Chem. Soc, Dalton Trans, p. 931 (1977). R. F. Heck, "Organotransition Metal Chemistry, A Mechanistic Approach." Academic Press, New York, 1974. D. S. Matteson, "Organometalhc Reaction Mechanisms." Academic Press, New York, 1974. B. L. Shaw and N. I. Tucker, in " Comprehensive Inorganic Chemistry " (A. F. TrotmanDickenson, ed.), Vol. 4, Ch. 53. Pergamon, Oxford, 1973. G. N. Schrauzer, ed., "Transition Metals in Homogeneous Catalysis." Dekker, New York, 1971. A. J. Deeming, Int. Rev. Sei., Inorg. Chem. Ser. Two 9, 271 (1974). G. Henrici-Olive and S. Olive, " Coordination and Catalysis." Verlag Chemie, Weinheim, 1977. C. A. Tolman, Chem. Soc. Rev. 1, 337 (1972). R. Zahradnik and S. Beran, J. Catal. 44, 107 (1976).

ADDITIONAL READING G. W. Parshall, Industrial applications of homogeneous catalysis. J. Mol. Catal. 4, 243 (1978). R. F. Heck, " Organotransition Metal Chemistry, A Mechanistic Approach." Academic Press, New York, 1974. A. P. Kozikowski and H. F. Wetter, Transition metals in organic synthesis. Synthesis p. 561 (1976). M. M. Taqui Khan and A. E. Martell, " Homogeneous Catalysis by Metal Complexes. Vol. 1: Activation of Small Inorganic Molecules."; Vol. 2: "Activation of Alkenes and Alkynes." Academic Press, New York, 1974. I. Wender and P. Pino, eds., "Organic Synthesis via Metal Carbonyls." Vol. 1, Wiley (Interscience), New York, 1968; Vol. 2, 1977. P. Wiseman, "An Introduction to Industrial Organic Chemistry." Wiley (Interscience), New York, 1972. H. Alper, ed., " Transition Metal Organics in Organic Synthesis." Academic Press, New York, 1976. D. Seyferth, ed., "New Applications of Organometallic Reagents in Organic Synthesis." Elsevier, New York, 1977. M. Tsutsui and R. Ugo, eds., " Fundamental Research in Homogeneous Catalysis." Plenum, New York, 1977.

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/. Metal Catalysis in Organic Chemistry

D. Forster and J. F. Roth, eds., Homogeneous catalysis, Adv. Chem. Ser. 132 (1974). P. N. Rylander and H. Greenfield, eds., "Catalysis in Organic Synthesis." Academic Press, New York, 1976. R. P. Hanzlik, "Inorganic Aspects of Biological and Organic Chemistry." Academic Press, New York, 1976. D. W. Slocum, ed., The place of transition metals in organic synthesis. Ann. N.Y. Acad. Sei. 295 (1977).