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Cationic iridium complexes with C2-symmetry binaphthalene-core disulfide ligands
Inorganica Chimica Acta 357 (2004) 2957–2964 www.elsevier.com/locate/ica
Cationic iridium complexes with C2-symmetry binaphthalene-core disulfide ligands Synthesis and catalytic activity in the hydrogenation of alkenes Montserrat Dieguez a,*, Aurora Ruiz a, Carmen Claver a, Franco Doro b, Maria G. Sanna b, Serafino Gladiali b,* a
Departament de Quımica Fısica i Inorganica, Universitat Rovira i Virgili, Pl. Imperial Tarraco 1, 43005 Tarragona, Spain b Dipartimento di Chimica, Universita di Sassari, Via Vienna 2, 07100 Sassari, Italy Received 20 January 2004; accepted 27 March 2004 Available online 12 May 2004
Abstract Mononuclear cationic Ir(I)-cyclooctadiene complexes containing three different C2 -symmetrical binaphthalene-templated sulfide ligands, featuring alkyl groups of increasing steric demand onto the donor centres, have been prepared and characterized. Variable temperature NMR spectra provide evidence that, regardless of the bulk of the alkyl substituent on the sulfur, the chelate coordination of the ligands proceeds in all cases with complete stereoselectivity at the newly generated S-stereocentres affording just one stereoisomer. This species features a seven-membered chelate ring in a frozen conformation where the diequatorial S-alkyl substituents are disposed in anti-relationship and the stereogenic S-donors display the same configuration. The oxidative addition of hydrogen to these complexes proceeds smoothly affording in every case one single cis-dihydride complex whose structure in one case has been cleared by correlated NMR spectra. The cationic complexes derived from these ligands are catalysts of modest value for the hydrogenation of a; b-unsaturated acid derivatives where they produce nearly racemic products in moderate yields. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Ir complexes; Chiral S-ligands; Stereoselectivity; Catalytic hydrogenation
1. Introduction Chiral ligands with sulfur donors are exceedingly underutilized in asymmetric catalysis in comparison with the corresponding counterparts containing P- or even Ndonor centres. Actually sulfur is a donor weaker than both phosphorus and nitrogen towards soft and hard metal ions, respectively, and is comparably more prone to labilization than these two donors. Probably in consequence of this behaviour, for a long time sulfur ligands have been considered poorly suited for supporting efficiently the stereorecognitive process necessary for the transfer of the chiral information from the metal to the * Corresponding authors. Tel.: +34-977-558046; fax: +34-977-559563 (M. Dieguez), Tel.: +39-79-229546; fax: + 39-79-229559 (S. Gladiali). E-mail addresses: [email protected] (M. Dieguez), [email protected] (S. Gladiali).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.03.039
substrate to proceed. In recent times, however, the utilization of sulfur donor ligands as chiral modifiers in transition metal catalyzed enantioselective reactions is attracting increased attention and the library of chiral ligands with S-donors has been growing steadily [1]. During the last 10 years, either jointly or separately, we have provided several contributions to this topic of asymmetric catalysis. In particular, our collaborative research efforts have been centred on the synthesis and applications of axially chiral bidentate ligands containing two homotopic or heterotopic sulfur donor centres featuring the same or different substituents. These li0 gands are basically derived from 1,1 -binaphthalene or similar atropisomeric diaryl-core scaffolds and allow for the chelate coordination to the metal through two neutral or anionic sulfur donors. C2 -symmetry ligands 1 with two equivalent S-donors have been prepared from the corresponding binaphthyl
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or biphenanthryl diols using in the key step the Newmann–Kwart rearrangement to generate the new carbon–sulfur bonds [2]. The relevant cationic mononuclear or neutral dinuclear Rh(I)-complexes have been characterized and screened as chiral catalysts in the asymmetric hydroformylation of styrene. A good catalytic activity and a high selectivity towards the branched aldehyde was observed with the mononuclear complexes, but the stereoselectivity was poor and the enantiomeric excess (e.e.) of the chiral product did not exceed 20% [3]. E.e.s even lower were recorded in the Pd-catalyzed asymmetric hydrocarboxylation of styrene [4] in the presence of similar C1 -symmetry thiolate-thioether binaphthalene ligands [5]. The modest outcomes in asymmetric catalysis of the above reported ligands are contrasted by more recent results recorded in the asymmetric Ir(I)-catalyzed hydrogenation of olefins [6] and in the Pd(II)-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate [7], where stereoselectivities as high as 68% and 81% e.e., respectively, have been obtained using C2 -symmetry thioethers as chiral inducers. These results prompted us to investigate the potential of our C2 -symmetry binaphthalene-templated sulfides in these processes. Here, we report some aspects of the coordination chemistry of the binaphthyl sulfide ligands 1–3 towards iridium and on the catalytic behaviour of the relevant complexes in the hydrogenation of a; bunsaturated acid derivatives. 2. Experimental 2.1. General methods All iridium complexes were synthesized using standard Schlenk techniques under a nitrogen atmosphere. The complex [Ir(cod)2 ]BF4 was prepared using methods described in the literature [8]. Ligands 1 and 2 were prepared as reported [3,9]. Solvents were dried over standard drying agents and were freshly distilled and deoxygenated prior to use. All other reagents were used as commercially supplied. Elemental analyses were performed on a Carlo Erba EA-1108 instrument. The 1 H and 13 C–{1 H} NMR
Fig. 1. Dithioethers ligands 1–3.
spectra were recorded on a Varian Gemini 300 and 400 MHz spectrometers. Chemical shifts are relative to SiMe4 (1 H and 13 C) as internal standard. All assignments in NMR spectra were determined by means of COSY and 13 C–1 H correlation experiments. Standard pulse sequences were employed for 1 H 2-D NOESY [10]. The phase-sensitive NOESY experiments used mixing times of 0.4 s. Proton T1 studies were performed using the standard inversion recovery 180°–s)90° pulse sequence method [11]. Infrared spectra were recorded on a Bruker EQUINOX 55 spectrophotometer. A VGAutospect was used for FAB mass spectral analyses. The matrix was m-nitrobenzyl alcohol. The reaction under 1 atm of H2 was performed in a previously described hydrogen vacuum line [12]. Gas chromatography analyses were performed in a Hewlett-Packard Model 5890A instrument. Enantiomeric excesses (e.e.) were measured using a fused silica capillary column 25 0.25 mm Permabond L-Chirasil-Val for the determination of e.e. for substrate 12, and in Chiraldex-GTA (30 m 0.25 mm) for substrates 10 and 11. 2.2. Preparation of disulfide ligand 3 A solution of (R)-binaphthyl dithiol (1 g, 3.1 mmol) and t-butanol (1.2 ml, 16 mmol) in AcOH (15 ml) containing acetic anhydride (2.15 ml) was stirred at 0 °C for 20 min under inert atmosphere. 70% Aqueous HClO4 (0.5 ml) was then added and stirring was continued at room temperature for 15 h. The solution was poured into water, the resulting solid was filtered out, washed with water and crystallized from diethyl ether–methanol: 0.82 g, 70%; m.p. 158–160 °C; 1 H NMR (d, CHCl3 ): 1.17 (18H, s); 7.0 (2H, d); 7.19 (2H, m); 7.43 (2H, m); 7.90 (6H, m). ½a25 D : )299.6 (c ¼ 0:5; CHCl3 ). Anal. Calc. for C28 H30 S2 : C, 78.09; H, 7.02; S, 14.89. Found: C, 78.25; H, 7.15; S, 14.62%. 2.3. Preparation of iridium complexes 2.3.1. [Ir(cod)(1)]BF4 (4) The compound was prepared by adding an excess of ligand (1) (38.1 mg, 0.11 mmol) to a dichloromethane (5 ml) solution of [Ir(cod)2 ]BF4 (49.7 mg, 0.1 mmol), which produces an immediate change from brown–red to yellow. After 30 min, the solvent was evaporated and the yellow solid cleaned twice with cold diethyl ether and vacuum dried (48 mg, 81%). FABþ : 647 m=z (M–BF4 ). Anal. Calc. for C30 H30 IrS2 BF4 : C, 49.30; H, 4.13; S, 8.73. Found: C, 48.30; H, 4.41; S, 8.61%. NMR details are reported in Table 1. 2.3.2. Synthesis of complexes [Ir(cod)(2)]BF4 (5) and [Ir(cod)(3)]BF4 (6). General procedure A solution of the dithioether ligand (0.08 mmol) in 3 ml dichloromethane was added to a solution of
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Table 1 The NMR spectroscopic data for complexes 4–6a
Position
4 1
3 4 5 6 7 8 S–Me Me S–CH S–C CH2 CH@
H
7.90(d, J ¼ 9.1) 8.23(d, J ¼ 9.1) 8.02(d, J ¼ 8.4) 7.59(t, J ¼ 7.2) 7.37(t, J ¼ 7.2) 7.03(d, J ¼ 8.8) 2.62(s)
1.80(m), 2.20(m) 2.40(m) 3.85(m), 4.65(m)
5 13
C
128.7 131.4 128.2 127.9 126.0 123.2 15.0
31.0, 32.0 73.0, 79.0
1
6 13
H
C
1
13
H
7.85(d, J ¼ 9.8) 8.25(d, J ¼ 9.8) 8.10(d, J ¼ 9.8) 7.65(t, J ¼ 6.5) 7.40(t, J ¼ 6.5) 7.10(d, J ¼ 11.7)
129.5 131.4 129.4 128.9 128.5 127.3
7.95 8.17 8.08 7.64 7.40 7.15
1.18(d, J ¼ 5.2), 1.21(d, J ¼ 5.2) 3.30(sp, J ¼ 5.2)
22.8, 23.2
1.1 (s)
1.65(m), 1.90(m), 2.15(m), 2.40(m) 4.10(m), 4.85(m)
(d,J ¼ 8.4) (d,J ¼ 8.4) (d,J ¼ 8.4) (t,J ¼ 7.6) (t,J ¼ 7.6) (d,J ¼ 8.4)
C
128.3 130.7 130.1 127.8 127.6 126.7 31.8
42.9 29.8, 33.7 72.2, 78.1
1.40(m), 1.60(m), 2.15(m), 2.45(m) 4.60 (m), 5.40(m)
60.0 27.4, 34.5 68.0, 74.6
a
Chemical shifts in ppm; coupling constants in Hz; room temperature; 1 H and 13 C NMR in CDCl3 . Abbreviations: s, singlet; d, doublet; t, triplet; sp, septuplet; m, multiplet.
[Ir(cod)2 ]BF4 (40 mg, 0.08 mmol) in 5 ml dichloromethane under nitrogen. The solution was stirred for 30 min and then hydrogen was bubbled inside the solution for 30 min. Subsequent addition of diethyl ether precipitated the desired complex. The resulting solid was thoroughly washed with diethyl ether in order to eliminate cyclooctadiene residues and vacuum dried. [Ir(cod)(2)]BF4 (5): colour: orange; Yield: 49%; FABþ : 743 m=z (M–BF4 +O2 ). Anal. Calc. for C34 H38 IrS2 BF4 : C, 51.80; H, 4.90; S, 9.30. Found: C, 51.70; H, 4.80; S, 8.10%. [Ir(cod)(3)]BF4 (6): colour: yellow; Yield: 46%; FABþ : 730 m=z (M–BF4 ). Anal. Calc. for C35 H42 IrS2 BF4 : C, 52.87; H, 5.18; S, 7.84. Found: C, 52.75; H, 4.99; S, 7.27%. 2.4. In situ preparation of cis-dihydridoiridium(III) complexes [Ir(H)2 (cod)(dithioether)]BF4 (dithioether ¼ 1–3) In a typical experiment, hydrogen was bubbled into an NMR tube containing a solution of 0.05 mmol of [Ir(cod)(dithioether)]BF4 in deuterated dichloromethane at )70 °C during 30 min. The tube was introduced into the NMR equipment at )80 °C and spectra were acquired (see text for 1 H NMR data and characterization). Relaxation time T1 was determined at )70 °C and then, temperature was increased to 25 °C. 2DNMR experiments were recorded at 25 °C for complexes [Ir(H)2 (cod)(2)]BF4 and [Ir(H)2 (cod)(3)]BF4 .
2.5. Hydrogenation of prochiral olefins In a typical run, a Schlenk tube was filled with a dichloromethane (6 ml) solution of substrate (1 mmol) and catalyst precursor (0.01 mmol). It was then purged three times with H2 and vacuum. The reaction mixture was then shaken under H2 (1 atm) at 293 K. Conversion and enantiomeric excesses were determined by gas chromatography for substrate, 11–12. For substrate 10 the conversion was measured by 1 H NMR.
3. Results and discussion 3.1. Synthesis of sulfide ligands Ligands 1 and 2 were prepared by alkylation of the relevant dithiolate anion with the suitable alkyl iodides according to reported procedures [3,9]. The t-butyl derivative 3 was apparently unknown in the literature and was prepared by Sn 1-alkylation of the dithiol. The pure disulfide was isolated in 70% yield as a white crystalline solid after crystallization. 3.2. Iridium complexes [Ir(cod)(dithioether)]BF4 Ligand 1 reacted with dichloromethane solutions of [Ir(cod)2 ]BF4 to form the corresponding [Ir(cod)(dithioether)]BF4 complex 4 by 1,5-cyclooctadiene substitution (Scheme 1(a)). In contrast, ligands 2 and 3 did not
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Scheme 1. Syntheses of complexes (a) 4 and (b) 5–6.
react with dichloromethane solutions of [Ir(cod)2 ]BF4 unless hydrogen was bubbled into the solution in order to reduce the amount of 1,5-cyclooctadiene present in solution and to favour its substitution (Scheme 1(b)). All the complexes were isolated as air stable solids by addition of diethyl ether. Complexes were characterized by elemental analysis, FAB mass spectrometry, IR and 1 H, 13 C NMR spectroscopy. The spectral assignments (Table 1) were based on information from 1 H–1 H COSY and 13 C–1 H correlation measurements, in combination with the 1 H–1 H NOESY experiments. The FAB mass spectra confirmed the formation of the mononuclear species in all cases. The IR spectra show a strong band between 1090 and 1050 cm1 and a medium band at 450 cm1 characteristic of non-coordinated BF 4 anion in cationic complexes [13]. Upon chelate coordination to the metal, both sulfur atoms of the dithioether ligand become stereogenic centres. As a consequence, the reaction could lead to a mixture of stereoisomers with different configurations of the sulfur stereocentres and different spatial arrangements of the substituents. The resulting outcome may become even more complicated than anticipated if different conformations of comparable energy are available to the seven-membered chelate ring of the Ir-complex. (see Figs. 1 and 3). Since ligands 1–3 have an R absolute configuration of binaphthalene backbone, three possible diastereomers can be obtained: R, RS , RS and R, SS , SS (both attributed to the anti isomers), and R, RS , SS or R, SS , RS (corresponding to syn isomers) (Fig. 2). Remarkably, for the three complexes prepared, variable temperature NMR (VT-NMR) (Table 1) measurements indicated that only one diastereomer, with an overall C2 symmetry, was present in solution.
Over the whole temperature range, the 1 H NMR spectra of each complex show the olefinic proton signals of the coordinated 1,5-cyclooctadiene ligand as two multiplets, while the 13 C NMR spectra reveal only two different olefinic and methylenic resonances for the three complexes. These results are in keeping with the presence in all the cases of a single entity endowed with a C2 symmetrical structure. The coordinated dithioether ligands gave rise to a NMR pattern as expected for a C2 symmetry complex where the two MeS, i PrS and t BuS groups of complexes 4, 5 and 6, respectively, are magnetically equivalent in the 1 H and 13 C NMR. Thus, for instance, just one signal for the MeS groups of 4 was observed in the 1 H and 13 C NMR spectra. For all the complexes, the aromatic part of the spectrum is characterized by the presence of six peaks (roughly four doublets and two multiplets) which corresponds to the typical pattern of a C2 -symmetrical 2,20 disubstituted binaphthalene. For all these complexes, the shape of the NMR spectra is not affected when the sample is cooled down to )80 °C and warmed up to 50 °C.
Fig. 2. Three possible diastereomers for complexes containing ligands 1–3.
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Scheme 2. Reactivity of 4–6 with H2 .
The NOE experiments pointed out that, irrespective of the nature of the alkyl substituent, the three complexes show the same interactions. Thus, irradiation of the methyl signals of the disulfide ligands in complexes 4, 5 and 6, respectively, causes significant NOE enhancement of the H3 proton on the naphthalene ring, which is consistent with an equatorial location of the alkyl substituent. The presence of these NOE contacts supports the view that the anti-disposed alkyl substituents at the sulfur atoms occupy a pseudoequatorial position in the distorted twisted chair conformation of the seven-membered chelate ring as a consequence of the S,S-configuration of the sulfur stereocentres. On the whole, the stereochemical features of these Ircomplexes 4, 5 and 6 exactly reproduce the properties which we have previously assessed for the prevailing stereoisomer of the Pd-dichloride complex of ligand 1 [14]. From this similarity, we can confidently attribute to complexes 4–6 the same twisted-chair frozen conformation of the seven-membered chelate ring, where both S-stereocentres display a pyramidal geometry and have
Fig. 3. (a) Schematic representation of diastereomer 4–6 with equatorial location of the sulfur substituents. The arrows show the cross peak signals. (b) Representation of the conformation adopted by the seven-membered chelate ring of diastereomer of 4–6. The absolute configuration of the stereogenic sulfur centres is indicated. The binaphthalene backbone (R configuration) and the additional ligands are omitted for clarity.
the same relative configuration, whereas the binaphthyl moiety has the opposite chiral notation. 3.3. Reactivity of the olefinic complexes with H2 The oxidative addition of hydrogen is the rate-determining step in the asymmetric hydrogenation of acrylic acid derivatives catalyzed by rhodium complexes with chiral diphosphines [15], diphosphinites [16], diphosphites [17] and phosphite-phosphine [18]. Aiming at a better understanding of the catalytic process, an investigation on the reactivity of the cationic disulfide complexes 4–6 with hydrogen was undertaken before testing the activity of these complexes in the catalytic hydrogenation of some benchmark olefins. Complexes 4–6 react with H2 at )70 °C to form the corresponding cis-dihydridoiridium(III) complexes [Ir(H)2 (cod)(1)]BF4 (7), [Ir(H)2 (cod)(2)]BF4 (8) and [Ir(H)2 (cod)(3)]BF4 (9) (Scheme 2). A similar behaviour has been observed for other dithioether iridium complexes [1,6]. Complexes 7–9 were stable in solution at 25 °C. As in these species, in addition to the diaryl backbone and to the sulfur atoms, even the metal is stereogenic, then the number of possible diastereomers in solution can be as high as 16. It is really remarkable that, even in this case, for the three dihydride complexes obtained 7– 9, the NMR spectra were consistent with the presence of only one single species in solution (Table 2). In the high-field region of the 1 H NMR spectra of the CD2 Cl2 solutions of any of the complexes 7–9, two metal hydride resonances, each integrating for one proton, can be distinguished (Table 2). The hydridic nature of these ligands was assessed by measuring T1 min with 1 H relaxation rates [19]. For all complexes the hydride resonances have T1 min values of around 300 ms in CD2 Cl2 at )70 °C and 400 MHz, which are consistent with classical hydrides. The patterns of the low-field region of the 1 H NMR spectrum are consistent with the anticipated
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Table 2 The 1 H NMR spectroscopic data for complexes 7–9a
Position
7
8
9
3, 30 4, 40 5, 50 6, 60 7, 70 8, 80 S–Me Me
7.8–8.4(br) 7.8–8.4(br) 7.8–8.4(br) 7.65(br) 7.25(br) 7.00(br) 1.80(s), 3.20(s),
8.12(d, J ¼ 8.4), 8.19(d, J ¼ 8.8) 7.97(d, J ¼ 8.4), 8.06(d, J ¼ 8.8) 7.70(d, J ¼ 8.0), 7.85(d, J ¼ 7.6) 7.54(t, J ¼ 7.6), 7.60(t, J ¼ 7.6) 7.30(t, J ¼ 7.6), 7.40(t, J ¼ 8.0) 6.94(d, J ¼ 8.0), 7.20(d, J ¼ 8.0)
7.8(d, J ¼ 8.1 7.85(d, J ¼ 8.3) 7.8–8.2(br) 7.8–8.2(br) 7.45(t, J ¼ 8.0), 7.59(t, J ¼ 8.4) 7.21(t, J ¼ 8.0), 7.26(t, J ¼ 8.0) 6.80(d, J ¼ 8.0), 7.15(d, J ¼ 8.0)
0.71(d, J ¼ 7.8), 0.93(d, J ¼ 7.8), 1.20(d, J ¼ 7.8), 1.52(d, J ¼ 7.8) 2.65(m), 3.98(m)
0.60(s), 1.45(s)
2.1–2.6(br) 3.92(br), 4.11(br) 4.49(br), 4.78(br)
1.8–2.4(br), 2.45(m), 2.65(m) 4.15(br), 4.26(br) 4.65(br), 4.84(br)
)13.13(s) )12.99(s)
)14.43(s) )13.99(s)
S–CH S–C CH2 CH@ H9 H10
2.1–3.0(br) 3.85(br), 4.00(br) 4.45(br), 4.65(br) )12.99(s) )12.82(s)
a Chemical shifts in ppm; coupling constants in Hz; room temperature; 1 H and 13 C NMR in CD2 Cl2 . Abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad.
C1 -symmetry of the complexes (Table 2). Thus, for each of the complexes 7, 8 and 9, the olefinic protons of the coordinated cyclooctadiene appear as four multiplets and the two S-alkyl substituents produced separate peaks in the 1 H NMR as expected for heterotopic groups in a C1 -symmetrical structure. Thus, two resonances were observed for the MeS groups of complex 7, while the i PrS groups of complex 8 gave rise to four doublets and two multiplets. The aromatic part of the spectrum is as well consistent with the C1 symmetry. For instance, complex 8 shows twelve separate peaks (roughly eight doublets and four triplets) while in the spectra of 7 and 9 the pattern is less clear-cut due to the presence of some overlap. The protonic spectra of all theses complexes did not show any significant change in the variable temperature experiments in the range between )90 and 25 °C. In particular, hydride, methyl, iso-propyl, and tert-butyl resonances did not undergo any apparent splitting, leading further support to the view that only one diastereoisomer is present. In the 2D-NOESY spectrum of complex 9, the methyl protons of the t BuS group at 0.6 ppm show close contacts with the hydride signal H-9, with the olefinic proton at 4.3 ppm and with the H-8 proton on the naphthalene ring. This last interaction is compatible only with an axial location of this t BuS group. The second t BuS group (1.45 ppm) shows cross peaks with the hydride H-10, with the olefinic proton at 4.7 ppm
and with the H-3 proton of the naphthalene ring. These interactions are consistent with an equatorial location of this second t BuS group. From all these data, an axialequatorial disposition of the substituents on the stereogenic sulfur centres of S,R-absolute configuration can be inferred for complex 9, provided that the seven-membered chelate ring maintains the original distorted twisted chair conformation. The 2D-NOESY spectra of complexes 7 and 8 were less clear-cut and did not allow to draw any sound conclusion on the location of the alkyl substituents. All attempts to isolate these cis-dihydridoiridium complexes led to the isolation of the corresponding olefinic iridium(I) complexes even at )70 °C under a hydrogen atmosphere. From this NMR study, we can confidently draw the conclusion that the oxidative addition of hydrogen to the Ir-complexes 4–6 proceeds with complete stereoselectivity affording in all the cases just one out of the sixteen possible diastereoisomeric dihydrides and that these species maintain their stereochemical identity in solution over a wide range of temperatures. It is worth to stress that this result contrasts with the normal behaviour of iridium complexes with alkylsulfide ligands which upon addition of hydrogen usually provide a mixture of the corresponding dihydrides [1]. This unprecedented result encouraged us to explore the potentiality of these complexes as chiral catalysts in asymmetric hydrogenation.
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4. Asymmetric hydrogenation The iridium complexes 4–6 were tested in the asymmetric hydrogenation of prochiral olefins 10–12. Selected results are shown in Table 3. As a general trend, the complexes displayed a modest-to-fair catalytic activity and led to disappointingly low stereoselectivities. (see Scheme 3). The hydrogenation of itaconic acid 10 was investigated at first. The reaction proceeded slowly at room temperature in methanol or dichloromethane solution under atmospheric pressure of H2 and in most cases the enantioinduction was practically null. The activity of the complex is strongly influenced by the nature of the solvent (entries 1–3), the fastest reaction being observed in a 9/1 mixture of dichloromethane-methanol (entry 3). This increase of the rate was not matched by a parallel improvement of the stereoselectivity the best e.e. recorded was as low as 5%. Using the catalyst prepared in situ by stirring a solution of [Ir(COD)2 ]BF4 and the dithioether ligand did not result in any significant variation of the efficiency of the process (entry 4). Use of onefold excess of ligand (entry 5) or of an increased hydrogen pressure (5 bar, data not shown) did not improve in the catalyst performance (activity and enantioselectivity). Complexes 5 and 6 showed a behaviour similar to the one observed for 4, but both were catalysts of lower activity. The decrease in the hydrogenation rate was particularly pronounced in the case of the iso-propyl derivative 5 (entry 6), whereas the tert-butyl substituted complex 6 was only half as active as the corresponding
Scheme 3. Asymmetric hydrogenation of prochiral olefins by Ir-complexes 4–6.
methyl substituted derivative 4 (compare entries 3 and 7). Similar results were observed in the hydrogenation of dimethyl itaconate 11 and methyl a-acetamido acrylate 12. With these substrates even in the best solvent (dichloromethane) the hydrogenation rates are lower than with itaconic acid. The hydrogenation products were all practically racemic (Table 3). 5. Conclusions The coordination of the dithioether ligands 1–3 to Ir(I)-centres is remarkable in that it occurs stereoselectively producing just one out of the several possible prospective stereoisomers which may be predicted from the coordination of two prostereogenic sulfur donors and from the possible interconversion of conformers of the seven-membered chelate ring. Thus, it appears that the formation of the new stereogenic centres at the Sdonor is effectively steered by the pre-existing chirality of the diaryl framework in such a way that it is selec-
Table 3 Asymmetric hydrogenation of substrates 10–12 with catalytic system 4–6a Entry
Precursor
Substrate
Solvent
% Conversion (t/h)b
%eec
1 2 3 4d 5e 6 7 8 9 10 11 12 13 14 15
4 4 4 4 4 5 6 4 4 4 5 6 4 5 6
10 10 10 10 10 10 10 11 11 11 11 11 12 12 12
CH2 Cl2 MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 MeOH CH2 Cl2 /MeOH CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2
65(20) 7(2) 100(0.5) 100(0.5) 100(0.5) 100(6) 100(1) 14(6) 5(6) 2(6) 3(6) 83(6) 2(6) 5(6) 7(6)
1(R) 1(R) 4(R) 3(R) 4(R) 5(R) 1(R) 1(R) 1(R) 1(R) 1(R) 2(R) 1(S) 1(S) 1(S)
a
Substrate/Ir ¼ 100; Ligand/Ir ¼ 1.1, solvent ¼ 6 ml, T ¼ 25 °C. % Conversion measured by 1 H NMR for substrate 10 and by GC for substrates 11–12. c % Enantiomeric excess measured by GC using a Chiraldex-G-TA for substrates 10–11 and a Permabond L-Chirasil-Val column for substrate 12. d The catalyst was prepared in situ by stirring a solution of [Ir(COD)2 ]BF4 and the dithioether ligand. e Ligand/Ir ¼ 2. b
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tively obtained only by the diastereoisomer featuring the same chiral notations at the stereogenic sulfur donors and the anti-disposition of the alkyl substituent on the S-donors. Albeit not unprecedented [1b,6,20], this result is of importance since it provides the appropriate background for the stereochemistry of following step of the catalytic cycle of the hydrogenation of the olefins, i.e., the oxidative addition of hydrogen, to be properly addressed. Much at our surprise, even this reaction apparently proceeds stereoselectively and in all the three cases one single dihydride was observed in solution. Common features of these dihydrides are the unusual stability in solution, which allowed a complete NMR characterization of these species; the cis-arrangement of two g1 -hydrogen ligands, resulting in classical Ir(III)dihydride complexes and the absence of dynamic behaviour due to conformational interconversion. The axial-equatorial location of the alkyl substituents displayed by the t-butyl substituted derivative 9 is probably due to the steric bulk of the substituent which is better accommodated by changing the original stereochemical relation of the alkyl substituents. This particular assembly of substituents apparently exerts some positive influence on the catalytic activity in the hydrogenation of olefins, as the t-butyl containing Ir-complex parallels the activity of the methyl complex and is significantly more active than the i-propyl derivative. In spite of the complete stereoselectivity associated to the above reported steps of the catalytic cycle and of the significative e.e.s recorded with Ir-catalysts of similar design, the hydrogenation of a; b-unsaturated acid derivatives with the title Ir(I)-complexes is practically devoid of stereoselectivity. As of now we cannot settle these differences, further work seems necessary for the reasons of this dichotomic behaviour to be cleared.
Acknowledgements This work was carried out in the frame of the EU project COST-D24 (0003/01). The Spanish group thanks Ministerio de Educaci on, Cultura y Deporte (BQU2001–0656) for financial support and the undergraduate student Laetitia Bertrand (France) for her valuable assistance in the synthesis and characterization of complexes 5 and 8. Financial support from MIUR (PRIN 2003033857) and from the University of Sassari is gratefully acknowledged by S.G.
References [1] (a) Reviews J.C. Bay on, C. Claver, A.M. Masdeu-Bult o, Coord. Chem. Rev. 193–195 (1999) 73; (b) A.M. Masdeu-Bult o, M. Dieguez, E. Martın, M. G omez, Coord. Chem. Rev. 242 (2003) 159. [2] For some recent advances and references on N.-K. rearrangement see V. Albrow, K. Biswas, A. Crane, N. Chaplin, T. Easun, S. Gladiali, B. Lygo, S. Woodward, Tetrahedron: Asymm. 14 (2003) 2813. [3] (a) C. Claver, S. Castill on, N. Ruiz, G. Delogu, D. Fabbri, S. Gladiali, J. Chem. Soc., Chem.Commun. (1993) 1833; (b) N. Ruiz, A. Aaliti, J. Fornies, A. Ruiz, C. Claver, C.J. Cardin, D. Fabbri, S. Gladiali, J. Organomet. Chem. 545/546 (1997) 79. [4] N. Ruiz, I. del Rıo, J.L. Jimenez, C. Claver, J. Fornies-Camer, C.C. Cardin, S. Gladiali, J. Mol. Catal. 143 (1999) 171. [5] D. Fabbri, S. Pulacchini, S. Gladiali, Synlett (1996) 1054. [6] (a) M. Dieguez, A. Orej on, A.M. Masdeu-Bult o, R. Echarri, S. Castill on, C. Claver, A. Ruiz, J. Chem. Soc., Dalton Trans. (1997) 4611; (b) M. Dieguez, A. Ruiz, C. Claver, M.M. Pereira, A.M.d’A. Rocha Gonsalves, J. Chem. Soc., Dalton Trans. (1998) 3517; (c) O. Pamies, M. Dieguez, G. Net, A. Ruiz, C. Claver, J. Chem. Soc., Dalton Trans. (1999) 3439; (d) O. Pamies, M. Dieguez, G. Net, A. Ruiz, C. Claver, Organometallics 19 (2000) 1488. [7] S. Jansat, M. Gomez, G. Muller, M. Dieguez, A. Aghmiz, C. Claver, A.M. Masdeu-Bult o, L. Flores-Santos, E. Martin, M.A. Maestro, J. Mahia, Tetrahedron: Asymm. 12 (2001) 1469. [8] M. Green, T.A. Kuc, S.H. Taylor, J. Chem. Soc. (1971) 2334. [9] D. Fabbri, G. Delogu, O. De Lucchi, J. Org. Chem. 58 (1993) 1748. [10] D.J. States, R.A. Haberkorn, D.J. Ruben, J. Magn. Reson. 48 (1982) 286. [11] D.G. Hamilton, R.H. Crabtree, J. Am. Chem. Soc. 110 (1998) 4126. [12] C. Cativiela, J. Fernandez, J.A. Mayoral, E. Melendez, R. Us on, L.A. Oro, M.J. Fernandez, J. Mol. Catal. 16 (1992) 19. [13] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1978. [14] S. Gladiali, D. Fabbri, L. Kollar, C. Claver, N. Ruiz, A. AlvarezLarena, J.F. Piniella, Eur. J. Inorg. Chem. 1 (1998) 113. [15] C.R. Landis, J. Halpern, J. Am. Chem. Soc. 109 (1987) 1746. [16] (a) T.V. RajanBabu, T.A. Ayers, A.L. Casalnuovo, J. Am. Chem. Soc. 116 (1994) 4101; (b) T.V. RajanBabu, T.A. Ayers, G.A. Halliday, K.K. You, J.C. Calabrese, J. Org. Chem. 62 (1997) 6012; (c) T.V. RajanBabu, B. Radetich, K.K. You, T.A. Ayers, A.L. Casalnuovo, J.C. Calabrese, J. Org. Chem. 64 (1999) 3429. [17] O. Pamies, G. Net, A. Ruiz, C. Claver, Eur. J. Inorg. Chem. (2000) 1287. [18] (a) S. Deerenberg, O. Pamies, M. Dieguez, C. Claver, P.C.J. Kamer, P.W.N.M. van Leeuwen, J. Org. Chem. 66 (2001) 7626; (b) O. Pamies, M. Dieguez, G. Net, A. Ruiz, C. Claver, J. Org. Chem. 66 (2001) 8364. [19] R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, John Wiley & Sons, New York, 1998. [20] A. Gomis, G. Net, A. Ruiz, C. Claver, A. Alvarez-Larena, Inorg. Chim. Acta 325 (2001) 58, and references cited therein.
Cationic iridium complexes with C2-symmetry binaphthalene-core disulfide ligands Synthesis and catalytic activity in the hydrogenation of alkenes Montserrat Dieguez a,*, Aurora Ruiz a, Carmen Claver a, Franco Doro b, Maria G. Sanna b, Serafino Gladiali b,* a
Departament de Quımica Fısica i Inorganica, Universitat Rovira i Virgili, Pl. Imperial Tarraco 1, 43005 Tarragona, Spain b Dipartimento di Chimica, Universita di Sassari, Via Vienna 2, 07100 Sassari, Italy Received 20 January 2004; accepted 27 March 2004 Available online 12 May 2004
Abstract Mononuclear cationic Ir(I)-cyclooctadiene complexes containing three different C2 -symmetrical binaphthalene-templated sulfide ligands, featuring alkyl groups of increasing steric demand onto the donor centres, have been prepared and characterized. Variable temperature NMR spectra provide evidence that, regardless of the bulk of the alkyl substituent on the sulfur, the chelate coordination of the ligands proceeds in all cases with complete stereoselectivity at the newly generated S-stereocentres affording just one stereoisomer. This species features a seven-membered chelate ring in a frozen conformation where the diequatorial S-alkyl substituents are disposed in anti-relationship and the stereogenic S-donors display the same configuration. The oxidative addition of hydrogen to these complexes proceeds smoothly affording in every case one single cis-dihydride complex whose structure in one case has been cleared by correlated NMR spectra. The cationic complexes derived from these ligands are catalysts of modest value for the hydrogenation of a; b-unsaturated acid derivatives where they produce nearly racemic products in moderate yields. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Ir complexes; Chiral S-ligands; Stereoselectivity; Catalytic hydrogenation
1. Introduction Chiral ligands with sulfur donors are exceedingly underutilized in asymmetric catalysis in comparison with the corresponding counterparts containing P- or even Ndonor centres. Actually sulfur is a donor weaker than both phosphorus and nitrogen towards soft and hard metal ions, respectively, and is comparably more prone to labilization than these two donors. Probably in consequence of this behaviour, for a long time sulfur ligands have been considered poorly suited for supporting efficiently the stereorecognitive process necessary for the transfer of the chiral information from the metal to the * Corresponding authors. Tel.: +34-977-558046; fax: +34-977-559563 (M. Dieguez), Tel.: +39-79-229546; fax: + 39-79-229559 (S. Gladiali). E-mail addresses: [email protected] (M. Dieguez), [email protected] (S. Gladiali).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.03.039
substrate to proceed. In recent times, however, the utilization of sulfur donor ligands as chiral modifiers in transition metal catalyzed enantioselective reactions is attracting increased attention and the library of chiral ligands with S-donors has been growing steadily [1]. During the last 10 years, either jointly or separately, we have provided several contributions to this topic of asymmetric catalysis. In particular, our collaborative research efforts have been centred on the synthesis and applications of axially chiral bidentate ligands containing two homotopic or heterotopic sulfur donor centres featuring the same or different substituents. These li0 gands are basically derived from 1,1 -binaphthalene or similar atropisomeric diaryl-core scaffolds and allow for the chelate coordination to the metal through two neutral or anionic sulfur donors. C2 -symmetry ligands 1 with two equivalent S-donors have been prepared from the corresponding binaphthyl
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or biphenanthryl diols using in the key step the Newmann–Kwart rearrangement to generate the new carbon–sulfur bonds [2]. The relevant cationic mononuclear or neutral dinuclear Rh(I)-complexes have been characterized and screened as chiral catalysts in the asymmetric hydroformylation of styrene. A good catalytic activity and a high selectivity towards the branched aldehyde was observed with the mononuclear complexes, but the stereoselectivity was poor and the enantiomeric excess (e.e.) of the chiral product did not exceed 20% [3]. E.e.s even lower were recorded in the Pd-catalyzed asymmetric hydrocarboxylation of styrene [4] in the presence of similar C1 -symmetry thiolate-thioether binaphthalene ligands [5]. The modest outcomes in asymmetric catalysis of the above reported ligands are contrasted by more recent results recorded in the asymmetric Ir(I)-catalyzed hydrogenation of olefins [6] and in the Pd(II)-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate [7], where stereoselectivities as high as 68% and 81% e.e., respectively, have been obtained using C2 -symmetry thioethers as chiral inducers. These results prompted us to investigate the potential of our C2 -symmetry binaphthalene-templated sulfides in these processes. Here, we report some aspects of the coordination chemistry of the binaphthyl sulfide ligands 1–3 towards iridium and on the catalytic behaviour of the relevant complexes in the hydrogenation of a; bunsaturated acid derivatives. 2. Experimental 2.1. General methods All iridium complexes were synthesized using standard Schlenk techniques under a nitrogen atmosphere. The complex [Ir(cod)2 ]BF4 was prepared using methods described in the literature [8]. Ligands 1 and 2 were prepared as reported [3,9]. Solvents were dried over standard drying agents and were freshly distilled and deoxygenated prior to use. All other reagents were used as commercially supplied. Elemental analyses were performed on a Carlo Erba EA-1108 instrument. The 1 H and 13 C–{1 H} NMR
Fig. 1. Dithioethers ligands 1–3.
spectra were recorded on a Varian Gemini 300 and 400 MHz spectrometers. Chemical shifts are relative to SiMe4 (1 H and 13 C) as internal standard. All assignments in NMR spectra were determined by means of COSY and 13 C–1 H correlation experiments. Standard pulse sequences were employed for 1 H 2-D NOESY [10]. The phase-sensitive NOESY experiments used mixing times of 0.4 s. Proton T1 studies were performed using the standard inversion recovery 180°–s)90° pulse sequence method [11]. Infrared spectra were recorded on a Bruker EQUINOX 55 spectrophotometer. A VGAutospect was used for FAB mass spectral analyses. The matrix was m-nitrobenzyl alcohol. The reaction under 1 atm of H2 was performed in a previously described hydrogen vacuum line [12]. Gas chromatography analyses were performed in a Hewlett-Packard Model 5890A instrument. Enantiomeric excesses (e.e.) were measured using a fused silica capillary column 25 0.25 mm Permabond L-Chirasil-Val for the determination of e.e. for substrate 12, and in Chiraldex-GTA (30 m 0.25 mm) for substrates 10 and 11. 2.2. Preparation of disulfide ligand 3 A solution of (R)-binaphthyl dithiol (1 g, 3.1 mmol) and t-butanol (1.2 ml, 16 mmol) in AcOH (15 ml) containing acetic anhydride (2.15 ml) was stirred at 0 °C for 20 min under inert atmosphere. 70% Aqueous HClO4 (0.5 ml) was then added and stirring was continued at room temperature for 15 h. The solution was poured into water, the resulting solid was filtered out, washed with water and crystallized from diethyl ether–methanol: 0.82 g, 70%; m.p. 158–160 °C; 1 H NMR (d, CHCl3 ): 1.17 (18H, s); 7.0 (2H, d); 7.19 (2H, m); 7.43 (2H, m); 7.90 (6H, m). ½a25 D : )299.6 (c ¼ 0:5; CHCl3 ). Anal. Calc. for C28 H30 S2 : C, 78.09; H, 7.02; S, 14.89. Found: C, 78.25; H, 7.15; S, 14.62%. 2.3. Preparation of iridium complexes 2.3.1. [Ir(cod)(1)]BF4 (4) The compound was prepared by adding an excess of ligand (1) (38.1 mg, 0.11 mmol) to a dichloromethane (5 ml) solution of [Ir(cod)2 ]BF4 (49.7 mg, 0.1 mmol), which produces an immediate change from brown–red to yellow. After 30 min, the solvent was evaporated and the yellow solid cleaned twice with cold diethyl ether and vacuum dried (48 mg, 81%). FABþ : 647 m=z (M–BF4 ). Anal. Calc. for C30 H30 IrS2 BF4 : C, 49.30; H, 4.13; S, 8.73. Found: C, 48.30; H, 4.41; S, 8.61%. NMR details are reported in Table 1. 2.3.2. Synthesis of complexes [Ir(cod)(2)]BF4 (5) and [Ir(cod)(3)]BF4 (6). General procedure A solution of the dithioether ligand (0.08 mmol) in 3 ml dichloromethane was added to a solution of
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Table 1 The NMR spectroscopic data for complexes 4–6a
Position
4 1
3 4 5 6 7 8 S–Me Me S–CH S–C CH2 CH@
H
7.90(d, J ¼ 9.1) 8.23(d, J ¼ 9.1) 8.02(d, J ¼ 8.4) 7.59(t, J ¼ 7.2) 7.37(t, J ¼ 7.2) 7.03(d, J ¼ 8.8) 2.62(s)
1.80(m), 2.20(m) 2.40(m) 3.85(m), 4.65(m)
5 13
C
128.7 131.4 128.2 127.9 126.0 123.2 15.0
31.0, 32.0 73.0, 79.0
1
6 13
H
C
1
13
H
7.85(d, J ¼ 9.8) 8.25(d, J ¼ 9.8) 8.10(d, J ¼ 9.8) 7.65(t, J ¼ 6.5) 7.40(t, J ¼ 6.5) 7.10(d, J ¼ 11.7)
129.5 131.4 129.4 128.9 128.5 127.3
7.95 8.17 8.08 7.64 7.40 7.15
1.18(d, J ¼ 5.2), 1.21(d, J ¼ 5.2) 3.30(sp, J ¼ 5.2)
22.8, 23.2
1.1 (s)
1.65(m), 1.90(m), 2.15(m), 2.40(m) 4.10(m), 4.85(m)
(d,J ¼ 8.4) (d,J ¼ 8.4) (d,J ¼ 8.4) (t,J ¼ 7.6) (t,J ¼ 7.6) (d,J ¼ 8.4)
C
128.3 130.7 130.1 127.8 127.6 126.7 31.8
42.9 29.8, 33.7 72.2, 78.1
1.40(m), 1.60(m), 2.15(m), 2.45(m) 4.60 (m), 5.40(m)
60.0 27.4, 34.5 68.0, 74.6
a
Chemical shifts in ppm; coupling constants in Hz; room temperature; 1 H and 13 C NMR in CDCl3 . Abbreviations: s, singlet; d, doublet; t, triplet; sp, septuplet; m, multiplet.
[Ir(cod)2 ]BF4 (40 mg, 0.08 mmol) in 5 ml dichloromethane under nitrogen. The solution was stirred for 30 min and then hydrogen was bubbled inside the solution for 30 min. Subsequent addition of diethyl ether precipitated the desired complex. The resulting solid was thoroughly washed with diethyl ether in order to eliminate cyclooctadiene residues and vacuum dried. [Ir(cod)(2)]BF4 (5): colour: orange; Yield: 49%; FABþ : 743 m=z (M–BF4 +O2 ). Anal. Calc. for C34 H38 IrS2 BF4 : C, 51.80; H, 4.90; S, 9.30. Found: C, 51.70; H, 4.80; S, 8.10%. [Ir(cod)(3)]BF4 (6): colour: yellow; Yield: 46%; FABþ : 730 m=z (M–BF4 ). Anal. Calc. for C35 H42 IrS2 BF4 : C, 52.87; H, 5.18; S, 7.84. Found: C, 52.75; H, 4.99; S, 7.27%. 2.4. In situ preparation of cis-dihydridoiridium(III) complexes [Ir(H)2 (cod)(dithioether)]BF4 (dithioether ¼ 1–3) In a typical experiment, hydrogen was bubbled into an NMR tube containing a solution of 0.05 mmol of [Ir(cod)(dithioether)]BF4 in deuterated dichloromethane at )70 °C during 30 min. The tube was introduced into the NMR equipment at )80 °C and spectra were acquired (see text for 1 H NMR data and characterization). Relaxation time T1 was determined at )70 °C and then, temperature was increased to 25 °C. 2DNMR experiments were recorded at 25 °C for complexes [Ir(H)2 (cod)(2)]BF4 and [Ir(H)2 (cod)(3)]BF4 .
2.5. Hydrogenation of prochiral olefins In a typical run, a Schlenk tube was filled with a dichloromethane (6 ml) solution of substrate (1 mmol) and catalyst precursor (0.01 mmol). It was then purged three times with H2 and vacuum. The reaction mixture was then shaken under H2 (1 atm) at 293 K. Conversion and enantiomeric excesses were determined by gas chromatography for substrate, 11–12. For substrate 10 the conversion was measured by 1 H NMR.
3. Results and discussion 3.1. Synthesis of sulfide ligands Ligands 1 and 2 were prepared by alkylation of the relevant dithiolate anion with the suitable alkyl iodides according to reported procedures [3,9]. The t-butyl derivative 3 was apparently unknown in the literature and was prepared by Sn 1-alkylation of the dithiol. The pure disulfide was isolated in 70% yield as a white crystalline solid after crystallization. 3.2. Iridium complexes [Ir(cod)(dithioether)]BF4 Ligand 1 reacted with dichloromethane solutions of [Ir(cod)2 ]BF4 to form the corresponding [Ir(cod)(dithioether)]BF4 complex 4 by 1,5-cyclooctadiene substitution (Scheme 1(a)). In contrast, ligands 2 and 3 did not
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Scheme 1. Syntheses of complexes (a) 4 and (b) 5–6.
react with dichloromethane solutions of [Ir(cod)2 ]BF4 unless hydrogen was bubbled into the solution in order to reduce the amount of 1,5-cyclooctadiene present in solution and to favour its substitution (Scheme 1(b)). All the complexes were isolated as air stable solids by addition of diethyl ether. Complexes were characterized by elemental analysis, FAB mass spectrometry, IR and 1 H, 13 C NMR spectroscopy. The spectral assignments (Table 1) were based on information from 1 H–1 H COSY and 13 C–1 H correlation measurements, in combination with the 1 H–1 H NOESY experiments. The FAB mass spectra confirmed the formation of the mononuclear species in all cases. The IR spectra show a strong band between 1090 and 1050 cm1 and a medium band at 450 cm1 characteristic of non-coordinated BF 4 anion in cationic complexes [13]. Upon chelate coordination to the metal, both sulfur atoms of the dithioether ligand become stereogenic centres. As a consequence, the reaction could lead to a mixture of stereoisomers with different configurations of the sulfur stereocentres and different spatial arrangements of the substituents. The resulting outcome may become even more complicated than anticipated if different conformations of comparable energy are available to the seven-membered chelate ring of the Ir-complex. (see Figs. 1 and 3). Since ligands 1–3 have an R absolute configuration of binaphthalene backbone, three possible diastereomers can be obtained: R, RS , RS and R, SS , SS (both attributed to the anti isomers), and R, RS , SS or R, SS , RS (corresponding to syn isomers) (Fig. 2). Remarkably, for the three complexes prepared, variable temperature NMR (VT-NMR) (Table 1) measurements indicated that only one diastereomer, with an overall C2 symmetry, was present in solution.
Over the whole temperature range, the 1 H NMR spectra of each complex show the olefinic proton signals of the coordinated 1,5-cyclooctadiene ligand as two multiplets, while the 13 C NMR spectra reveal only two different olefinic and methylenic resonances for the three complexes. These results are in keeping with the presence in all the cases of a single entity endowed with a C2 symmetrical structure. The coordinated dithioether ligands gave rise to a NMR pattern as expected for a C2 symmetry complex where the two MeS, i PrS and t BuS groups of complexes 4, 5 and 6, respectively, are magnetically equivalent in the 1 H and 13 C NMR. Thus, for instance, just one signal for the MeS groups of 4 was observed in the 1 H and 13 C NMR spectra. For all the complexes, the aromatic part of the spectrum is characterized by the presence of six peaks (roughly four doublets and two multiplets) which corresponds to the typical pattern of a C2 -symmetrical 2,20 disubstituted binaphthalene. For all these complexes, the shape of the NMR spectra is not affected when the sample is cooled down to )80 °C and warmed up to 50 °C.
Fig. 2. Three possible diastereomers for complexes containing ligands 1–3.
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Scheme 2. Reactivity of 4–6 with H2 .
The NOE experiments pointed out that, irrespective of the nature of the alkyl substituent, the three complexes show the same interactions. Thus, irradiation of the methyl signals of the disulfide ligands in complexes 4, 5 and 6, respectively, causes significant NOE enhancement of the H3 proton on the naphthalene ring, which is consistent with an equatorial location of the alkyl substituent. The presence of these NOE contacts supports the view that the anti-disposed alkyl substituents at the sulfur atoms occupy a pseudoequatorial position in the distorted twisted chair conformation of the seven-membered chelate ring as a consequence of the S,S-configuration of the sulfur stereocentres. On the whole, the stereochemical features of these Ircomplexes 4, 5 and 6 exactly reproduce the properties which we have previously assessed for the prevailing stereoisomer of the Pd-dichloride complex of ligand 1 [14]. From this similarity, we can confidently attribute to complexes 4–6 the same twisted-chair frozen conformation of the seven-membered chelate ring, where both S-stereocentres display a pyramidal geometry and have
Fig. 3. (a) Schematic representation of diastereomer 4–6 with equatorial location of the sulfur substituents. The arrows show the cross peak signals. (b) Representation of the conformation adopted by the seven-membered chelate ring of diastereomer of 4–6. The absolute configuration of the stereogenic sulfur centres is indicated. The binaphthalene backbone (R configuration) and the additional ligands are omitted for clarity.
the same relative configuration, whereas the binaphthyl moiety has the opposite chiral notation. 3.3. Reactivity of the olefinic complexes with H2 The oxidative addition of hydrogen is the rate-determining step in the asymmetric hydrogenation of acrylic acid derivatives catalyzed by rhodium complexes with chiral diphosphines [15], diphosphinites [16], diphosphites [17] and phosphite-phosphine [18]. Aiming at a better understanding of the catalytic process, an investigation on the reactivity of the cationic disulfide complexes 4–6 with hydrogen was undertaken before testing the activity of these complexes in the catalytic hydrogenation of some benchmark olefins. Complexes 4–6 react with H2 at )70 °C to form the corresponding cis-dihydridoiridium(III) complexes [Ir(H)2 (cod)(1)]BF4 (7), [Ir(H)2 (cod)(2)]BF4 (8) and [Ir(H)2 (cod)(3)]BF4 (9) (Scheme 2). A similar behaviour has been observed for other dithioether iridium complexes [1,6]. Complexes 7–9 were stable in solution at 25 °C. As in these species, in addition to the diaryl backbone and to the sulfur atoms, even the metal is stereogenic, then the number of possible diastereomers in solution can be as high as 16. It is really remarkable that, even in this case, for the three dihydride complexes obtained 7– 9, the NMR spectra were consistent with the presence of only one single species in solution (Table 2). In the high-field region of the 1 H NMR spectra of the CD2 Cl2 solutions of any of the complexes 7–9, two metal hydride resonances, each integrating for one proton, can be distinguished (Table 2). The hydridic nature of these ligands was assessed by measuring T1 min with 1 H relaxation rates [19]. For all complexes the hydride resonances have T1 min values of around 300 ms in CD2 Cl2 at )70 °C and 400 MHz, which are consistent with classical hydrides. The patterns of the low-field region of the 1 H NMR spectrum are consistent with the anticipated
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Table 2 The 1 H NMR spectroscopic data for complexes 7–9a
Position
7
8
9
3, 30 4, 40 5, 50 6, 60 7, 70 8, 80 S–Me Me
7.8–8.4(br) 7.8–8.4(br) 7.8–8.4(br) 7.65(br) 7.25(br) 7.00(br) 1.80(s), 3.20(s),
8.12(d, J ¼ 8.4), 8.19(d, J ¼ 8.8) 7.97(d, J ¼ 8.4), 8.06(d, J ¼ 8.8) 7.70(d, J ¼ 8.0), 7.85(d, J ¼ 7.6) 7.54(t, J ¼ 7.6), 7.60(t, J ¼ 7.6) 7.30(t, J ¼ 7.6), 7.40(t, J ¼ 8.0) 6.94(d, J ¼ 8.0), 7.20(d, J ¼ 8.0)
7.8(d, J ¼ 8.1 7.85(d, J ¼ 8.3) 7.8–8.2(br) 7.8–8.2(br) 7.45(t, J ¼ 8.0), 7.59(t, J ¼ 8.4) 7.21(t, J ¼ 8.0), 7.26(t, J ¼ 8.0) 6.80(d, J ¼ 8.0), 7.15(d, J ¼ 8.0)
0.71(d, J ¼ 7.8), 0.93(d, J ¼ 7.8), 1.20(d, J ¼ 7.8), 1.52(d, J ¼ 7.8) 2.65(m), 3.98(m)
0.60(s), 1.45(s)
2.1–2.6(br) 3.92(br), 4.11(br) 4.49(br), 4.78(br)
1.8–2.4(br), 2.45(m), 2.65(m) 4.15(br), 4.26(br) 4.65(br), 4.84(br)
)13.13(s) )12.99(s)
)14.43(s) )13.99(s)
S–CH S–C CH2 CH@ H9 H10
2.1–3.0(br) 3.85(br), 4.00(br) 4.45(br), 4.65(br) )12.99(s) )12.82(s)
a Chemical shifts in ppm; coupling constants in Hz; room temperature; 1 H and 13 C NMR in CD2 Cl2 . Abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad.
C1 -symmetry of the complexes (Table 2). Thus, for each of the complexes 7, 8 and 9, the olefinic protons of the coordinated cyclooctadiene appear as four multiplets and the two S-alkyl substituents produced separate peaks in the 1 H NMR as expected for heterotopic groups in a C1 -symmetrical structure. Thus, two resonances were observed for the MeS groups of complex 7, while the i PrS groups of complex 8 gave rise to four doublets and two multiplets. The aromatic part of the spectrum is as well consistent with the C1 symmetry. For instance, complex 8 shows twelve separate peaks (roughly eight doublets and four triplets) while in the spectra of 7 and 9 the pattern is less clear-cut due to the presence of some overlap. The protonic spectra of all theses complexes did not show any significant change in the variable temperature experiments in the range between )90 and 25 °C. In particular, hydride, methyl, iso-propyl, and tert-butyl resonances did not undergo any apparent splitting, leading further support to the view that only one diastereoisomer is present. In the 2D-NOESY spectrum of complex 9, the methyl protons of the t BuS group at 0.6 ppm show close contacts with the hydride signal H-9, with the olefinic proton at 4.3 ppm and with the H-8 proton on the naphthalene ring. This last interaction is compatible only with an axial location of this t BuS group. The second t BuS group (1.45 ppm) shows cross peaks with the hydride H-10, with the olefinic proton at 4.7 ppm
and with the H-3 proton of the naphthalene ring. These interactions are consistent with an equatorial location of this second t BuS group. From all these data, an axialequatorial disposition of the substituents on the stereogenic sulfur centres of S,R-absolute configuration can be inferred for complex 9, provided that the seven-membered chelate ring maintains the original distorted twisted chair conformation. The 2D-NOESY spectra of complexes 7 and 8 were less clear-cut and did not allow to draw any sound conclusion on the location of the alkyl substituents. All attempts to isolate these cis-dihydridoiridium complexes led to the isolation of the corresponding olefinic iridium(I) complexes even at )70 °C under a hydrogen atmosphere. From this NMR study, we can confidently draw the conclusion that the oxidative addition of hydrogen to the Ir-complexes 4–6 proceeds with complete stereoselectivity affording in all the cases just one out of the sixteen possible diastereoisomeric dihydrides and that these species maintain their stereochemical identity in solution over a wide range of temperatures. It is worth to stress that this result contrasts with the normal behaviour of iridium complexes with alkylsulfide ligands which upon addition of hydrogen usually provide a mixture of the corresponding dihydrides [1]. This unprecedented result encouraged us to explore the potentiality of these complexes as chiral catalysts in asymmetric hydrogenation.
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4. Asymmetric hydrogenation The iridium complexes 4–6 were tested in the asymmetric hydrogenation of prochiral olefins 10–12. Selected results are shown in Table 3. As a general trend, the complexes displayed a modest-to-fair catalytic activity and led to disappointingly low stereoselectivities. (see Scheme 3). The hydrogenation of itaconic acid 10 was investigated at first. The reaction proceeded slowly at room temperature in methanol or dichloromethane solution under atmospheric pressure of H2 and in most cases the enantioinduction was practically null. The activity of the complex is strongly influenced by the nature of the solvent (entries 1–3), the fastest reaction being observed in a 9/1 mixture of dichloromethane-methanol (entry 3). This increase of the rate was not matched by a parallel improvement of the stereoselectivity the best e.e. recorded was as low as 5%. Using the catalyst prepared in situ by stirring a solution of [Ir(COD)2 ]BF4 and the dithioether ligand did not result in any significant variation of the efficiency of the process (entry 4). Use of onefold excess of ligand (entry 5) or of an increased hydrogen pressure (5 bar, data not shown) did not improve in the catalyst performance (activity and enantioselectivity). Complexes 5 and 6 showed a behaviour similar to the one observed for 4, but both were catalysts of lower activity. The decrease in the hydrogenation rate was particularly pronounced in the case of the iso-propyl derivative 5 (entry 6), whereas the tert-butyl substituted complex 6 was only half as active as the corresponding
Scheme 3. Asymmetric hydrogenation of prochiral olefins by Ir-complexes 4–6.
methyl substituted derivative 4 (compare entries 3 and 7). Similar results were observed in the hydrogenation of dimethyl itaconate 11 and methyl a-acetamido acrylate 12. With these substrates even in the best solvent (dichloromethane) the hydrogenation rates are lower than with itaconic acid. The hydrogenation products were all practically racemic (Table 3). 5. Conclusions The coordination of the dithioether ligands 1–3 to Ir(I)-centres is remarkable in that it occurs stereoselectively producing just one out of the several possible prospective stereoisomers which may be predicted from the coordination of two prostereogenic sulfur donors and from the possible interconversion of conformers of the seven-membered chelate ring. Thus, it appears that the formation of the new stereogenic centres at the Sdonor is effectively steered by the pre-existing chirality of the diaryl framework in such a way that it is selec-
Table 3 Asymmetric hydrogenation of substrates 10–12 with catalytic system 4–6a Entry
Precursor
Substrate
Solvent
% Conversion (t/h)b
%eec
1 2 3 4d 5e 6 7 8 9 10 11 12 13 14 15
4 4 4 4 4 5 6 4 4 4 5 6 4 5 6
10 10 10 10 10 10 10 11 11 11 11 11 12 12 12
CH2 Cl2 MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 /MeOH CH2 Cl2 MeOH CH2 Cl2 /MeOH CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2
65(20) 7(2) 100(0.5) 100(0.5) 100(0.5) 100(6) 100(1) 14(6) 5(6) 2(6) 3(6) 83(6) 2(6) 5(6) 7(6)
1(R) 1(R) 4(R) 3(R) 4(R) 5(R) 1(R) 1(R) 1(R) 1(R) 1(R) 2(R) 1(S) 1(S) 1(S)
a
Substrate/Ir ¼ 100; Ligand/Ir ¼ 1.1, solvent ¼ 6 ml, T ¼ 25 °C. % Conversion measured by 1 H NMR for substrate 10 and by GC for substrates 11–12. c % Enantiomeric excess measured by GC using a Chiraldex-G-TA for substrates 10–11 and a Permabond L-Chirasil-Val column for substrate 12. d The catalyst was prepared in situ by stirring a solution of [Ir(COD)2 ]BF4 and the dithioether ligand. e Ligand/Ir ¼ 2. b
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tively obtained only by the diastereoisomer featuring the same chiral notations at the stereogenic sulfur donors and the anti-disposition of the alkyl substituent on the S-donors. Albeit not unprecedented [1b,6,20], this result is of importance since it provides the appropriate background for the stereochemistry of following step of the catalytic cycle of the hydrogenation of the olefins, i.e., the oxidative addition of hydrogen, to be properly addressed. Much at our surprise, even this reaction apparently proceeds stereoselectively and in all the three cases one single dihydride was observed in solution. Common features of these dihydrides are the unusual stability in solution, which allowed a complete NMR characterization of these species; the cis-arrangement of two g1 -hydrogen ligands, resulting in classical Ir(III)dihydride complexes and the absence of dynamic behaviour due to conformational interconversion. The axial-equatorial location of the alkyl substituents displayed by the t-butyl substituted derivative 9 is probably due to the steric bulk of the substituent which is better accommodated by changing the original stereochemical relation of the alkyl substituents. This particular assembly of substituents apparently exerts some positive influence on the catalytic activity in the hydrogenation of olefins, as the t-butyl containing Ir-complex parallels the activity of the methyl complex and is significantly more active than the i-propyl derivative. In spite of the complete stereoselectivity associated to the above reported steps of the catalytic cycle and of the significative e.e.s recorded with Ir-catalysts of similar design, the hydrogenation of a; b-unsaturated acid derivatives with the title Ir(I)-complexes is practically devoid of stereoselectivity. As of now we cannot settle these differences, further work seems necessary for the reasons of this dichotomic behaviour to be cleared.
Acknowledgements This work was carried out in the frame of the EU project COST-D24 (0003/01). The Spanish group thanks Ministerio de Educaci on, Cultura y Deporte (BQU2001–0656) for financial support and the undergraduate student Laetitia Bertrand (France) for her valuable assistance in the synthesis and characterization of complexes 5 and 8. Financial support from MIUR (PRIN 2003033857) and from the University of Sassari is gratefully acknowledged by S.G.
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