New Low-Temperature NMR Studies Establish the Presence of a

Jul 6, 2010 - Synopsis. The important enantioselective hydroformylation catalyst [(R,S)-Binaphos](CO)2RhH is a mixture of two rapidly interconverting ...
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Organometallics 2010, 29, 3362–3367 DOI: 10.1021/om100323q

New Low-Temperature NMR Studies Establish the Presence of a Second Equatorial-Apical Isomer of [(R,S)-Binaphos](CO)2RhH )

Dante A. Castillo Molina,† Charles P. Casey,‡ Imke M€ uller,§ Kyoko Nozaki,^ and ,†, Christoph J€akel* †

)

Catalysis Research Laboratory (CaRLa), Ruprecht-Karls-Universit€ at Heidelberg, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany, ‡Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, §BASF SE, GVC/S-B009, 67056 Ludwigshafen, Germany, ^Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656, Japan, and BASF SE, CZE/DD-E100, 67056 Ludwigshafen, Germany Received April 20, 2010

The important enantioselective hydroformylation catalyst [(R,S)-Binaphos](CO)2RhH has been reexamained by low-temperature NMR spectroscopy. Both 1H and 31P NMR spectroscopy at -90 C allow direct observation of a mixture of two apical-equatorial chelates. The major chelate, 1eq,ap, was shown to have an equatorial phosphine and an apical phosphite. Its structure was unambiguously assigned using low-temperature 31P NMR spectroscopy with selective decoupling of aromatic hydrogens but not of the rhodium hydride, which showed a 225 Hz trans phosphite to RhH coupling. The equilibrium constant for [1eq,ap]/[1ap,eq] was determined over a wide temperature range. Introduction The preparation of enantiomerically pure compounds for use as drugs or agrochemicals has become increasingly important in the last decades, as it has often been observed that the use of only a single enantiomer of a racemic compound provides the desired level of biological activity.1 In this sense, the synthesis of complex molecules with pharmaceutical properties through the use of asymmetric hydroformylation has received increasing attention in recent years with the development of new, clean, and atom-efficient synthetic routes.2 In spite of its evident advantages and importance, asymmetric hydroformylation of olefins was not intensively studied in the years after its discovery in 1972,3 as the results were still far from an industrial application due to difficulties in controlling the regio- and enantioselectivities and the limited substrate scope for any single ligand. Thus, early efforts were focused on the preparation of catalytic systems able to achieve high selectivity for the desired molecules.4 A historically important breakthrough in the rhodiumcatalyzed asymmetric hydroformylation came in 1993 with *To whom correspondence should be addressed. E-mail: christoph.jaekel@ basf.com. (1) Powell, J. R.; Ambre, J. J.; Ruo, T. I. The Efficacy and Toxicity of Drug Stereoisomers. In Drug Stereochemistry; Wainer, I. W.; Drayer, D. E., Eds.; Marcel Dekker: New York, 1988; pp 245-270. (2) (a) Botteghi, C.; Paganelli, S.; Schionato, A.; Marchetti, M. Chirality 1991, 3, 355. (b) Chaudhari, R. V. Curr. Opin. Drug Discovery Dev. 2008, 11, 820. (3) Botteghi, C.; Consiglio, G.; Pino, P. Chimia 1972, 26, 141. (4) For reviews in asymmetric hydroformylation, see: (a) Gladiali, S.; Bay on, J. C.; Claver, C. Tetrahedron: Asymmetry 1995, 6, 1453. (b) Agbossou, F.; Carpentier, J.-F.; Mortreux, A. Chem. Rev. 1995, 95, 2485. (c) Breit, B.; Seiche, W. Synthesis 2001, 1. (d) Breit, B. Acc. Chem. Res. 2003, 36, 264. (e) Claver, C.; Dieguez, M.; Pamies, O.; Castillon, S. Top. Organomet. Chem. 2006, 18, 35. (f) Klosin, J.; Landis, C. R. Acc. Chem. Res. 2007, 40, 1251. pubs.acs.org/Organometallics

Published on Web 07/06/2010

the development of the (R,S)-Binaphos ligand (Scheme 1, left top).5 (R,S)-Binaphos was created with the aim of combining the high stereocontrol achieved with the rigid BINAP ligand with the high activity of a phosphite ligand. Since then, this phosphine-phosphite ligand has been successfully employed in the asymmetric hydroformylation of a large variety of substrates to achieve high activity and stereoselectivity.6 Hence, (R,S)-Binaphos has been considered a benchmark ligand in this area. The initial NMR spectral studies of [(R, S)-Binaphos](CO)2RhH were interpreted as resulting from the presence of a single species with an apical-equatorial chelate with the phosphite ligand in an apical position trans to hydride.5,7 The presence of a single species was thought to be an important contributor to the high enantioselectivity achieved with (R,S)-Binaphos. Further investigations on asymmetric hydroformylation have focused on improving the occasional low regioselectivity of (R,S)-Binaphos by designing related phosphinephosphite ligands (Scheme 1).8-13 Unfortunately, the rhodium-catalyzed hydroformylations with these ligands often (5) Sakai, N.; Mano, S.; Nozaki, K.; Takaya, H. J. Am. Chem. Soc. 1993, 115, 7033. (6) Nozaki, K. Chem. Rec. 2005, 5, 376. (7) Nozaki, K.; Sakai, N.; Nanno, T.; Higashijima, T.; Mano, S.; Horiuchi, T.; Takaya, H. J. Am. Chem. Soc. 1997, 119, 4413. (8) Kless, A.; Holz, J.; Heller, D.; Kadyrov, R.; Selke, R.; Fischer, C.; B€ orner, A. Tetrahedron: Asymmetry 1996, 7, 33. (9) Deerenberg, S.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Organometallics 2000, 19, 2065. (10) Pamies, O.; Net, G.; Ruiz, A.; Claver, C. Tetrahedron: Asymmetry 2001, 12, 3441. (11) Arena, C. G.; Faraone, F.; Graiff, C.; Tiripicchio, A. Eur. J. Inorg. Chem. 2002, 711.  (12) Rubio, M.; Suarez, A.; Alvarez, E.; Bianchini, C.; Oberhauser, W.; Peruzzini, M.; Pizzano, A. Organometallics 2007, 26, 6428. (13) Robert, T.; Abiri, Z.; Wassenaar, J.; Sandee, A. J.; Romanski, S.; Neud€ orfl, J.-M.; Schmalz, H.-G.; Reek, J. N. H. Organometallics 2010, 29, 478. r 2010 American Chemical Society

Article Scheme 1. (R,S)-Binaphos5,7 and Examples of PhosphinePhosphite Ligands, As Reported by Faraone (A),11 B€orner (B),8 Pizzano (C),12 Ruiz (D),10 van Leeuwen (E),9 and Reek (F)13

provided only low to moderate enantioselectivities. On the other hand, Zhang has used Binaphos as a model in designing a new series of phosphine-phosphoramidite ligands that can be employed in highly regio- and enantioselective hydroformylations.14 The performance of the new phosphine-phosphite ligands in catalysis has frequently been related to the coordination mode in the corresponding active species (phosphinephosphite)(CO)2RhH.7,9-12 Apical-equatorial coordination is commonly seen, and mixtures of isomers with either apical phosphite, Ieq,ap, or apical phosphine, Iap,eq (Scheme 2), or with a predominance of one isomer have been seen. It is now known that Ieq,ap and Iap,eq rapidly equilibrate at room temperature and give rise to exchange-averaged NMR spectra. The best evidence for the presence of a mixture of isomers comes from direct observation of two species by low-temperature 1 H or 31P NMR spectroscopy. In addition, a strong temperature dependence of the two JPH coupling constants provides evidence for temperature-dependent equilibria between the two isomers. The JPH coupling constants are sensitive to the ratio (14) Zhang, X.; Cao, B.; Yan, Y.; Yu, S.; Ji, B.; Zhang, X. Chem.; Eur. J. 2010, 16, 871.

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Scheme 2. Observed Coordination Modes for (Phosphine-Phosphite)(CO)2RhH

of isomers since trans-HRhP coupling is much larger than cis-HRhP (and usually of opposite sign). The assignment of the major conformer requires determination of whether the large JPH is associated with the phosphite or phosphine ligand. This requires measurement of a low-temperature 31 P NMR spectrum without proton decoupling or with selective decoupling of aromatic hydrogens but not the rhodium hydride. In 2000, van Leeuwen spectrally characterized (ligand E)(CO)2RhH as a single Iap,eq isomer with an apical phosphine,9 whereas Nozaki had proposed an Ieq,ap configuration with an apical phosphite for [(R,S)-Binaphos](CO)2RhH.5,7 van Leeuwen’s low-temperature 1H and 31P NMR spectra showed a single isomer having temperature-independent JPH coupling constants of 102 and 2 Hz; observation of 1H NMR spectra with selective decoupling of either the phosphine or phosphite phosphorus established that the large coupling was to the phosphine, which therefore must be trans to H in an Iap,eq conformation. This favored conformation is consistent with the observation that, for electronically dissymmetric diphosphines, the phosphorus atom bearing electronwithdrawing substituents prefers the equatorial position.15 In 2001, Ruiz studied (ligand D)(CO)2RhH by room-temperature 1 H and 31P NMR spectroscopy; intermediate values of JPH coupling constants of 76 and 33 Hz provided evidence for a rapidly equilibrating mixture of Iap,eq and Ieq,ap conformers.10 In the absence of low-temperature NMR and of decoupling experiments, the major isomer was suggested to be Iap,eq, having an apical phosphine. In 2007, Pizzano found temperaturedependent JPH coupling constants for (ligand C)(CO)2RhH complexes that provided evidence for a mixture of Iap,eq and Ieq,ap conformers; in the absence of decoupling experiments, the major conformer was suggested to be Iap,eq, with an apical phosphine.12 In 2010, Reek found intermediate room-temperature JPH coupling constants of 95 and 45 Hz for (ligand F)(CO)2RhH; at -50 C, the coupling constants changed substantially and the hydride signal broadened.13 These observations require a rapidly equilibrating mixture of Iap,eq and Ieq,ap conformers. Reek used 1H-coupled 31P NMR spectroscopy to establish that the phosphine has the larger JPH and therefore is trans to RhH in the major conformer (Iap,eq). In light of these studies showing that the better electron donor phosphine ligand preferentially occupied the apical position, we thought it important to reexamine the evidence behind Nozaki’s assignment of [(R,S)-Binaphos](CO)2RhH as a single 1eq,ap conformer with an apical phosphite (Scheme 3).5,7 Examination of the published data leads to the following picture. The room-temperature 1H NMR spectrum of [(R,S)Binaphos](CO)2RhH (1) in C6D6 showed a single hydride resonance with JPH coupling constants of 160 and 23 Hz;7 at the minimum, this shows the predominance of one apicalequatorial chelate over another. The room-temperature 31P NMR in CDCl3 (obtained with decoupling of the aromatic, (15) Casey, C. P.; Paulsen, E. L.; Beuttenmueller, E. W.; Proft, B. R.; Matter, B. A.; Powell, D. R. J. Am. Chem. Soc. 1999, 121, 63.

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Castillo Molina et al. Scheme 3

Figure 1. 1H and 31P{1H} NMR spectra (left and right, respectively) for complex 1 recorded at 25 C (top) and -90 C (bottom) in toluene-d8.

but not rhodium hydride resonances, which was not clearly stated in the communication) showed a phosphite resonance at δ 183.5 with JPH = 159 Hz and a phosphine resonance at δ 25.5 with JPH = 21 Hz;5 this provided evidence that the major (or only) species has the phosphite in an apical position trans to the rhodium hydride (1eq,ap). A variable-temperature 31P{1H} NMR spectrum in toluene-d8 exhibited two phosphorus resonances between room temperature and 60 C, broadening of the resonances at -50 C, and resharpening of the resonances at -90 C.5,7 In the communication, the authors suggested that this was due to a fluxional process, implying the presence of two isomers;5 but in the full paper,7 the authors cited the spectra as evidence for a single species and the “reason for the peak broadening is unclear”. Due to the unique behavior of complex 1, the significance of (R,S)-Binaphos in the design of efficient ligands for asymmetric hydroformylation in subsequent years, and our interest in catalytic hydroformylation,16 we decided to reinvestigate the NMR spectroscopy of 1. Here we report direct observation of a mixture of two apical-equatorial chelates by low-temperature 1H and 31P NMR spectroscopy, unambiguous assignment of the major chelate as 1eq,ap from low-temperature 31P NMR spectra with selective decoupling of aromatic hydrogens but not of the rhodium hydride, and determination of the equilibrium constant for [1eq,ap]/[1ap,eq] over a wide temperature range.

Results Complex 1 was prepared by reaction of (R,S)-Binaphos with Rh(acac)(CO)2 in toluene-d8 to give [(R,S)-Binaphos](16) Scheuermann nee Taylor, C. J.; Jaekel, C. Adv. Synth. Catal. 2008, 350, 2708.

(acac)Rh followed by exposure to CO/H2 as described previously.5,7 The 1H and 31P{1H} NMR spectra of 1 at room temperature (Figure 1, top; Table 1) were similar to those reported by Nozaki.7 The 500 MHz 1H NMR spectrum showed a single hydride resonance at δ -9.06 (ddd, 2JPH = 161 Hz, 2JP’H = 22 Hz, 1JRhH = 9 Hz), and the 203 MHz 31 P{1H} NMR spectrum exhibited two different resonances at δ 184.3 (dd, 1JRhP = 181 Hz, 2JPP =39 Hz) for the phosphite and at δ 26.5 (dd, 1JRhP=120 Hz, 2JPP=39 Hz) for the phosphine. The greater sensitivity of the 500 MHz spectrometer enabled the detection of a second isomer of 1 at -90 C (Figure 1, bottom; Table 1). A 6:1 ratio of two hydride resonances was detected at δ -8.28 (br d, 2JPH =225 Hz) and -9.46 (br d, 2 JPH =118 Hz); 1JRhH coupling and a smaller 2JPH were not observed due to broadening of the signals. Similarly, the 31 P{1H} NMR spectrum provided additional evidence for two isomers of 1; two phosphite resonances at δ 188.4 (br s, major) and 182.9 (br s, minor) and two phosphine resonances at δ 27.9 (br s, minor) and 23.4 (dd, 2JPP = 24 Hz, 1JRhP = 126 Hz, major) were observed. Nozaki’s earlier low-temperature 31P NMR spectra of 1 had not allowed detection of the small phosphorus resonances of the minor isomer due to their low intensity and diffuseness.7 On the basis of previous observations of two isomers of (phosphine-phosphite)(CO)2RhH complexes,10-12 we assigned the structures of the two species as the apical-equatorial and equatorial--apical isomers 1ap,eq and 1eq,ap and set out to determine which was the major isomer. We switched to the lower melting solvent CD2Cl2 to obtain better resolved low-temperature NMR spectra. The roomtemperature 1H and 31P{1H} NMR spectra of 1 in CD2Cl2

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Table 1. Summary of Coupling Constants for Complex 1 Obtained from Variable-Temperature NMR Experimentsa 2

temp (C)

JPH

2

JPO3H

2

1

JPHb

JRhH

2

JPPO3

1

JRhPO3

1

JRhP

1

JRhPO3b

Toluene-d8 25 -90

1

H 31 P{1H} 31 P{1H}c 1 H 31 P{1H}

22

161

21

156 225

9 39 39

181 183

120 120

118 24

126

CD2Cl2 25

1

H P{1H} P{1H}c 1 H 31 P{1H} 1 H 31 P{1H} 31 P{1H}c 31 31

-90 -100

27

150

10

27 -11

146 226

117

11

-11

225

117

11

-9

224

38 38

185 185

117 118

34

154

127

225

34 34

144 144

127 127

230 224

a Values in Hz for the mixture of conformers (at 25 C) or the major species (at lower temperatures) unless noted; P = phosphine and PO3 = phosphite. b Corresponding to the minor species. c Partially decoupled 31P{1H} NMR.

Figure 2. Fully decoupled 31P{1H} and partially decoupled 31P{1H} NMR spectra (left and right, respectively) for complex 1 recorded at -100 C in CD2Cl2. Note the additional large JPH coupling in the δ 187 phosphite resonance of the major isomer and the greater line width of the δ 27 phosphine resonance of the minor isomer when the Rh hydride is not decoupled.

were similar to those obtained in toluene-d8 (Table 1 and Supporting Information). While a 6:1 ratio of conformers was seen by 1H NMR in toluene-d8 at -90 C, a 3:1 ratio was observed in CD2Cl2 at -90 C.17 To establish the nature of the donor atom trans to hydride in the two conformers, the fully decoupled 31P{1H} NMR spectrum and the 31P NMR spectrum with selective decoupling of the aromatic hydrogens but not of the rhodium hydride were obtained at -100 C in CD2Cl2 (Figure 2, Table 1). In the partially decoupled spectrum, the phosphite resonance of the major isomer at δ 187.3 has an additional very large coupling to the rhodium hydride (224 Hz), consistent with a large phosphite to trans rhodium hydride coupling, and the phosphine resonance of the major isomer at δ 21.2 has only a small additional coupling (-9 Hz), consistent with a small phosphine to cis rhodium hydride coupling. Taken together, this provides conclusive evidence that the major conformer is 1eq,ap, having an apical-equatorial chelate with the phosphite trans to an apical hydride and an equatorial phosphine cis to the hydride. Similarly, in the partially decoupled spectrum, the phosphine resonance of the minor isomer at δ 26.5 is significantly broader (ω1/2 increases to 228 Hz from 177 Hz), while the phosphite resonance of the minor isomer at δ 182.4 shows no significant additional broadening (ω1/2 367 Hz compared with 376 Hz with full decoupling). This is consistent with a large phosphine to trans rhodium hydride coupling and a small phosphite to cis rhodium hydride coupling in the minor conformer. The minor conformer is therefore assigned as (17) This solvent-dependent change in equilibrium constant reflects a small energy difference (ΔΔG = 1.0 kJ mol-1).

1ap,eq, with an apical-equatorial chelate with the phosphine trans to an apical hydride and an equatorial phosphite cis to the hydride. Variable-temperature 1H NMR spectra of 1 between 45 and -90 C provided evidence for interconversion of the two species (Figure 3). The multiplet for the rapidly equilibrating mixture of 1eq,ap and 1ap,eq seen at room temperature began to broaden at -5 C and split into two separate multiplets at -50 C. Upon further cooling to -80 C, the separate multiplets assigned to 1eq,ap and 1ap,eq sharpened. Integration of the separate hydride multiplets at low temperature allowed measurement of the equilibrium constant. Measurement of the average 2JPH coupling constants of the hydride resonance above 10 C allowed determination of Keq at these temperatures.18 In a rapid equilibrating mixture of 1eq,ap and 1ap,eq, the observed coupling constants are the weighted average of the coupling constants of the two isomers and can be used to compute the mole fraction of 1eq,ap and 1ap,eq present;15 for example, at 10 C,

ð2 JPH at 10 CÞ ¼ χ1ap, eq ð2 JPH in 1ap, eq at - 100 CÞ þ χ1eq, ap ð2 JPH in 1eq, ap at - 100 CÞ The equilibrium constant varied from 2.8 at -100 C to 2.3 at 45 C; a van’t Hoff plot gave ΔH = -0.6 kJ mol-1 and ΔS = 4.9 J K-1 mol-1.18 The similar stability of 1eq,ap and 1ap,eq is supported by density functional theory calculations at the B3LYP/def-TZVP//BP86/ (18) See Supporting Information.

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Figure 3. Hydride region of 1H NMR spectra of 1 in CD2Cl2 recorded from 45 to -90 C.

def-SV(P) level of theory. 1eq,ap was computed to be -2.8 kJ mol-1 (ΔH) more stable than 1ap,eq, clearly indicating the very similar stability of the two conformers.

Discussion The use of low-temperature NMR was critical in finding evidence for two equatorial-apical conformers of [(R,S)Binaphos](CO)2RhH (1), since the two conformers are in rapid equilibrium and only an exchange-averaged spectrum is obtained at room temperature. In some cases, temperature dependence of two JPH coupling constants can provide evidence for two isomers, but in the case of 1, there was no significant change in coupling constants. When intermediate values of JPH are observed, they provide evidence for an equilibrating mixture of equatorial-apical conformers. Unequivocal assignment of the structure of an equatorialapical conformer requires knowledge of the JPH values for each ligand. This information can be obtained by low-temperature 31 P NMR with selective decoupling of the aromatic hydrogens but not RhH resonances or by 1H NMR with selective decoupling of either the phosphite or phosphine 31P resonance. The observation that the JPH for a phosphite trans to a RhH is much larger (225 Hz) than for a phosphine trans to a RhH (117 Hz) could be useful in structure determination. The results reported here are fully consistent with Nozaki’s earlier room-temperature 1H and 31P NMR spectra,5,7 where the two JPH coupling constants (159 and 21 Hz) are now understood as the weighted average of couplings for the two conformers 1eq,ap and 1ap,eq. The temperature dependence of Nozaki’s 31P NMR spectra7 can now be seen as exchangeaveraged resonances at room temperature and above, exchange-broadened spectra at -50 C, and then sharpening of resonances of the major conformer in the slow exchange limit at -90 C, where the low signal-to-noise ratio precluded observation of the minor conformer.

Castillo Molina et al.

Qualitative bonding and steric effect concepts provide a framework for understanding the relative stabilities of the equatorial-apical and apical-equatorial conformers of phosphine-phosphite chelates in d8 trigonal-bipyramidal complexes such as 1eq,ap and 1ap,eq. Hoffmann’s extended H€ uckel calculations showed that in trigonal-bipyramidal d8 low-spin complexes the apical bonds are stronger, that stronger σ-donors prefer the apical site, and that π-interactions are greatest for equatorial substituents.19 This accounts for the strong preference for the hydride ligand to occupy an apical position. As a stronger σ-donor, a phosphine would be expected to prefer an apical site, but this preference is diminished (or possibly reversed) by interaction with the trans hydride. In the valence bond description of trigonal-bipyramidal d8 complexes only the energetically accessible 4s and 3d metal orbitals are employed; two sd hybrid orbitals are used for σ-bonding and four filled d-orbitals are used for π-backbonding.20 The apical ligands in a trigonal bipyramid are bound more strongly with three-center-four-electron bonding than the equatorial ligands, with weaker four-centersix-electron bonding; consideration of σ-bonding only and the strong trans influence of the H ligand should yield a slight preference for phosphite in the apical position and phosphine in the equatorial position. However, π-bonding is better to ligands in the equatorial plane that accept electrons from three filled d-orbitals than to apical ligands that accept electrons from only two filled d-orbitals; this would favor placing the phosphite in the equatorial position. Overall, the balance of σ- and π-bonding effects makes prediction of the major conformer a difficult call. Thus, electronic effects are probably not large and must compete with steric effects. The apical position in 1 is more crowded (it has 2 CO’s at ∼90) than the equatorial position (it has 2 CO’s at ∼120 and a negligible interaction with the sterically very small hydride at ∼90). Therefore, the sterically larger phosphines will prefer equatorial sites. For Binaphos complexes 1eq,ap and 1ap,eq, the observation of a small preference for 1eq,ap over 1ap,eq is consistent with a steric preference for phosphine in an equatorial position together with a difficult to assess electronic effect. In summary, both qualitative consideration of steric and electronic effects and quantitative DFT computations are consistent with the similar experimentally observed stability of 1eq,ap and 1ap,eq. Previously, the very high enantioselectivities obtained using (R,S)-Binaphos were attributed in part to the presence of only a single conformer of the catalyst (1eq,ap). Since we now know that significant amounts of both 1eq,ap and 1ap,eq are present and their ratio depends on solvent, explanations will necessarily be complicated. To more fully understand the origin of enantioselectivity, one needs to know which step in the hydroformylation is irreversible and product determining. Nozaki’s kinetic studies of [(R,S)-Binaphos]/Rh hydroformylation showed no rate dependence on H2 pressure and inhibition by CO pressure, and deuterioformylation studies that showed no label incorporation into recovered alkene are consistent with product-determining irreversible hydride addition to coordinated alkene.21 It will also be important (19) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365. (20) Landis, C. R.; Firman, T. K.; Root, D. M.; Cleveland, T. J. Am. Chem. Soc. 1998, 120, 1842. (21) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Organometallics 1997, 16, 2981.

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to know which (R,S)-Binaphos conformer is the major species in this step and which conformer reacts faster. To understand the complexity of the problem, consider the possibility that hydride migration to [(R,S)-Binaphos](CO)(alkene)RhH is product determining. There are eight different alkene complexes that need to be considered: either equatorial-apical or apical-equatorial conformations of the chelate, two different equatorial sites for alkene coordination, and two different alkene enantiofaces to coordinate.18 The relative amounts of these eight isomers and their relative rates of irreversible alkene insertion into the Rh-H bond will determine the observed enantioselectivity. It should be recalled that cases exist where the minor isomer in catalysis can be so kinetically active that it controls the observed enantioselectivity; a prime example of this is the rhodium-chiraphos-catalyzed hydrogenation of enamides.22

Experimental Section General Procedures. All experiments were carried out under argon unless noted. High-pressure NMR spectra were recorded using a 5 mm OD quick pressure valve (QPV) NMR tube on a Bruker DRX 500 MHz FT spectrometer at 25 C unless noted. 1 H NMR spectra were referenced to the residual solvent signal (1H: CHDCl2, δ 5.32; toluene-d7, δ 2.03). (22) (a) Chan, A. S. C.; Pluth, J. J.; Halpern, J. J. Am. Chem. Soc. 1980, 102, 5952. (b) Landis, C.; Halpern, J. J. Am. Chem. Soc. 1987, 109, 1746. (c) Feldgus, S.; Landis, C. R. J. Am. Chem. Soc. 2000, 122, 12714. (23) Ahlrichs, R.; B€ar, M.; H€aser, M.; Horn, H.; K€ olmel, C. Chem. Phys. Lett. 1989, 162, 165. (24) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. Phys. Rev. B 1986, 33, 8822 (erratum: 1986, 34, 7406). (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (25) Sch€ afer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.

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Chemicals were treated as follows: CD2Cl2 and toluene-d8 (Aldrich) were stored over molecular sieves; Rh(acac)(CO)2 (Acros) and CO/H2 (50:50, Air Liquide) were used as received. (R,S)-Binaphos was prepared as published.7 High-Pressure NMR Experiments. A 5 mm OD QPV NMR tube was charged with Rh(acac)(CO)2 (0.007 g, 0.03 mmol), (R, S)-Binaphos (0.020, 0.026 mmol), and toluene-d8 or CD2Cl2 (0.5 mL). Intensive bubbling was observed and a yellow solution formed. After three freeze-pump-thaw cycles, the NMR tube was charged with CO/H2 (50:50, 4 bar) and sealed. After 2 h at room temperature, the NMR spectra were recorded. DFT Calculations of Complex 1. The geometry optimization and energy calculations were performed with the Turbomole program suite.23 Geometries were optimized at the BP86/defSV(P)24,25 level of theory within the efficient RI-J approximation.26 Single-point energies were obtained from these structures at the B3LYP/def-TZVP27,28 level of theory. Vibrational analysis was done at the BP86/def-SV(P) level of theory. No imaginary frequencies were obtained. Enthalpies were obtained from the Turbomole program suite.

Acknowledgment. The work of D.A.C.M. and C.J. at CaRLa of Heidelberg University was co-financed by Heidelberg University, the state of Baden-W€ urttemberg, and BASF. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. € (26) Eichkorn, K.; Treutler, O.; Ohm, H.; H€aser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283 (erratum: 1995, 242, 652). (27) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can J. Phys. 1980, 58, 1200. (28) Sch€afer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.