ARTICLE pubs.acs.org/Organometallics
Asymmetric Transfer Hydrogenation of Imines and Ketones Using Chiral RuIICl(η6-p-cymene)[(S,S)-N-TsDPEN] as a Catalyst: A Computational Study Jirí Vaclavík,*,† Marek Kuzma,*,‡ Jan Prech,† and Petr Kacer† † ‡
Department of Organic Technology, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic Laboratory of Molecular Structure Characterization, Institute of Microbiology, v.v.i., Academy of Sciences of the Czech Republic, Víde nska 1083, 142 20, Prague 4, Czech Republic
bS Supporting Information ABSTRACT: Density functional theory (DFT) computational methods were used to investigate the increasingly popular ionic mechanistic concept for the asymmetric transfer hydrogenation of imines on the chiral catalyst RuIICl(η6-p-cymene)[(S,S)N-p-tosyl-1,2-diphenylethylenediamine]. On application of the ionic mechanism, the reaction preferentially affords the (R)-amine product, which is in agreement with the experimental observations. Calculated transition state structures for the hydrogenation of protonated 1-methyl-3,4-dihydroisoquinoline are discussed together with their preceding and following energy minima. Stabilization of the favorable transition state by a CH/π interaction between the η6-p-cymene ligand and the substrate molecule is explored in depth to show that both C(sp2)H/π is more probable than C(sp3)H/π in this molecular system. Finally, transition state geometries for the asymmetric transfer hydrogenation of acetophenone are proposed, which take the “standard” six-membered cyclic form.
’ INTRODUCTION The asymmetric transfer hydrogenation (ATH) of imines has remained a rather unexplored topic since the first successful approach disclosed by Noyori et al. in 1996, despite its indubitable usefulness, especially in pharmaceutical technology.1 Noyori showed that the Ru(II)Cl(η6-p-cymene)[N-p-tosyl-1,2-diphenylethylenediamine] ([RuCl(η6-p-cymene)TsDPEN] 1) catalyst in a HCOOH/triethylamine mixture was able to efficiently hydrogenate substituted isoquinolines with great selectivity.2 Almost simultaneously, the same system was reported to reduce ketones superbly,3 suggesting that the mechanisms of asymmetric hydrogenation of CdN and CdO bonds should be alike.4 The mechanism of the asymmetric reduction of ketones was extensively discussed by Noyori and co-workers in a computational study in 2001, which proposed that the reaction proceeds via six-membered transition states (TSs) in the outer coordination sphere of ruthenium.5 However, this mechanistic concept is not compatible with the transfer hydrogenation of imines, as pointed out by Wills et al.6 on the basis of their experimental results and previous work (see references therein). According to the original mechanism, the (S,S)-2 complex would give an (S)configured product, which conforms with the results for ketones but disagrees with experimental observations for the asymmetric reduction of imines. The key element explaining this contrast seems to be the fact that an imine can only be reduced under acidic conditions, which supports the notion of requisite imine protonation,7 even though this is still not entirely confirmed.8,9 In accordance with the latest evidence regarding the ketone and imine reduction mechanistic pathways,9 11 both posited r 2011 American Chemical Society
mechanisms are outlined in Schemes 1 and 2. The first step is a gradual transformation of the Ru Cl species 1 to a hydride Ru H 2 which may occur in two ways. One involves the abstraction of an HCl molecule by the base present, which was initially proposed by Noyori.12 However, due to the presence of formic acid in the solution, the base (triethylamine) is protonated to a great extent and the transformation of 1 to 2 might therefore proceed in another manner, commencing with an equilibriumcontrolled dissociation of 1 to a solvate 4.10,11 Nonetheless, the mechanism of formation of Ru hydride 2 from the precatalyst 1 has so far been investigated only in the KOH/methanol or 2-propanol hydrogenation systems5,13 and the usage of the HCOOH/triethylamine H2-donor mixture has remained unexplored. Hence, Schemes 1 and 2 only suggest possible routes according to latest findings. Scheme 1 focuses on transfer hydrogenation of acetophenone (5), affording chiral 1-phenylethanol (6), which follows the wellknown mechanistic concept published earlier.5,13 The substrate molecule 5 approaches the Ru hydride 2 and is hydrogenated to 6 by transferring both the proton and hydride from 2, which leads to formation of the 16e Ru complex 3. Scheme 2 reflects the process of hydrogenation of 1-methyl3,4-dihydroisoquinolinium (8), affording chiral 1-methyl-1,2,3,4tetrahydroisoquinoline (9). This proceeds by a hydride transfer only, giving a cationic structure of the solvate 4, which is assumed Received: March 25, 2011 Published: August 26, 2011 4822
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Organometallics Scheme 1. Suggested Pathways for Asymmetric Transfer Hydrogenation of Acetophenone (5) Using 1 in the Presence of Triethylamine (TEA) and Formic Acida
a
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Scheme 2. Suggested Pathways for Asymmetric Transfer Hydrogenation of 1-Methyl-3,4-Dihydroisoquinoline (7) Using 1 in the Presence of Triethylamine (TEA) and Formic Acida
The chelate phenyl substituents are omitted for the sake of clarity.
to create an ion pair with a chloride anion present in the solution, while the Ru atom is believed to be solvated.10 Given the recent evidence for an ionic mechanism of imine hydrogenation and the fact that it has not yet been examined computationally, this paper focuses on a full-scale molecular modeling of the system, mainly exploring the imine pathway and comparing it with a different (ketone) reduction mechanism.
’ RESULTS AND DISCUSSION The aim of this study was to investigate the currently popular ionic mechanism by means of computational chemistry. The structure optimizations, frequency analyses, and energy calculations were conducted using density functional theory (DFT) at the B3LYP level. However, single-point energy values were also obtained employing the Møller Plesset perturbation (MP2) electron correlation method, as the energies obtained this way were considered more precise and reliable. Hence, all singlepoint energies were calculated at both the MP2//DFT and DFT//DFT levels. The ruthenium compounds have been calculated without structural simplifications, and therefore they have the potential to provide plausible steric information for the system. Since such in-depth calculations result in demanding hardware requirements, the simplest imine substrate, 1-methyl-3,4-dihydroisoquinoline (7), was selected for this proof-of-concept mechanistic study. Scheme 3 shows the resulting MP2//DFT (DFT//DFT) energy profiles of asymmetric transfer hydrogenation of 7 in the 1/HCOOH-triethylamine system, proceeding via an ionic pathway. The energy plots are complemented with values of Gibbs free energy, and all energy values are summarized in Table 1. In the following section, each step of the reaction coordinate is discussed in detail. Step I, which represents the isolated Ru hydride molecule 2 and the protonated isoquinolinium substrate 8 (Figure 1), was assigned the reference energy value of 0 kJ/mol, and the rest of
a
The chelate phenyl substituents are omitted for the sake of clarity.
the diagram was plotted relative to this step. The model of Ru hydride 2 was obtained by a DFT structure optimization using the coordinates from a single-crystal X-ray structure of 1.2 However, given that the η6-p-cymene ligand may adopt different orientations, several options needed to be taken into account: namely, those where the methyl group was close to the hydride atom (as in structure 2, which was further used for modeling of transition states) or where the 2-propyl was at the hydride side (2b, which had been suggested by X-ray structure analyses2,12 but not considered optimal for use in the models of transition states, vide infra). Another Ru hydride geometry, 2c, was obtained using X-ray diffraction data published earlier,12 although the orientation of the p-toluenesulfonyl group was rather disadvantageous, since it would most probably sterically hinder the approach of the substrate molecule to the complex active site. As a result, structure 2 (Figure 1) was chosen as a representative Ru hydride geometry to be used in step I and further. It needs to be said that all three geometries considered (2, 2b, 2c) differed in their MP2//DFT single-point energies by no more than an almost negligible value of 1.01 kJ/mol, which did not give exclusive priority to any of these. For the geometries of 2b and 2c, please refer to the Supporting Information. In comparison with the following steps, the energy of step I is considerably higher, which can be explained by the consideration of the isolated molecule of the protonated imine 8 in the 4823
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Scheme 3. Resulting Single-Point Energy Profiles at the Levels MP2/6-31G(d,p)/LANL2DZ//B3LYP/6-31G(d,p)/LANL2DZ (i.e. MP2//DFT, Solid Line) and B3LYP/6-31G(d,p)/LANL2DZ//B3LYP/6-31G(d,p)/LANL2DZ (i.e. DFT//DFT, Dashed Line)a
a The diagrams outline possible reduction pathways proceeding either via transition state fav11a, employing a C(sp3)H/π interaction (route A), or via fav11b, stabilized by a C(sp2)H/π interaction (route B). DFT//DFT computed Gibbs free energies are also given, showing the differences between routes A and B. The corresponding structures are given at the bottom, together with their MP2//DFT (DFT//DFT) energies in kJ/mol relative to step I.
calculations. Naturally, the imine favors forming a salt with the formate anion, which significantly contributes to its stabilization. Having compared the MP2//DFT single-point energies of the salt (i.e., 1-methyl-3,4-dihydroisoquinolinium formate) and isolated ions (the 1-methyl-3,4-dihydroisoquinolinium cation 8 and the formate anion), a striking difference in energy (563.56 kJ/ mol) was obtained in favor of the iminium formate salt. Furthermore, molecule 8 was calculated in the same way as all the structures appearing in the mechanism and thus lacks any form of
solvation which would further stabilize it. To conform to this expectation, the cationic molecule 8 was reoptimized using the IEFPCM14 method of solvation in acetonitrile, which lowered its MP2//DFT single-point energy by 210.21 kJ/mol. Therefore, both salt formation and solvation stabilize the cationic species 8 to a great extent, and the deliberate omission of these factors explains the high energy of step I. However, this step was not deemed crucial in the reaction mechanism and, thus, the imine cation instability was not examined further. 4824
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Table 1. Values of Relative MP2//DFT and DFT//DFT Total Electronic and Relative DFT//DFT Gibbs Free Energies of the Individual Steps of Routes A and B step I
II
III
IV
V
77.5
87.4
92.6
83.6
88.3
32.6
42.6
40.9
44.9
32.1 29.1
31.1 25.8
64.1
77.5
87.4
92.6
84.0
86.5
32.6
42.6
45.1
48.1
32.1
31.1
Route A rel energy (MP2//DFT), kJ/mol 0.0 rel energy (DFT//DFT), kJ/mol 0.0 rel ΔG (DFT//DFT), kJ/mol
0.0
127.6 127.6 75.2
Route B rel energy (MP2//DFT), kJ/mol 0.0 128.0 rel energy (DFT//DFT), kJ/mol 0.0 rel ΔG (DFT//DFT), kJ/mol
0.0
123.3 70.9
75.9
75.9 64.1
Figure 2. Calculated structures of energy minima 10a and 10b with their single-point energies at the MP2//DFT (DFT//DFT) level in au. Hydrogen atoms, except for the essential ones, are omitted for clarity.
Scheme 4. Sites in the η6-p-Cymene Ligand Allowing the Formation of a CH/π Interaction
Figure 1. Calculated structures of [RuH(η6-p-cymene)TsDPEN] (2) and 1-methyl-1,2,3,4-tetrahydroisoquinolinium (8) with their singlepoint energies at the MP2//DFT (DFT//DFT) level in au.
Step II represents the energy minimum preceding the transition states. According to the particular transition states used in the diagrams (vide infra), the corresponding minima 10a and 10b (Figure 2) were localized by IRC calculations and inserted into the energy profiles in Scheme 3. The substrate molecule 8 is
attracted to the Ru hydride species 2 by an interaction between the =NH+ group of 8 and an oxygen of the SO2 fragment of 2. This interaction appears to be highly important in the mechanism, as it retains the substrate at the active site of the ruthenium complex. However, it may also be affected by present anionic species (Cl , HCOO ), which have not been included in our calculations. Step III reflects the favorable and disfavorable transition states, which formed the primary objective of this work. It is a wellknown fact that the reaction proceeds preferentially via the more stable favorable transition state (favTS), while the other enantiomer of the product results from the disfavorable transition state (disTS).15 The TS structures have been solved in order to maintain the experimentally observed enantioselectivity, being (R)-9 when using the (S,S)-TsDPEN ligand. Moreover, the protonation of the imine rules out the six-membered pericyclic TS structures, since only one hydrogen is being transferred, 4825
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Figure 3. Calculated structures of favorable transition states fav11a, fav11b, fav11c, and fav11e, together with their single-point MP2//DFT (DFT// DFT) energies in au. Hydrogen atoms, except for the essential ones, are omitted for clarity. Where applicable, CH/π interactions are shown as dotted lines highlighted in green with corresponding interatomic distances in Å. The total SCF electronic density surfaces for fav11a and fav11b are partially displayed with applied MO isovalues of 0.02 and densities 0.006 and 0.004, respectively.
which opens up multiple routes for the approach of the substrate to the complex due to its free rotation around the Ru H C axis. As a result, possible TS structures had to be considered with the aim of finding the most stable ones: i.e., those formed with the highest probability. The lower energy (i.e., higher probability of formation) of a favTS is reached by a rather weak, yet critical CH/π interaction between the imine substrate molecule and the η6-aromatic ligand of 2.16 The structure of the η6-p-cymene ligand theoretically allows four different CH/π interactions (Scheme 4), which is inherently connected with a possible rotation of the p-cymene moiety. Therefore, two different structures employing C(sp3)H/π interactions were investigated, utilizing either the CH3 group (A) or the methine of the 2-propyl group (D), and two more C(sp2)H/π interactions were considered involving either of the aromatic hydrogens (B, C). Transition states employing sites A and B in a CH/π attraction (further denoted as fav11a and fav11b) were found to be energetically very similar (difference 0.47 kJ/mol, with fav11b being the more stable one). The energy of the type C transition state (fav11c) was higher than in the case of fav11b (4.80 kJ/ mol). Unfortunately, the structure of D-favTS (fav11d) failed to converge in the optimization process, and therefore, its formation was considered rather improbable. A video of the unsuccessful optimization is included in the Supporting Information. To sum up, structures fav11a and fav11b (Figure 3) were chosen as the most probable candidates to be included in the energy profiles shown in Scheme 3 (route A or B, respectively). However, despite
their almost equal single-point energies, differences were found on the Gibbs free energy scale (Scheme 3). TS fav11b was characterized by a lower relative free energy (difference 12.8 kJ/mol) compared to fav11a, which expressed the possible preference of this TS over fav11a. Accordingly, the Gibbs energy of activation for fav11b was lower by 17.1 kJ/mol. For both transition states, the total electron density surface was calculated and is involved in Figure 3, graphically emphasizing the CH/π interaction. The C(sp3)H/π interaction was found to be shorter (2.79 and 2.89 Å for fav11a) than C(sp2)H/π (3.14 and 3.08 Å in the case of fav11b). For fav11a and fav11b, intrinsic reaction coordinate (IRC) calculations were performed in order to identify the corresponding energy minima preceding and following the transition states (i.e., steps II and IV in Scheme 3). Generally, for an endothermic reaction, the TS structure is closer to the product structure, and vice versa.17 On observing that the TS structures had a high resemblance to the reaction products, the IRC calculation in this direction consisted of only a small number of points and the product minima were found easily (step IV, vide infra). However, in the opposite direction, even 500 points were not sufficient for complete IRC calculations. Therefore, energy minima were modeled separately, starting from the 500th step of the IRC calculation. Denoted 10a and 10b, these were used in the corresponding energy profiles as steps II. One additional favTS structure was investigated with respect to the aforementioned substrate rotation around the Ru H C axis. This transition state (fav11e, Figure 3) lacked a CH/π 4826
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Organometallics attraction between the substrate and p-cymene and, more importantly, the =NH+ 3 3 3 OdS interaction, which was caused by a different route of approach of the substrate. However, this structure was found to be significantly less stable than the other investigated favTSs (by 53.37 kJ/mol in comparison with fav11b) and supported the necessity of both interactions. The disfavorable transition state (disTS) afforded the minor, undesirable product enantiomer (S)-9. Given the p-cymene rotation and multiple possibilities for the substrate approach, several disTS structures had to be found, similar to the case of favTSs. Although two of these structures were not enforced by the CH/π attraction (these were labeled dis11a and dis11b, as their p-cymene orientation was similar to those in fav11a and fav11b, respectively), its employment was not impossible, as shown in the third structure dis11c. It should be emphasized that
Figure 4. Calculated geometry of dis11a with its MP2//DFT (DFT// DFT) single-point energy in au units. Hydrogen atoms, except for the essential ones, are omitted for clarity.
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this was only allowed by following the ionic reaction mechanism and would not be possible in the ketone-type six-membered pericyclic transition states, which strictly enable only a single route of substrate approach (vide infra). Interestingly, while in favTSs the CH/π interaction serves to stabilize the structures, in this case dis11c was found to be less stable than dis11a by 34.96 kJ/mol. We ascribe this to the fact that dis11c lacks the important =NH+ 3 3 3 OdS interaction, which stabilizes the substrate molecule at the active site. Figure 4 gives the structure of dis11a, while figures of structures dis11b and dis11c can be found in the Supporting Information. Therefore, the TS structures were fully consistent with the product configuration, since no disTS (leading to (S)-9 as a minor product enantiomer) which would be formed with higher probability than fav11b (leading to (R)-9 as a major product enantiomer, observed experimentally18) was encountered. As a result of the transition states, cationic intermediates (energy minima) form step IV of Scheme 3. Using IRC calculations from fav11a, fav11b, and dis11a, the corresponding geometries 12a, 12b, and 12c were determined and are shown in Figure 5. These structures were not likely to exist in any stable form and were assumed to disintegrate, ending in step V, which represents a sum of the isolated cationic species from the Ru solvate 4 and the product 9. The solvate 4 is a direct outcome from the hydrogenation process and exists in equilibrium with the 16e Ru complex 3.10 Structures 4 and 9 are also depicted in Figure 5. In order to investigate various routes of hydrogen transfer in the ATH of imines, the structures of six-membered cyclic transition states were also calculated following the concerted pathway for ketone reduction.5 These TSs (fav13a and dis13a) are depicted in Figure 6, and we presume that the reaction will not preferentially proceed via these structures for several reasons. First, it is obvious that fav13a leads to an enantiomer with
Figure 5. Calculated geometries of the cationic intermediates (12a, 12b, 12c), Ru solvate cation 4, and products (R)-9 and (S)-9. Single-point energies at the MP2//DFT (DFT//DFT) are given in au. Hydrogen atoms, except for the essential ones, are omitted for clarity. 4827
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Figure 6. Calculated structures of six-membered cyclic transition states fav13a and dis13a regarding the transfer hydrogenation of unprotonated 7. Single-point energies at the MP2//DFT (DFT/DFT) levels are given in au. Hydrogen atoms, except for the essential ones, are omitted for clarity.
configuration opposite to that observed experimentally: i.e., mainly (S)-9 would be formed instead of the correct (R)-9, when (S,S)-2 is used. Second, the imine molecule is not protonated, which as a consequence does not allow the reaction to proceed (as discussed above). Finally, the MP2//DFT singlepoint energy difference between fav13a and dis13a is so high (43.2 kJ/mol) that the reaction would proceed with absolute enantioselectivity, which is not observed.18 All these concerns discriminate the six-membered cyclic TSs from being feasible candidates for a correct reaction mechanism and therefore were not examined any further. Complementarily to the proposed mechanisms of imine asymmetric reduction, the transition states for the hydrogenation of acetophenone 5 were calculated. In a similar fashion, A and B favTSs and one disTS (fav14a, fav14b, and dis14a) were optimized, where the six-membered cyclic structures were eligible owing to their accordance with the experimentally observed major product enantiomer (S)-9. These structures are displayed in Figure 7. It can be seen that a proton is transferred from the NH2 moiety of the TsDPEN ligand, which supports the latest evidence on the necessity of the presence of a N H proton.19 In contrast, in the imine ionic mechanism, the NH2 group does not participate directly in the hydrogen transfer. As this work was mostly focused on imine reduction, no further steps upon the reaction coordinates of ketone hydrogenation were explored. Some basic ideas can be drawn from this computational study. Assuming that an imine substrate needs to be protonated for the
Figure 7. Calculated structures of transition states fav14a, fav14b, and dis14a appearing in the asymmetric transfer hydrogenation of acetophenone 5. Single-point energies at MP2//DFT (DFT/DFT) levels are given in au. Hydrogen atoms, except for the essential ones, are omitted for clarity.
reaction to occur, transition states fav11b and dis11a represent viable structures which are in agreement with experimental data. Nevertheless, they substantially differ from the original sixmembered cyclic concept which has been shown infeasible for imine substrates but functional when applied in the ATH of ketones. The preference of an ionic mechanism in imine hydrogenation can be related to an interaction between the protonated substrate and the TsDPEN ligand, which is not observed in the case of the classical pericyclic TSs, owing to their different structure. In fact, it is enabled by the imine protonation and most probably it contributes to stabilization of the structures involved in the reaction coordinate.
’ CONCLUSIONS On the basis of the latest findings concerning the mechanism of asymmetric transfer hydrogenation of imines using chiral RuIICl(η6-arene)[(S,S)-N-TsDPEN] complexes, this computational study sought to explore a pathway that was essentially 4828
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Organometallics different from that applicable to ketone reduction. After a detailed analysis of transition state structures and their preceding and following energy minima, two energy diagrams were constructed using the most likely structures. This being the first computational study of such a system using the η6-p-cymene instead of a symmetric η6-benzene or η6-1,2,3,4,5,6-hexamethylbenzene ligand, an effort was made to determine whether the C(sp3)H/π or C(sp2)H/π interactions took place in the favorable transition states. Interestingly, both possibilities were almost equal in terms of single-point energy, but the pathway involving a C(sp2)H/π attraction was favored owing to its lower Gibbs free energy. The interaction between the protonated nitrogen atom of the substrate and the sulfonyl fragment of the TsDPEN ligand was identified as highly important for the stabilization of all transition states and energy minima. This observation represents another key feature of the ionic mechanism which is absent in the original mechanism operating for the ATH of ketones. For a comparison, the original mechanism has also been examined for both an imine and a ketone (acetophenone). Whereas the full-scale models of structures appearing in the ATH of acetophenone corresponded well both to previous experimental and theoretical works, they were shown to be infeasible in the case of imine hydrogenation. Computational studies of this kind provide a valuable insight into the reaction mechanism. Endowed with a deeper understanding of how asymmetric transfer hydrogenations might proceed, we are subsequently able to adjust the reaction conditions in order to reach the optimal outcome.
’ COMPUTATIONAL METHODS The geometry optimizations, frequency jobs, and single-point energy calculations were performed using the standard Gaussian0320a package at the DFT level of theory employing the restricted B3LYP21 combination of functionals. The standard polarized 6-31G(d,p) basis set was used for all atoms except ruthenium, which was described by the LANL2DZ22 basis set (for 18 valence electrons) together with the LANL2DZ effective core potential (ECP). In addition, single-point energies were computed also at the MP2 level using the previously DFToptimized structures (an MP2/6-31G(d,p)/LANL2DZ//DFT//631G(d,p)/LANL2DZ approach, abbreviated MP2//DFT) to obtain more reliable energy values. All structures were confirmed by frequency analyses to be either true minima (no imaginary vibrations found) or transition states (one imaginary vibration). All intrinsic reaction coordinate (IRC)23 calculations were carried out using the Gaussian0920b package. These were computed separately in the forward and reverse directions, each set up with a maximum of 500 points in order to localize corresponding energy minima. If an IRC calculation failed to reach the potential energy surface minimum, its last available point was used for a manual optimization toward a minimum.
’ ASSOCIATED CONTENT
bS Supporting Information. Tables and figures giving computational data for each structure, text giving the complete ref 20, and a movie showing the movement of one of the structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ ACKNOWLEDGMENT This work has been financially supported by the Grant Agency of the Czech Republic (Grant GACR 104/09/1497), the Ministry of Education, Youth, and Sports (MEYS) of the Czech Republic (Grant CEZ: MSM 604 613 7301), and a research intent (AV0Z50200510). Access to the MetaCentrum computing facilities, provided under the program “Projects of Large Infrastructure for Research, Development, and Innovations” (LM2010005) funded by the MEYS, is gratefully acknowledged. ’ REFERENCES (1) Andrews, I.; Cui, J.; DaSilva, J.; Dudin, L.; Dunn, P.; Hayler, J.; Hinkley, B.; Hughes, D.; Kaptein, B.; Kolis, S.; Lorenz, K.; Mathew, S.; Rammeloo, T.; Wang, L.; Wells, A.; White, T.; Xie, C.; Zhang, F. Org. Process Res. Dev. 2009, 13, 397. (2) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916. (3) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 2521. (4) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97. (5) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931. (6) Martins, J. E. D.; Clarkson, G. J.; Wills, M. Org. Lett. 2009, 11, 847. (7) Åberg, J. B.; Samec, J. S. M.; B€ackvall, J.-E. Chem. Commun. 2006, 2771. (8) Blackmond, D. G.; Ropic, M.; Stefinovic, M. Org. Process Res. Dev. 2006, 10, 457. (9) Fleury-Bregeot, N.; de la Fuente, V.; Castillon, S.; Claver, C. ChemCatChem 2010, 2, 1346. (10) Sandoval, C. A.; Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Noyori, R. Chem. Asian J. 2006, 1 2, 102. (11) Sandoval, C. A.; Bie, F.; Matsuoka, A.; Yamaguchi, Y.; Naka, H.; Li, Y.; Kato, K.; Utsumi, N.; Tsutsumi, K.; Ohkuma, T.; Murata, K.; Noyori, R. Chem. Asian J. 2010, 5, 806. (12) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285. (13) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466. (14) For an exhaustive review on computational solvation models, see this review: Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (15) See e.g.: Balcells, D.; Maseras, F. New J. Chem. 2007, 31, 333. (16) Yamakawa, M.; Yamada, I.; Noyori, R. Angew. Chem., Int. Ed. 2001, 40, 2818. (17) MacNeil, P. A.; Roberts, N. K.; Bosnich, B. J. Am. Chem. Soc. 1981, 103, 2273. erveny , (18) Gulamhussen, A. M.; Kacer, P.; Prech, J.; Kuzma, M.; C L. React. Kinet. Catal. Lett. 2009, 97, 335. (19) Soni, R.; Cheung, F. K.; Clarkson, G. C.; Martins, J. E. D.; Graham, M. A.; Wills, M. Org. Biomol. Chem. 2011, 9, 3290. (20) (a) Frisch, M. J.; et al. Gaussian 03, revision E.01; Gaussian, Inc., Wallingford, CT, 2004. (b) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (21) (a) Lee, C.; Yang, W.; Parr, R. Phys. Rev. B 1988, 37, 785–789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (23) Fukui, K. Acc. Chem. Res. 1981, 14, 363.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.K.);
[email protected] (J.V.). 4829
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