Competition at Chiral Metal Surfaces: Fundamental Aspects of the Inversion of Enantioselectivity in Hydrogenations on Platinum Norberto Bonalumi, Angelo Vargas, Davide Ferri, Thomas Bu¨ rgi, ² Tamas Mallat, and Alfons Baiker* Contribution from the Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Ho¨nggerberg HCI, CH-8093 Zurich, Switzerland Received January 21, 2005; E-mail: baiker@chem.ethz.ch Abstract: O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al2O3. To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R-S switch system is provided. Introduction Chiral recognition on metal surfaces is gaining growing attention and has stimulated various fundamental and applied research, 1-4 given the potential technological relevance of devices that enable stereochemical control. Adsorption of a chiral organic compound from solution onto a metal surface is a simple and efficient approach to create a chiral surface. 5 This strategy has been widely used in (supported) metal-catalyzed heterogeneous enantioselective hydrogenation reactions. 6-12 Chirally modified metals are synthetically useful solid catalysts that offer over 90% ee in some reactions including the hydrogenation of R- and -functionalized ketones 13-16 and 2-pyrones. 17 An attractive feature of these catalysts is that the major enantiomer can be switched (i) by changing the reaction conditions or the metal catalyst and (ii) by changing the chiral modifier. Examples for the first category include the inversion of enantioselectivity in the asymmetric hydrogenation of activated ketones by changing the solvent composition, including water and strong acid additives. 18-23 In the hydrogenation of methyl pyruvate in the presence of cinchonidine (CD, Scheme 1) as chiral modifier, (R)-lactate was obtained on Pt/SiO 2 , whereas the S-enantiomer was produced on iron oxide-supported Pd. 18 Inversion of enantioselectivity by structural variation of the modifier was first reported by Orito, 24 who produced the opposite enantiomer in the Pt-catalyzed hydrogenation of R-ketoesters by substitution of CD to its pseudo-enantiomer ² Universite´ de Neuchaˆtel, Institut de Chimie, Av. de Bellevaux 51, CH-Neuchaˆtel, Switzerland. (1) Sholl, D. S.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771. (2) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2002, 124, 8984. (3) Raval, R. Curr. Opin. Solid State Mater. Sci. 2003, 7, 67. 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