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Enantioselection on Heterogeneous Noble Metal Catalyst: Proline-Induced Asymmetry in the Hydrogenation of Isophorone on Pd Catalyst

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journal contribution
posted on 2015-09-23, 00:00 authored by Laura Rodríguez-García, Konrad Hungerbühler, Alfons Baiker, Fabian Meemken
In the (S)-proline-mediated asymmetric hydrogenation of isophorone (IP) on supported Pd catalyst, excellent enantio­selectivity is achieved, with an enantio­meric excess of up to 99%. The role of the hetero­geneous catalyst has been the subject of a controversial debate, and the current mechanistic understanding cannot explain the observed enantio­selectivity of this catalytic system. The lack of in situ information about the role of the hetero­geneous catalyst has prompted us to investigate the surface processes occurring at the methanol–Pd catalyst interface using attenuated total reflection infrared spectroscopy. Time-resolved monitoring of the homo­geneous solution and of the catalytic solid–liquid interface coupled with catalytic data provides crucial information on the catalytically relevant enantio­differentiating processes. While the condensation of IP and the corresponding chiral product 3,3,5-trimethyl­cyclohexanone with the chiral amine is connected to the enantio­differentiation, it was found that the crucial enantio­selectivity-controlling steps take place on the metal surface, and the reaction has to be classified as hetero­geneous asymmetric hydrogenation. The presented spectroscopic and catalytic results provide strong evidence for the existence of two competing enantio­selective processes leading to opposing enantio­selection. Depending on surface coverage of the Pd catalyst, the reaction is controlled either by kinetic resolution ((S)-pathway) or by chiral catalysis ((R)-pathway). Steering the hydrogenation on the (R)-reaction pathway requires sufficient concentration of IP-(S)-proline condensate, as this chiral reactive intermediate becomes the most abundant surface species, inhibiting the competing kinetic resolution. The unraveled (R)-reaction pathway emphasizes an intriguing strategy for inducing chirality in hetero­geneous asymmetric catalysis.

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