A generalized two-point H-bonding model for catalytic stereoselective hydrogenation of activated ketones on chirally modified platinum

The asymmetric hydrogenation of alpha-ketoesters on cinchona-modified supported platinum particles is a prototype reaction in heterogeneous chiral catalysis. The catalysis literature shows that the reaction is highly metal-specific, that it displays rate-enhancement with respect to the racemic reaction on the nonmodified surface, and that the observed stereoselectivity is a sensitive function of substrate and modifier structure. This set of observations has proven difficult to rationalize within the context of existing models for the mechanism of the Orito reaction. The most widely discussed mechanistic models are based on the formation of chemisorbed 1: 1 complexes through H-bonding between the quinuclidine function of the cinchona modifier and the prochiral, keto-carbonyl, function of the substrate. Recent surface science studies, as well as advances in the area of C-(HO)-O-... hydrogen bonding, suggest that chemisorption-induced polarization may lead to an aromatic-carbonyl H-bonding interaction between the aromatic anchor of the modifier and the coadsorbed substrate. By specifying that the aromatic C-(HO)-O-... interaction is to the prochiral carbonyl and that it is accompanied by a H-bonding interaction between the ester carbonyl and the quinuclidine function, we show that it is possible to rationalize essentially all of the catalysis literature for the Orito reaction in terms of a single molecular mechanism. The generality of the proposed mechanistic model is demonstrated by addressing data from the literature for a representative range of substrates, modifiers, solvents, and metals. Results of catalytic tests on an asymmetric diketone substrate are presented in support of the model.