Enantioselective hydrogenation of α-keto esters over cinchona-Pt/Al2O3 catalyst.: Molecular modelling of the substrate-modifier interaction

The substrate-modifier interaction involved in the enantioselective hydrogenation of alpha-keto esters over cinchonidine-Pt/Al2O3 catalyst was investigated by molecular modelling. The model is based on our earlier kinetic and nuclear magnetic resonance (NMR) results as well as on analogies found in experimental organic chemistry. The model suggests the formation of a weak complex between the modifier and the substrate in the liquid phase. In the above complex, the modifier provides a specific shielding effect (SE). Due to the particular character of shielding, the alpha-keto ester can interact with the metal surface only by its unshielded site. If the reactivity of the substrate in the shielded [substrate-modifier] complex is higher than that of the free substrate, pronounced enantio-differentiation (ED) should be observed. In one of the shielded forms, the proper directionality of the quinuclidine nitrogen towards the keto carbonyl group provides the increased reactivity of the keto carbonyl group. The 'shielding effect' model can explain both the ED and the rate acceleration (RA) effect observed in the hydrogenation of methyl and ethyl pyruvate, methyl benzoylformate, pantolactone and trifluoroacetophenone (TFAP) over cinchonidine-Pt/Al2O3 catalyst. The 'shielding effect' model is the first model which can elucidate the substrate specificity of the above enantioselective hydrogenation reaction. (C) 1999 Elsevier Science B.V. All rights reserved.