Donor-acceptor interactions in the enantioselective hydrogenation of α-ketoesters

The origin of enantiodiscrimination in the hydrogenation of methyl pyruvate (MP) on cinchona alkaloid modified by Pt has been mainly ascribed to interactions between the modifier and the substrate. In the present work, the role of these substrate-modifier interactions on the stabilization of intermediate complexes is discussed on the basis of ab initio MP2/6-31G(d) and MP2/6-31G(d,p) calculations. The amines ammonia, trimethylamine and quinuclidine are employed as model for the cinchona alkaloid. Our results show that MP interacts with the amines via a donor-acceptor complex with a stabilization energy that increases from ammonia, to trimethylamine and to quinuclidine, being in the last case on the order of 4.0 kcal mol(-1) after correction for BSSE and inclusion of solvent effects. NBO analysis of the interacting orbitals confirms the nitrogen lone pair of the amines as a donor and the antibonding (C-O)* orbital of the U-keto carbonyl as the acceptor. These results give support for experimental observations that interactions between the basic quinuclidine moiety of cinchonidine and the MP molecule may control the stereoselectivity of the catalytic process. (C) 2004 Elsevier B.V. All rights reserved.