Model for enantioselective hydrogenation of alpha-ketoesters over chirallymodified platinum revisited: Influence of alpha-ketoester conformation

Interaction complexes between cinchonidine modifier and methyl pyruvate rea ctant proposed for the enantioselective hydrogenation over platinum catalys ts have been calculated using ab initio methods. For s-trans-methyl pyruvat e it was found that the complex yielding (R)-methyl lactate upon hydrogenat ion was more stable than the corresponding pro-(S) complex. The calculated energy difference of 1.8 kcal/mol corresponds to an enantiomeric excess of 92%, in good agreement with experiment. For the analogous complexes of s-ci s-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of p ro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded comple xes with cinchonidine are considerably more stable than the corresponding s -trans complexes. However, the predicted low enantiomeric excess for the s- cis conformer is in contrast with experiment. Possible reasons for this beh aviour are discussed. (C) 2000 Academic Press.