Model for Enantioselective Hydrogenation of α-Ketoesters over Chirally Modified Platinum Revisited: Influence of α-Ketoester Conformation

Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the enantioselective hydrogenation over platinum catalysts have been calculated using ab initio methods. For s-trans-methyl pyruvate it was found that the complex yielding (R)-methyl lactate upon hydrogenation 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-cis-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of pro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded complexes 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 behaviour are discussed.