Model of reactant-modifier interaction in enantioselective hydrogenation of ethyl pyruvate on platinum-cinchona catalysts: Extension to synthetic chiral modifiers

The previously proposed model for reactant-modifier interaction in the enan tioselective hydrogenation of activated carbonyl compounds over platinum ch irally modified by cinchona alkaloids has been extended to platinum modifie d by synthetic pyrrolidinyl-naphthyl-ethanol modifiers. As in the case of c inchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is a chieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one int eraction between modifier and reactant was calculated at the ab initio leve l. A comparison of the results for different modifiers leads to the conclus ion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of t he (R)-product is traced to the fact that the pro-(S) complex leading to th e (S)-product upon hydrogenation is more destabilised due to repulsive inte raction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanis m of enantiodifferentiation. (C) 2001 Academic Press.