Asymmetric amplifying phenomena in enantioselective addition of diethylzinc to benzaldehyde

Abstract

BACKGROUND In 1986, Puchot et al. established rigorously that the earlier proposal of Wynberg and Feringa concerning the effect of enantiomers on one another had general validity. Puchot et al. showed experimentally that the general assumption, namely, that products could never have an e.e. greater than that of the chiral auxiliary (catalyst, modifier, etc), was invalid. The basis for this apparent anomaly, as was discussed theoretically in 1976, is the possible association of enantiomeric catalyst molecules with new (better and worse) catalyst properties. Early proof of association phenomena of chiral molecules to form diastereomers (particularly, chiral hydroxy amines) had been obtained by a classic experiment of Williams et al. The latter showed that in a solution of enantiomerically impure quinine, distinct NMR absorptions could be detected for the ( + ) ( + ), ( – ) ( – ), and ( + ) ( – ) “dimeric complexes” in achiral solvents. It is, of course, not surprising (except in the sense that we did not recognize this much earlier!) that an efficient chiral catalyst, which by its very behavior must complex strongly with the achiral substrates, also complexes with itself. It should also come as no surprise, therefore, that such associative dimers (or trimers, etc.) might show different (better or worse) catalytic properties.