ASYMMETRIC LEWIS ACID-DIENOPHILE COMPLEXATION - SECONDARY ATTRACTION VERSUS CATALYST POLARIZABILITY

A series of five X-ray crystal structures of chiral Lewis acid-dienoph ile complexes support a two-point-binding chiral recognition mechanism for these asymmetric Diels-Alder catalysts. As the arene moieties of the alkyldichloroborane Lewis acids are made more polarizable through the progression of phenyl to 3,5-dimethylphenyl to 3,5-dichlorophenyl to 3,5-dibromophenyl to naphthyl, the polar ester group of the boron-b ound methyl crotonate is drawn in toward the arene of the catalyst due to an enhanced dipole-induced-dipole attraction. The catalyst with th e most polarizable arene (1-naphthyl) gives higher enantioselectivity (up to 99.5% ee) in Diels-Alder reactions than the catalyst with the l east polarizable arene (phenyl), demonstrating that this effect transl ates to the transition state.