Asymmetric boronate addition to o -quinone methides: Ligand exchange, solvent effects, and Lewis acid catalysis

Density functional theory calculations suggest that asymmetric boronate addition to o-quinone methides proceeds via a Lewis acid catalyzed process through a closed six-membered transition structure. The BINOL-derived catalyst undergoes an exchange process with the original ethoxide boronate ligands. This activation mode successfully accounts for the sense and level of enantioselectivity observed experimentally. A qualitative model which accurately predicts the observed enantioselectivity has been developed and is consistent with results from our study of ketone propargylation demonstrating the reaction models generality. The effects of replacing the BINOL framework with H₈-BINOL have been rationalized.