A Lewis Acid-Controlled Enantiodivergent Epoxidation

Two epoxidation catalysts one of which consists of two VANOL ligands and an aluminum and the other of two VANOL ligands and a boron were compared. Both catalysts are highly effective in the catalytic asymmetric epoxidation of a variety of aromatic and aliphatic aldehydes with diazoacetamides giving high yields and excellent asymmetric inductions. The aluminum catalyst is effective at 0 °C and the boron catalyst at –40 °C. Although both the aluminum and boron catalysts of (R)-VANOL give very high asymmetric inductions (up to 99% ee), they give opposite enantiomers of the epoxide. The mechanism, rate- and enantioselectivity-determining step, and origin of enantiodivergence is evaluated using DFT calculations.