A Lewis Acid-Controlled Enantiodivergent Epoxidation of Aldehydes

Two
epoxidation catalysts, one of which consists of two VANOL ligands
and an aluminum and the other that consists 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 are evaluated using density functional
theory calculations.