Enantioselective epoxidation of olefins with molecular oxygen catalyzed by gold(III): A dual pathway for oxygen transfer

A chiral gold(III) complex has been prepared that performs the epoxidation of olefins in the presence of O2, PhIO, or bleach. Catalytic experiments with 18O show that O2 is activated on the catalyst and can be directly incorporated into the epoxide through a non-radical mechanism that probably involves formation of gold, oxo, or peroxo species. In addition to this, there is a parallel radical mechanism operating that yields α, β-unsaturated ketones and alcohols as subproducts. Electrochemical and UV–Vis experiments confirmed the occurrence of a Au(III)/Au(I) redox cycle during the catalytic epoxidation in a mechanism sustained by molecular oxygen.