Asymmetric Epoxidation of Alkenes Catalyzed by a Cobalt Complex

Asymmetric epoxidation of alkenes catalyzed by nonheme chiral Mn–O and Fe–O catalysts has been well established, but chiral Co–O catalysts for the purpose remain virtually undeveloped due to the oxo wall. Herein is first reported a chiral cobalt complex to realize the enantioselective epoxidation of cyclic and acyclic trisubstituted alkenes by using PhIO as the oxidant in acetone, wherein the tetra-oxygen-based chiral N,N′-dioxide with sterically hindered amide subunits plays a crucial role in supporting the formation of the Co–O intermediate and enantioselective electrophilic oxygen transfer. Mechanistic studies, including HRMS measurements, UV–vis absorption spectroscopy, magnetic susceptibility, as well as DFT calculations, were carried out, confirming the formation of Co–O species as a quartet Co(III)-oxyl tautomer. The mechanism and the origin of enantioselectivity were also elucidated based on control experiments, nonlinear effects, kinetic studies, and DFT calculations.