Covalently anchored chiral Mn(III) salen-containing ionic species on mesoporous materials as effective catalysts for asymmetric epoxidation of unfunctionalized olefins

A kind of chiral Mn(III) salen-containing ionic species was prepared and covalently anchored on mesoporous materials, including SBA-15, MCM-48, MCM-41, and amorphous SiO2, via imidazole modification. These catalysts were characterized by N2 sorption, XRD, FTIR, DR UV–vis, and elemental analysis. The results showed that the chiral Mn(III) salen complex was successfully immobilized inside the channels of the modified mesoporous materials and that the long-range mesoporous ordering of parent supports was maintained after the immobilization. These prepared catalysts were evaluated in the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as oxidant. Comparable or even higher enantioselectivity than that of homogeneous counterpart was obtained with similar catalytic activity. Moreover, these heterogeneous catalysts can complete epoxidation rapidly, thus providing notably high turnover frequencies, attributed in part to the ionic property of the active centers. The SBA-15-based catalyst showed the best performance in the epoxidation reaction. Furthermore, these immobilized catalysts were stable and could be recycled five times without loss of activity.