A highly efficient and recyclable catalyst-dendrimer supported chiral salen Mn(III) complexes for asymmetric epoxidation

A novel series of PAMAM dendrimer-supported chiral salen Mn(III) complexes has been synthesized and applied to the enantioselective epoxidation of unfunctionalized olefins. Technologies of characterization clearly suggested the axial grafting of the chiral salen Mn(III) complex on the surface of PAMAM dendrimers through a phenoxy group spacer. It was found that the peripherally modified dendrimers benefited from the cooperativity between neighboring sites confined to the surface of the dendrimers, thereby enhancing the asymmetric induction of the catalyst. Higher enantioselectivity for PAMAM dendrimer-supported chiral salen Mn(III) complexes over the homogeneous counterpart was observed in the enantioselective epoxidation of unfunctionalized olefins. Furthermore, the dendritic supports imparted selective solubility properties to the catalysts. These catalysts could be easily recovered from the reaction solution by using solvent precipitation, and could be reused for several times without loss of activity and enantioselectivity.