Ionic liquid-functionalized salen Mn(III) complexes as tunable separation catalysts for enantioselective epoxidation of styrene

A series of novel chiral salen Mn(III) complexes functionalized by ionic liquid (IL) of 1-propylamine-3-methylimidazolium tetrafluoroborate were synthesized through the reaction of amino group (NH2) of the IL with chloromethyl (CH2Cl) in the salen ligand at one side of the 5 position (complex 2) and at two sides of the 5, 5′ position (complex 3), as well as by direct axial coordination between NH2 of the IL and metal center of the salen Mn(III) complex (complex 4). All of the synthesized complexes were well characterized, and their performance in the enantioselective epoxidation of styrene was investigated systematically. Under optimum reaction conditions, a 99% styrene epoxide yield with 50% enantiometric excess (ee) could be obtained over the complex 2. Furthermore, the IL-functionalized chiral salen Mn(III) complexes of 2 and 3 could be conveniently separated from the reaction system by simple precipitation in hexane and subsequently used without significant loss of activity and enantioselectivity.