Chiral Mn(III) salen complexes covalently bonded on modified MCM-41 and SBA-15 as efficient catalysts for enantioselective epoxidation of nonfunctionalized alkenes

Abstract

Chiral Mn(III) salen complex supported onto modified mesoporous supports (MCM-41 and SBA-15) were prepared using 3-aminopropyltriethoxysilane as a reactive surface modifier by a covalent grafting method. The supported catalysts showed higher chiral induction (ee, 71%) for enantioselective epoxidation of styrene and 4-chlorostyrene in presence of pyridine N-oxide (PyNO) as axial base using aqueous NaOCl (12%) as an oxidant than seen in its homogeneous counterpart 1 (ee, 48%). SBA-15-based catalyst 3, with a larger pore diameter, was found to be more active than MCM-41-supported catalyst 2. In addition, bulkier alkenes like indene, 1,2-dihydronaphthalene, and 2,2-dimethylchromene were efficiently epoxidized with these supported catalysts (ee up to 96%), and the results were comparable to those for the homogeneous system. The performance of catalysts 2 and 3 was retained for four reuse experiments.