Chiral MnIII (Salen) Covalently Bonded on Modified ZPS-PVPA and ZPS-IPPA as Efficient Catalysts for Enantioselective Epoxidation of Unfunctionalized Olefins

Chiral MnIII (salen) complex supported on modified ZPS-PVPA (zirconium poly(styrene-phenylvinylphosphonate)) and ZPS-IPPA (zirconium poly(styrene-isopropenyl phosphonate)) were prepared using -CH2Cl as a reactive surface modifier by a covalent grafting method. The supported catalysts showed higher chiral induction (ee: 72%-83%) compared with the corresponding homogeneous catalyst (ee: 54%) for asymmetric epoxidation of alpha-methylstrene in the presence of 4-phenylpyridine N-oxide (PPNO) as axial base using NaClO as an oxidant. ZPS-PVPA-based catalyst 1, with a larger pore diameter and surface area, was found to be more active than ZPS-IPPA-based catalyst 2. In addition, bulkier alkene-like indene, was efficiently epoxidized with these supported catalysts (ee: 96%-99%), the results weremuch higher than those for the homogeneous system(ee: 65%). Moreover, the prepared catalysts were relatively stable and can be recycled at least eight times without significant loss of activity and enantioselectivity.