Asymmetric epoxidation of unfunctionalized olefins accelerated by thermoresponsive self-assemblies in aqueous systems

Abstract

A novel thermoresponsive surfactant-type chiral salen Mn^III catalyst was developed by axially grafting “smart” poly(N-isopropylacrylamide) (PNIPAAm) onto the metal center of a neat chiral salen Mn^III complex. Characterization data suggests thermoresponsive micellization behavior of the obtained catalyst in water. The chiral metallomicellar catalyst acted as a nanoreactor to carry out asymmetric epoxidation of unfunctionalized olefins in water and dramatically accelerated reaction rates. Outstanding catalytic efficiency was observed in the nanoreactor system. In particular, quantitative conversion (99%) of styrene with high enantioselectivity (39%) was achieved over 0.8 mol% of the catalyst within 3 min, giving an unprecedented TOF value (2.48 × 10³ h⁻¹) which is significantly higher than that obtained over previously reported homogeneous or heterogeneous systems. Moreover, the catalyst could be easily recovered by thermocontrolled separation and reused with high activity for five cycles.