Synthesis of Chiral Salen Mn (III) Complex Immobilized on Phenoxy-modified AlPS-PVPA as Catalysts for Epoxidation of Olefins

Abstract

Chiral epoxides are versatile intermediates that can be readily converted into a wide variety of enantiomerically pure compounds by means of region-and stereo-selective ring opening reactions. The asymmetric epoxidation of unfunctionalized olefins is an important approach for synthesizing optically active epoxides, and thus is widely used in the synthesis of fine chemicals, such as pharmaceuticals, agrochemicals and perfumes. Chiral salen Mn (III) complexes have demonstrated activities and selectivities for the enantioselective epoxidation of unfunctionalized olefins under homogeneous conditions. Compared with the homogeneous asymmetric catalysts, the heterogeneous ones have the advantages of easy catalyst/product separation and simple catalyst recycling. And more and more interests have been focused on the studies of heterogenization of chiral complexes. New types of supported catalysts are obtained by anchoring chiral salen Mn (III) complex on a series of phenoxy-modified aluminium poly (styrene-phenylvinylphosphonate)-phosphate (AlPS-PVPA) in the text. All the prepared catalysts are characterized by FT-IR, UV-vis, XPS, SEM, TG and elemental analysis. The catalytic capabilities are investigated with m-CPBA as an oxidant and with indene and α-methylstyrene as substrates for asymmetric epoxidation of unfunctionalized olefins. The supported catalysts indicate superior catalytic activities in the asymmetric epoxidation of α-methylstyrene and indene with m-CPBA as oxidative system, compared with the corresponding homogeneous catalyst (ee, > 97% vs. 54% and > 99% vs. 65%). The steric properties of the linkages really play vital impacts on the configuration of the transition state for the asymmetric reactions. Contrary to most of the literatures reported, the results show that the heterogeneous catalysts 3a~3d exhibit excellent catalytic activities, and their conversions and eevalues increase remarkably in the absence of N-methylmorpholine N-oxide (NMO) under the same catalytic conditions. The structures of the immobilized cat-alysts similar to the N-oxide ligand act as axial ligands leading to the unusual phenomenon. Simultaneously, additives are generally regarded as axial ligands on the transition metal catalyst, which make for activating the catalyst either toward oxidation or toward reactivity with the olefin. Thus, there is a steric hindrance when the N-oxide ligand is added and the optimal geometric configuration of the reactive intermediate salen Mn (V)=O was altered. It is steric hindrance that makes olefins approaching salen Mn (V)=O difficult and lower ee values are obtained. Furthermore, these catalysts are easily separated and are relatively stable and reused nine times without significant loss of activities.