Stable chiral salen Mn(III) complexes with built-in phase-transfer capability for the asymmetric epoxidation of unfunctionalized olefins using NaOCl as an oxidant

Abstract

A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at two sides of 5,5′-position. Technologies of characterization well suggested the presence of polyether chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction rates in water/organic biphasic systems. Enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complexes with NaOCl gave >99% conversions within 60min. The enantiomeric excess (ee) for the epoxides was in the range of 68–93%, except for styrene (ee, 35%) and α-methylstyrene (ee, 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction mixture by control of the solvent.