Asymmetric epoxidation of styrene and chromenes catalysed by chiral (salen)Mn(III) complexes with a pyrrolidine backbone

Abstract

A series of chiral (pyrrolidine salen)Mn(III) complexes were synthesized from N aza -substituted ( R , R )- N , N ′ -bis(3,5-di- tert-butylsalicylidene)-3,4-diaminopyrrolidine salen ligands. High yields and comparable enantioselectivity relative to Jacobsen's catalyst were achieved in asymmetric epoxidation of styrene and chromenes using NaClO/PPNO and m-CPBA/NMO as the oxidant systems. Complexes 1– 3, featuring a tertiary amine unit, displayed higher activity than complex 4, bearing an amide unit and Jacobsen's catalyst in the NaClO/PPNO aqueous/organic biphasic system. The influence of CH 3I on epoxidation of 6-nitro-2,2-dimethylchromene catalysed by 1– 4 in the biphasic medium was studied to explore the catalysts with built-in phase-transfer capability. The alkenes' access pathway is discussed on the basis of the steric effect of the N aza -substituent in the pyrrolidine backbone of complexes 1– 3 on the enantioselectivity of epoxidation.