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

A series of chiral (pyrrolidine salen)Mn(III) complexes were synthesized from NazaNaza-substituted (R,RR,R)-N,N′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 CH3I 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 NazaNaza-substituent in the pyrrolidine backbone of complexes 1–3 on the enantioselectivity of epoxidation.