Catalytic asymmetric epoxidation of non-functionalised alkenes using polymeric Mn(III) Salen as catalysts and NaOCl as oxidant

Abstract

Enantioselective epoxidation of non-functionalised alkenes using polymeric Mn(III) Salen complex in presence of PyN–O using NaOCl gave excellent (>99%) epoxide yield with high chiral induction in case of nitro and cyano chromene and the catalyst could be reused up to five cycles. Polymeric chiral Mn(III) Salen complexes 1a , 1b were synthesised by the interaction of 1 S ,2 S -(+)-l,2-diaminocyclohexane, 1 R ,2 R -(+)-l,2-diphenyl ethylenediamine with 5,5′-methylene-di-3- tert -butylsalicylaldehyde. Number of repetitive units and molecular weight of the polymeric ligands were found to be between 10 and 12 and M n = ∼5200 and ∼5400, respectively by VPO. Enantioselective epoxidation of chromenes, indene and styrene mediated by complexes 1a and 1b (2 mol%) as catalyst using NaOCl as an oxidant gave >99% epoxide yields except for cyclochromene where the yield is in the range of 50–61% in 2.5–12 h. The enantiomeric excess (ee) for the product epoxide was found to be in the range of 75 to >99%, except for styrene (ee, 32–56%). The catalysts were easily recovered by precipitation and were re-used up to five times with some loss of activity while there was no loss of enantioselectivity in the product epoxide. The effect of axial bases, oxidants and phase transfer reagents on activity and enantioselectivity of the catalytic system was also studied.