Reusable chiral macrocyclic Mn(III) salen complexes for enantioselective epoxidation of nonfunctionalized alkenes

Abstract

A series of new chiral monomeric and dimeric macrocyclic Mn(III) salen complexes 1–4 with trigol linker were synthesized, characterized (by microanalysis, IR spectroscopy, UV–vis. spectroscopy, optical rotation, and mass spectrometry), and used as catalysts in the enantioselective epoxidation of styrene, cis β-methyl styrene, indene, and chromenes in the presence of several N-oxides as an axial base and NaOCl as an oxidant at 0°C. With the use of chiral dimeric macrocyclic catalyst 3 (2.5mol%), enantio-pure epoxides were achieved in excellent yields (>99%) and enantioselectivities (ee up to 98% in selected cases). The recycling was demonstrated with complex 4 (recyclable up to six cycles studied with retention of enantioselectivity) in the asymmetric epoxidation of styrene. The kinetic investigation with complex 4 for the epoxidation of styrene as the representative substrate showed the first-order dependence on the catalyst and the oxidant but independent on the initial concentration of the substrate.