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

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 (2.5 mol%), 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.