Enantioselective epoxidation of nonfunctionalized alkenes catalyzed by recyclable new homochiral dimeric Mn(III) salen complexes

Two recyclable dimeric MnIII salen complexes with geminal methyl groups on the linking carbon atom at the 5,5′ position of the salen units were synthesized. They gave a high epoxide yield (>99%) and enantiomeric excess up to >99% in 2 to 11 h at 2 mol% catalyst loading in the enantioselective epoxidation of nonfunctionalized alkenes using NaOCl as the oxidant in the presence of Py NO as the axial base. The epoxidation reaction was successful even at a catalyst loading of 0.4 mol%, but the reaction took longer. The catalyst was recovered easily and recycled five times in a simple separation process. To understand the mechanism of the catalytic reaction, the kinetic investigation was carried out with different concentrations of the catalyst, the oxidant, and the substrate and with styrene as the representative substrate. The epoxidation of styrene was first order with respect to the catalyst and the oxidant but did not depend on the initial concentration of the substrate. An appropriate mechanism of the epoxidation reaction is proposed.