Journal of Catalysis, Vol.286, 41-50, 2012
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 beta-methyl styrene, indene, and chromenes in the presence of several N-oxides as an axial base and NaOCl as an oxidant at 0 degrees 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. (C) 2011 Elsevier Inc. All rights reserved.
Keywords:Enantioselective;Chiral Mn(III) macrocyclic salen complexes;Chiral epoxides;Nonfunctionalized alkenes;Kinetic investigation