Enantioselective arene epoxidation under mild conditions by Jacobsen catalyst: The role of protic solvent and co-catalyst in the activation of hydrogen peroxide

The epoxidation of arenes was achieved in high yield and with high enantioselectivity using the system Jacobsen catalyst:hydrogen peroxide:co-catalyst, ethanol as reaction solvent at 40 degrees C. The effect on the catalytic performance of the use of protic (ethanol) and aprotic solvents and of co-catalysts with different acid-base properties was analyzed, as well as, different reaction temperatures, using as substrates indene, 6-cyan-2,2'-dimethylchromene, styrene and alpha-methylstyrene. The protic solvent showed a positive effect enhancing catalytic performances when compared with the aprotic solvent. For amphoteric co-catalysts (ammonium acetate, 2-methylimidazole and imidazole) it was observed the highest substrate conversions, whereas for basic co-catalysts (1-methylimidazole, 4-methylmorpholine N-oxide and pyridine), higher ee% and relatively lower C% were observed. Moreover, the reactions at 40 C showed higher enantiomeric induction than those performed at room or lower temperatures. The catalytic data are in accordance with a multi-step mechanism for hydrogen peroxide activation by the Mn(salen) complex with the formation of two catalytic active intermediates, existing in different extension depending on the reaction conditions: a hydroperoxy intermediate or oxo-metallocomplex. The activating effect of the protic solvent ethanol vs aprotic solvent was explained by the formation of hydrogen bonds between the solvent and the catalytic active intermediates. (C) 2013 Elsevier B.V. All rights reserved.