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 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 α-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.