Enantioselective arene epoxidation under mild conditions by Jacobsen catalyst: The role of protic solvent and co-catalyst in the activation of hydrogen peroxide
•Arene epoxidation was obtained by Jacobsen catalyst using H2O2 in ethanol at 40 °C•Use of co-catalyst combined with protic solvent enhanced catalytic efficiency•Catalytic reactivity is explained by formation of oxo- or hydroperoxy species•Role of protic solvent associated with hydrogen bonding with active species•Best C % was obtained with ammonium acetate and best ee% with 1-methylimidazole
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.
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Journal: Applied Catalysis A: General - Volumes 460–461, 10 June 2013, Pages 116–123