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

Authors: Mariana Rocha, Susana L.H. Rebelo, Cristina Freire

Abstract:
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 -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.

Keywords:
Jacobsen catalyst
Asymmetric epoxidation
Hydrogen peroxide
Ethanol
Mechanism

Published in: Applied catalysis A: General  (Volumes 460–461, Pages 1-132, 10 June 2013)

Publisher: Elsevier      

ISSN Information: 0926-860X

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

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