Solvent and oxidant effects on the Au/TiO2-catalyzed aerobic epoxidation of stilbene
Résumé
Molecular oxygen can be used as the main oxidant to selectively epoxidize trans-stilbene over Au/TiO2 catalysts, in a free-radical process. However, the nature of the radical initiator has a critical influence on the reaction selectivity. tert-Butylhydroperoxide (TBHP, catalytic amount) leads to high yields of epoxide; on the other hand, hydrogen peroxide and di-tert-butylperoxide merely cause degradation of trans-stilbene. The choice of the solvent is also critical. Amongst the selected solvents. only alkyl-substituted cyclohexanes lead to high yields of epoxide, despite the poor dispersion of the catalytic powder. Other solvents. including the more polar ones and cyclohexane, are significantly less efficient, both in terms of total activity and epoxide production. The latter does not go beyond the yield expected from the potential stoichiometric reaction between TBHP and trans-stilbene (5%). On the basis of these results, an aerobic epoxidation mechanism is proposed in which molecular oxygen is activated by a substituted cyclohexyl radical produced by abstraction of a tertiary hydrogen atom from the solvent molecule by a tert-butylperoxy radical. (C) 2008 Elsevier B.V. All rights reserved.