Epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen using ruthenium(III) aquo ion as catalyst: a kinetic study

Abstract

Kinetics of the epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen catalysed by Ru(III) ion, [RuCl₂H(₂O)₄]⁺ at pH 2.0 in a mixed water-dioxane solvent medium are reported. The reactions were studied manometrically as a function of catalyst, substrate and dioxygen concentrations. For all the substrates studied, the rate of oxidation is first order with respect to catalyst and substrate concentrations and one-half order with respect to dioxygen concentration. The oxidations yielded epoxide as the only product in all cases; these were identified and analysed by GLC technique. Based on the kinetics, a mechanism involving homolytic cleavage of the O-O (peroxo) bond with concerted transfer of the oxygen atom to the substrate was proposed. The presence of methyl group on the cyclohexene ring (reaction 2) and an increase in the number of carbon atoms in the ring for cyclooctene (reaction 3) cause a decrease in the rates of epoxidation. The rate of epoxide formation and the corresponding yields for reactions (1), (2) and (3) decreases in the order: cyclohexene > methylcyclohexene > cyclooctene.