Kinetics of the epoxidation of cyclohexene by molecular oxygen catalyzed by dichlorotetraaquoruthenium(iii) in the presence of the reductants 1-ascorbic acid and ethanol

Abstract

Epoxidation of cyclohexene by oxygen atom transfer from coordinated dioxygen is catalyzed by dichlorotetraaquoruthenium(III) [RuCl₂(H₂O)₄]⁺ 1a in a 1:1 (v/v) mixture of water-ethanol and water-1,4-dioxan at 35°C and μ=0.1 M KCl. The rate of epoxidation of cyclohexene by molecular oxygen was also investigated using dichlorodiaquoruthenium(III)-ascorbate chelate [RuIIICl₂(H₂O)₂(HA)] 1 as catalyst under identical conditions in 1:1 water-1,4-dioxan mixture, and was found to be more rapid than that of catalyst 1a. These reactions, which act as models for monooxygenases, proceed by the insertion of one of the oxygen atoms of the coordinated dioxygen into the substrate with the formation of the oxidized product, the other oxygen atom being released as OH⁻. The rate of the epoxidation reaction is first order with respect to the concentration of cyclohexene, catalyst and molecular oxygen in all the cases. Inverse first order and zero order dependences in hydrogen ion concentration were observed in the presence and absence of ascorbic acid, respectively. The dependence in the concentration of reductants [ascorbic acid] and [ethanol] is first and zero order, respectively. A comparative study of the epoxidation of cyclohexene from product analysis, kinetic and mechanistic points of view is reported and discussed in detail.