Reactivity of active oxygen species generated in the EuCl3 catalytic system for monooxygenation of hydrocarbons

Abstract

The reactivity of active oxygen species generated from O₂ in the EuCl₃–Zn–McCO₂H catalytic system for the monooxygenation of various hydrocarbons has been studied in detail. In the case of the oxygenation of alkanes, a large difference between the reactivities of primary, secondary and tertiary C–H bonds (1:6:19) is observed. The kinetic isotope effect (KIE) between C–H and C–D (2.6) for the oxidation of cyclohexane, the relative conversion rate of cooxidation of cyclopentane and cyclohexane (C₅/C₆= 0.7) and the non-retention of the configuration of tertiary C–H bonds suggest that the cleavage of the C–H bond is a key step in forming the alkyl radical intermediate by H˙ abstraction in the oxidation of alkanes. In the case of epoxidation of cis-hex-2-ene and trans-hex-2-ene, the cis- and trans-configurations are not preserved in their epoxides. This suggests that the eooxidation proceeds through alkyl radical intermediates. In the case of the hydroxylation of toluene, the regioselectivity in the formations of cresols (o : m :p, 13 : 1 : 16) suggests a strong electrophilicity of active oxygen species. The electrochemical studies for oxidation of cyclohexane mediated by EuCl₃ over a glassy carbon cathode suggest that the formation of Eu²⁺ and O₂^– is important for the cyclohexane oxidation.