Mechanism of C–H bond oxidation by a monooxoruthenium(V) complex

Abstract

The complex [Ru^vL(O)]²⁺{HL =[2-hydroxy-2-(2-pyridyl)ethyl]bis[2-(2-pyridyl)ethyl]amine} was found to be a very active stoichiometric oxidant towards various hydrocarbons. Oxidation of alcohols was studied either in 0.1 mol dm^–3 HClO₄ or CH₃CN, the observed rate law being rate =k[Ru^v][alcohol]. The second-order rate constants for various alcohols follow the order benzylic > secondary > primary alcohols. Kinetic isotope effects (k_H/k_D) of 5.9 and 5.3 were found for benzyl alcohol and PrⁱOH respectively. Similar kinetic studies for the oxidation of tetrahydrofuran gave k_H/k_D= 6.8. Stoichiometric oxidation of saturated alkanes in acetonitrile gave tertiary alcohols exclusively or ketones when only methylene groups are present. The measured k_H/k_D for the oxidation of cyclohexane determined from competitive oxidation experiments was 5.3. No cyclohexyl chloride was found when the oxidation of cyclohexane was carried out in the presence of CCl₄. The mechanism of C–H bond oxidation by Ru^v=O is discussed.