Oxidations by the reagent ‘H2O2–vanadium complex–pyrazine-2-carboxylic acid’. Part 4. Oxidation of alkanes, benzene and alcohols by an adduct of H2O2 with urea

Abstract

In the presence of catalytic amounts of a vanadium complex (Bu₄ⁿVO₃) and pyrazine-2-carboxylic acid, the adduct H₂O₂·urea, in MeCN solution at 22–60 °C, oxidizes cyclohexane to yield mainly cyclohexyl hydroperoxide which may be easily converted into cyclohexanol by the action of triphenylphosphine (at room temperature after the reduction the ratio cyclohexanol : cyclohexanone is 25 : 1). Linear and branched alkanes are also oxidized by this reagent to give the corresponding alcohols, the regioselectivity being low; e.g., for oxidation of hexane C(1) : C(2) : C(3)= 1.0 : 6.8 : 6.0. The effective activation energy of cyclohexane oxidation is ∼ 70 kJ mol^–1. Dependencies of initial rate of the reaction on concentration of substrate and the components of the reagent have been obtained. The postulated mechanism includes, as the key stage, the abstraction of a hydrogen atom from alkane, RH, to produce radical R˙ which then rapidly reacts with O₂ to afford ROO˙. The hydrogen atom is most probably abstracted by hydroxyl radical formed in the interaction of H₂O₂ with the vanadium complex. Benzene is oxidized by the reagent under consideration to yield phenol, and alcohols are transformed into aldehydes or ketones.