One-electron oxidation of closed-shell molecules. Part 4. Acid-induced oxidative cleavage of substituted 1,2,2,2-tetraphenylethanones (benzpinacolones) with diaroyl peroxides

Abstract

The acid-induced oxidative cleavage of anispinacolone [1,2,2,2-tetrakis-(p-methoxyphenyl)ethanone] with diaroyl peroxides in 1,2-dichloroethane–trifluoroacetic acid (TFA) has been investigated. The principal two products after work-up are tris-(p-methoxyphenyl)methanol and p-methoxybenzoic acid; the latter was found as anhydrides in the reaction mixture. Free-radical formation in the course of the cleavage was verified by polymerization of added acrylonitrile in the oxidation by bis-(3,5-dinitrobenzoyl) peroxide. When the oxidation of [1,2-¹³C₂]anispinacolone (90%¹³C) by dibenzoyl peroxide was carried out in a ¹³C n.m.r. probe, an emission peak, assigned to p-methoxybenzoic trifluoroacetic anhydride, was observed. The logarithms of the rate constants for oxidation of p-substituted benzpinacolones by dibenzoyl peroxide were linearly correlated with the oxidation potentials of the benzpinacolones. These results are consistent with a single-electron transfer (s.e.t.) pathway from benzpinacolones to dibenzoyl peroxide. The oxidation is first-order in each reactant and is promoted by TFA. The effect of TFA is accounted for by two factors, (i) assisted O–O bond cleavage of the peroxide radical anion by TFA, and (ii) the formation of protonated peroxide, a more powerful oxidizing species. The former factor is dominant at lower TFA concentrations (<0.05M), the latter at higher concentrations.