One-electron oxidation of closed-shell molecules. Part 1. Oxidative cleavage of para-substituted benzpinacolones by cerium(IV) ammonium nitrate

Abstract

The stoicheiometry and kinetics of the Ce^IV oxidation of 1,2,2,2-tetrakis-(p-methoxyphenyl)ethanone (1a) have been investigated at 25.0 °C in acetic acid as solvent by the use of cerium(IV) ammonium nitrate as oxidant. The reaction occurs with the stoicheiometry 2 Ce^IV+ An₃C·CO·An → 2 Ce^III+ An₃C⁺+ An·CO⁺(An =p-methoxyphenyl), affording tris-(p-methoxyphenyl)methanol and anisic acid after work-up. The kinetics were monitored by u.v.–visible spectrophotometry of the tris-(p-methoxyphenyl)methyl cation, showing that the rate-determining step is first order in each reactant. Evidence was found for the formation of p-methoxybenzoyl radical by means of spin-trapping with 2,3,5,6-tetrakis(trideuteriomethyl)nitrosobenzene. The rate constants at 25.0 °C, the oxidation potential, as measured by cyclic voltammetry, and the charge transfer energy with tetracyanoethylene have been measured for five para-substituted benzpinacolones (substituents MeO, Me, and Cl), including (1a). A good correlation of the logarithms of the rates with the oxidation potentials or with the charge transfer energies was found, indicating that a single electron-transfer step between the substrate and Ce^IV is rate determining. The formation of a radical cation of the benzpinacolone in the rate-determining step and subsequent rapid cleavage into the triarylmethyl cation and the aroyl radical are proposed as a possible mechanism.