Mechanistic investigation of the anodic oxidation of 3,4,5-trimethoxytoluene in acetonitrile

Preparative scale oxidation of 3,4,5-trimethoxytoluene in dry or wet acetonitrile leads predominantly to dimeric biphenyl products where the methoxy group in position 4 of a monomer has been formally displaced by the other monomer. Kinetic investigations show that the coupling of the two initially formed radical cations is faster than the nucleophilic attack of water or acetonitrile. The ensuing dication very rapidly expels a proton and generates a dimeric cation, observable in cyclic voltammetry but unstable on a longer time scale; elimination of methanol followed by addition of various nucleophiles present in the medium is responsible for the formation of the biphenyl products. The same reaction scheme is followed in the presence of a nonnucleophilic base, except that the evolution of the monocationic dimeric intermediate, which occurs as the second stage, is now significantly faster. Digital simulation of the experimental voltammograms recorded at various potential scan rates and concentrations support the kinetic schemes.