Voltammetric investigation of the anodic dimerization of p-halogenoanilines in DMF -: Reactivity of their electrogenerated cation radicals

The electrodimerization mechanisms of p-chloro- and p-bromoaniline were studied in unbuffered DMF medium. By combined application of conventional and fast voltammetry (100 kV s(-1) range), the primary radical cation intermediates, formed by the one electron oxidation of each p-halogenoaniline were characterized. The overall reaction path involves a dimerization via a N-C bond formation and de-halogenation at the para position. A detailed mechanistic investigation demonstrates that this proceeds through a fast reversible deprotonation of the primary radical cation followed by the subsequent N-C bond formation between the resulting radical and its parent radical cation which is the rate-determining step of the sequence (e-p-RRC-p kinetic sequence). The effect of a relatively strong, but weakly nucleophilic base, 2,6-lutidine, has been also investigated and confirmed the involvement of the fast deprotonation pre-equilibria. The fast voltammetric experiments were simulated and the apparent rate constants for the overall deprotonation/dimerization sequence obtained on the basis of peak potential shift analysis were thus confirmed. (C) 1999 Elsevier Science S.A. All rights reserved.