A theoretical study on reaction mechanism of oxidative coupling reaction of p-phenylenediamine with phenol: a proposal of the route via a [5,5]-sigmatropic rearrangement

We have studied the mechanism for the reaction of p-benzoquinone diimine wi th phenolate, the model reaction of a key elementary step in oxidative coup ling of N,N-dialkyl-p-phenylenediamine with phenolic compounds, using ab in itio molecular orbital and density functional theory methods and analyzed t he reaction path in detail by the intrinsic reaction coordinate calculation . On the basis of the calculated results, we propose a new reaction mechani sm for formation of leuco-indoaniline through the suprafacial [5,5]-sigmatr opic rearrangement via an intermediate which has not been so far observed e xperimentally. After the intermediate is formed regioselectively under char ge control with no barrier, the rearrangement proceeds easily under orbital control into leuco-indoaniline via a rotational isomer. The activation ene rgy for this rearrangement was strongly dependent on the electron correlati on effects because of the stacking interaction between the two benzene ring s in the transition state, and the most reliable activation energy of appro ximately 7 kcal/mol was obtained by the B3LYP method with the 6-31 + G** ba sis set. (C) 1999 Elsevier Science B.V. All rights reserved.