Oxidation of 2,3-dimethylindole by peroxophosphates

Abstract

The kinetics of the oxidation of 2,3-dimethylindole 1 by peroxodiphosphoric, PDP, and peroxomonophosphoric, PMP, acids have been investigated in sulfuric acid (0.1–1 mol dm^–3) and 20%v/v methanol–water solutions. On the basis of the spectral and kinetic results and by analogy with the mechanism previously proposed for the oxidation of 1 by peroxomono- and peroxodi-sulfate anions, we have concluded that the reactions proceed through the following steps: an electrophilic attack of the peroxide bond at the C-3 atom of the indole ring to form an indoleninic intermediate, a second peroxo attack on the enaminic tautomer of this intermediate and the hydrolysis of the product to finally give 3-methylindole-2-carbaldehyde. The analysis of the dependence of the reaction rates with acidity shows that, under our experimental conditions, neutral H₄P₂O₈ and cationic H₄PO₅⁺ are, respectively, the active species mainly involved in the oxidation processes.