Chemical and electrochemical reduction of 2H-indole-3,5-dione and -dione 3-imine N-oxides

Abstract

Five of the title N-oxides 1 when reduced either chemically (phenylhydrazine, hydrazobenzene) or electrochemically (DMF–TEAP–protonating agent) were found to afford the corresponding, 5-hydroxyindoles 6. The reduction process takes place in two steps leading initially to 1,5-dihydroxyindoles 4 which undergo loss of water and are eventually converted into the products. EPR spectroscopic studies of the chemical and electrochemical reduction led to the characterization of the intermediate radical anions and neutral radicals, and a mechanism based on these results as well as on polarographic and cyclic voltammetric studies is proposed for the process.