Radical intermediates in the peroxidation of indoles

Abstract

2-Substituted and 1,2-disubstituted indoles react with m-chloroperbenzoic acid and hydrogen peroxide in the presence of acid or calcium chloride affording 2- and 3-(3-oxoindol-2-yl)indoles; whereas 2,3-disubstituted indoles, reacting with the same oxidants, lead to the formation of products typical of pentaatomic ring opening. The reaction mechanisms are discussed in terms of electron transfer processes based on the redox potentials of the reagents, the Marcus theory and the reaction products distribution. The reactions of 1-hydroxy-2-phenylindole, which yield 2-phenylisatogen (2-phenyl-3-oxo-3H-indole 1-oxide), bisnitrone and 3-(3-oxoindol-2-yl)indole are also explained by an electron transfer mechanism depending on the oxidant and on the conditions of the reaction. The structures of 2- and 3-(3-oxoindol-2-yl)indoles have been elucidated by X-ray analysis.