Electrophilic substitution in indoles. Part 12. Kinetic studies of the rearrangement of 3,3-disubstituted indolenines to 2,3-disubstituted indoles

Abstract

The rearrangements of 3-alkyl-3-methylindolenine salts to the corresponding 2-alkyl-3-methylindoles have been studied over a range of temperatures by u.v. methods in isopropyl alcohol containing hydrochloric acid. The relative migratory aptitudes of the various alkyl groups were as follows: Me, 1; Et, 14; Prⁱ, 120; allyl, 400; p-nitrobenzyl, 188; benzyl, 1 520. Attempts to prepare the analogous p-methylbenzyl and p-methoxybenzyl-3-methylindolenines by alkylation of the 3-methylindole Grignard reagent gave only the 2-benzylindoles owing to the much higher migratory aptitude of the p-methylbenzyl and p-methoxybenzyl groups. Spiro[cyclopentane-3′-indolenine] hydrochloride rearranged to tetrahydrocarbazole at ca. 7400 times the rate of rearrangement of 3,3-dimethylindolenine hydrochloride to 2,3-dimethylindole. Thermodynamic parameters for the various reactions were determined in view of their relevance to the mechanism of electrophilic substitution in 3-substituted indoles.