Site specificity of [3,3] sigmatropic rearrangements of 3-allyl- and 3-(prop-2-ynyl)-3H-indoles

Abstract

A series of 3-allyl-, 3-(prop-2-ynyl)-. and 3-(3-methylbut-2-enyl)-3H-indoles have been prepared by direct alkylation, and by alkylation of the appropriate indole Grignard reagent. Thermal rearrangement of these 3H-indoles and their N-methyl anhydro-bases demonstrates that the preferred migration terminus for a [3,3] sigmatropic rearrangement is, if present, a 2-alkyl group, and the rearrangement then occurs via the enamine tautomer of the 3H-indole. Allylic inversion of the migrating substituent is demonstrated by substituent labelling or deuteriation. When enamine formation is blocked the migration is diverted to nitrogen. Attempts to involve the 4-position of the indole nucleus were unsuccessful.