Reactivity of indoles, benzo[b]thiophenes, benzo[b]furans, isoindoles, benzo[c]thiophenes and isobenzofurans

This chapter develops a description of the chemistry of indole, then discusses modifications necessary to rationalise the chemistry of the benzo[b]furan and benzo[b]thiophene analogues. The reactivity of N-magnesioindoles, which result from displacement of the active N-hydrogen with a Grignard reagent, or the analogous zinc derivatives, are rather different from that of the sodium, potassium and lithium salts. The ready electron availability in the heterocyclic ring means that indoles are rather easily (aut)oxidised in the five-membered ring. Reductions can be made selective for either ring: in acid solution dissolving metals attack the heteroring, and the benzenoid ring can be selectively reduced by Birch reduction. The chemical behaviour of isoindole, benzo[c]thiophene and isobenzofuran is dominated by their lack of a ‘complete’ benzene ring: these three heterocycles undergo cycloaddition processes across the 1,3-positions with great facility, because the products do now have a regular benzene ring.