On the reactivity of 3-bromo-2-nitrobenzo[b]thiophene with nucleophiles: elucidation of the base-catalysed mechanism with rearrangement

The reactivity of 3-bromo-2-nitrobenzo[b]thiophene (1) with several (anionic and neutral) nucleophiles has been examined. Only with neutral, weak nucleophiles (as anilines) 1 gives, in the presence of non-nucleophilic bases (triethylamine or potassium carbonate), together with the ‘expected’ 3-amino-2-nitrobenzo[b]thiophenes (3) also the ‘unexpected’ 2-amino-3-nitrobenzo[b]thiophenes (4). The composition of the final isomeric mixture depends on the base added (nature and quantity) and on the solvent used. The results demonstrate the relevance of base-catalysis and support a reaction pathway involving the formation of an anionic intermediate (B) which undergoes addition of a second molecule of nucleophile to give (C) in which migration of the nitro group occurs through a three membered ring (D or E) formed by loss of bromide ion.