An unexpected reaction of 2-alkynylaryldiazonium tetrafluoroborate with sulfur dioxide

An unexpected result from the reaction of 2-alkynylaryldiazonium tetrafluoroborate with sulfur dioxide is described. In the presence of morpholin-4-amine, this transformation catalyzed by copper(I) bromide proceeds through insertion of sulfur dioxide and intramolecular 5-endo cyclization, leading to benzo[b]thiophene 1,1-dioxides in moderate to good yields.