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Base-promoted transformation of 4-(3-nitroaryl)-1,2,3-selenadiazoles via intermediate formation of eneselenolates followed by 5-exo-trig cyclization is reported. The regiochemistry of the intramolecular cyclization is condition-dependent. In the presence of an oxidant, the oxidative nucleophilic substitution of the hydrogen (ONSH, S(N)Ar(H)) pathway, by oxidative aromatization of the rapidly formed σ(H)-adduct, takes place. In the absence of oxidant, the reaction proceeds via intermediate formation of the σ(Cl)-adduct, following nucleophilic aromatic substitution of the halogen (S(N)Ar(Cl)) pathway.

A reaction of 4-(2-mercaptophenyl)-1,2,3-thiadiazoles with an oxidant in the presence of 1.1 equiv of base afforded good yields of benzo[4,5]thieno[3,2-d][1,2,3]thiadiazoles via the intramolecular oxidative nucleophilic substitution of a hydrogen (ONHS) pathway. The reaction of 4-(2-mercaptophenyl)-1,2,3-thiadiazoles in the presence of ≥2 equiv of base gave 2-mercaptobenzo[b]thiophenes via an anionic ring-opening/ring-closure pathway.

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