Kinetic and equilibrium studies of the reactions of 4-nitrobenzofurazan and some derivatives with sulfite ions in water. Evidence for the Boulton–Katritzky rearrangement in a σ-adduct

Abstract

The reactions with sulfite ions in water of 4-nitrobenzofurazan, 1, 4-nitrobenzofuroxan, 2, and three 4-nitro-7-substituted benzofurazans have been examined by ¹H NMR spectroscopy and stopped-flow spectrophotometry. For each substrate the initial reaction occurs at the 5-position to give σ-adducts which have considerably higher thermodynamic stability than the corresponding adduct of 1,3,5-trinitrobenzene. In the cases of 1 and 2 isomerisation of the 5-adducts occurs slowly to yield adducts carrying sulfite at the 7-position. ¹H NMR measurements on 2 labelled with ¹⁵N provide evidence that here the rearrangement involves an intramolecular Boulton–Katritzky mechanism.