σ-Adduct formation and oxidative substitution in the reactions of 4-nitrobenzofurazan and some derivatives with hydroxide ions in water

The reactions of hydroxide ions with 4-nitrobenzofurazan, 1a, 4-nitrobenzofuroxan, 1b, and with three 4-nitro-7-substituted benzofurazans have been examined using 1H NMR and UV-visible spectroscopies. In each case initial reaction is at the 5-position to give an anionic sigma-adduct. Kinetic and equilibrium results are reported. NMR spectra show that in the case of 1a oxidation of the anionic adduct yields 4-nitro-5-hydroxybenzofurazan. In the case of 1b rearrangement of the 5-hydroxy adduct to the thermodynamically more stable 7-hydroxy adduct, possibly by a Boulton-Katritzky mechanism, precedes oxidation. When the 7-substituent in the 4-nitrobenzofurazan is Cl, OMe or OPh the eventual product is 7-hydroxy-4-nitrobenzofurazan produced by nucleophilic displacement of the substituent.