The remarkable effect of 7-amino substituents on the reactivity of 4-nitrobenzofurazans with nucleophiles

The reactions of four 4-nitrobenzofurazans (4a–4d) substituted at the 7 position with (a) N-ethyl, (b) N-butyl, (c) piperidino, or (d) pyrrolidino groups have been examined with sulfite ions and with hydroxide ions in water–DMSO (80:20, v/v). Addition of sulfite at the 5 position gives σ adducts with equilibrium constants ca. 105 lower than that for the formation of the corresponding adduct from 4-nitrobenzofurazan. These reductions are attributed to the stabilization of the parent molecules 4a–4d by conjugative interaction between the 4 and 7 substituents. In alkaline solution, 4a and 4b yield the conjugate bases while 4c and 4d suffer nucleophilic substitution to give 7-hydroxy-4-nitrobenzofurazan. Reactivity here is relatively high because of the iminium ion character of the substrates. Key words: benzofurazans, σ adducts, nucleophilic reactivity, substitution.