σ complexes as biophysical and biochemical probes. Part III. Competitive demethylation and σ-complex formation in reaction of 4,6-dinitro-7-methoxybenzofuroxan with nucleophiles

Abstract

4,6-Dinitro-7-methoxybenzofuroxan (4) reacts with MeONa–MeOH or with Et2NH competitively by σ-complex formation and demethylation, by nucleophilic reaction at the aromatic and aliphatic carbon centers, respectively. Carbonate ion in CHCl3 in the presence of 18-crown-6 polyether also results in rapid demethylation. The facile leaving group ability in these SN2 displacements is consistent with the large acidity constant of 4,6-dinitro-7-hydroxybenzofuroxan (pKa ca. −3.7), i.e., 4pK units stronger than picric acid. Some unusual features exhibited in the nmr spectral characteristics of the σ-complexes are accounted for in terms of stereoelectronic effects.