Methanol Attack on Highly Electrophilic 4,6-Dinitrobenzofurazan and 4,6-Dinitrobenzofuroxan Derivatives. A kinetic Study

Article abstract of DOI:10.1021/jo00196a015

Methanol readily adds to the unsubstituted 7-carbons of 4,6-dinitrobenzofurazan (DNBZ) and 4,6-dinitrobenzofuroxan (DNBF) as well as the methoxy-bearing carbon of 7-methoxy-4,6-dinitrobenzofurazan (MDNBZ) to form the most stable methoxyl ? adducts known in this solvent.The formation and decomposition of the adducts are subject to general base and general acid catalysis, respectively, with β (α) values of ca.0.50, indicating a concerted mechanism.The results suggest that methoxide ion behaves as a base catalyst for methanol addition rather than as a nucleophile.The intrinsic rate constants k₀ (in the Marcus sense) have been determined for the derivatives.Although the equilibrium constant for methanol addition to MDNBZ is somewhat greater than for addition to DNBZ and DNBF, the k₀ value for the latter derivatives is about 10-fold higher than that for MDNBZ.This indicates a higher intrinsic barrier for attack at a methoxy-bearing than at an unsubstituted carbon, in qualitative agreement with previously reported patterns in the benzene series.The high reactivity of DNBZ, DNBF, and MDNBZ toward methanol and other bases, together with the very high stability of the resulting adducts, emphasizes the superelectrophilic character of these electron-deficient aromatics.