Transient Nitronic Acid Formation in the Acid-Catalyzed Decomposition of Nitrobenzofuroxan and Nitrobenzofurazan ?-Adducts in Methanolic Solution. A Kinetic Study

Article abstract of DOI:10.1021/jo00223a031

Kinetic data for the acid-ctalyzed and uncatalyzed decomposition of the dioxolane spiro complexes of 7-(2-hydroxyethoxy)-4-nitrobenzofurazan and -benzofuroxan and of the 7,7-dimethoxy complexes of 7-methoxy-4-nitrobenzofurazan and -benzofuroxan have been obtained over a large pH range in methanol.The ring opening of the spiro complexes but not the methoxide ion departure from the dimethoxy adducts is found to be appreciably catalyzed by carboxylic acids.The corresponding Broensted α coefficients are equal to about 0.5, indicating concerted acid catalysis.At low pH, i.e., pH < 5.5, a fast equilibrium protonation of the para-like NO2 group of the adducts precedes the decomposition process.The pK_a values associated with the ionization of the resulting nitronic acids are all very similar and of the order of 4.2 - 4.5, as compared with estimated pK_a values of about 1 - 2 for analogous nitronic acids of picryl ?-complexes.Kinetic data for the formation of the adducts are also reported.The marked differences observed in the rates of formation and decomposition of the similarly stable benzofurazan and benzofuroxan spiro adducts are interpreted in terms of electrostatic effects connected with the presence of the N-oxide group in the transition states for the benzofuroxan reactions.