Concurrent Methoxide Ion Attack at the 5- and 7-Carbons of 4-Nitrobenzofurazan and 4-Nitrobenzofuroxan. A Kinetic Study in Methanol

Article abstract of DOI:10.1021/jo01301a024

In methanolic solution,methoxide ions attack both the 5- and 7-carbons of 4-nitrobenzofurazan (5) and 4-nitrobenzofuroxan (8) to give the Meisenheimer-type complexes 6, 9 and 7, 10.The kinetics of the interactions have been studied by the stopped-flow method.The formation of the 5-methoxyl adducts 6 and 9 is found to always precede that of the thermodynamically more stable 7-methoxyl isomers 7 and 10.The results indicate that the para-like 4-nitro group is more efficient than the furazan and the furoxan rings in delocalizing the negative charge in 7 and 10.The kinetic and thermodynamic parameters Δ H⁰, Δ S⁰, Δ H(excit.), and Δ S(excit.) or the various reactions have been determined.It appears that only a large positive entropy change, Δ S⁰ (about +80 J/mol*K), is responsible for both the formation and the greater stability of the 7-methoxyl complexes 7 and 10 as compared to that of their 5-methoxyl analogues.Stabilization of 7 and 10 by association of the 4-nitro group with the potassium counterion is suggested.