Kinetic and thermodynamic acidities of substituted 1-benzyl-1-methoxy-2-nitroethylenes. Strong reduction of the transition state imbalance compared to other nitroalkanes

Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z = m-NO2, m-CF3, m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50% water (v/v) at 20 degreesC. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH- and piperidine and on the reversible deprotonation of 2-NO2 by piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine in the same solvent. These data allowed a determination of the Bronsted coefficients a. (dependence on acidity of 2-Z) and beta (dependence on amine basicity). The fact that alpha > beta indicates the presence of a transition state imbalance which, however, is much smaller than that for the deprotonation of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study of the deprotonation of Fischer carbene complexes are discussed.