Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

Abstract

The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a−c (X = NO2, H, MeO) by a variety of bases have been measured in 50% H2O−50%Me2SO (v/v) at 25 °C. Comparison of Bronsted βB values for the ionization of each carbon acid by phenoxide and carboxylate bases and αCH values for deprotonation of 3a−c by individual buffers bases indicates that the reaction proceeds through strongly imbalanced transition states. The intrinsic reactivities of 3a−c, as determined from the Bronsted plots for phenoxide ion reactions, are typical for the formation of resonance-stabilized polynitrobenzyl-type carbanions, but the intrinsic rate constant k0 decreases regularly on going from the less acidic p-methoxyacetophenone derivative 3c to the more acidic p-nitroacetophenone derivative 3a. This trend is attributed to the fact that the contribution of the benzoyl moiety to the resonance stabilization of the resulting carbanion C-3 is completely negligible for the p-methoxy compound but not for the p-nitro compound. An extensive 1H and 13C NMR study of the ionization of 3a−c confirms this proposal, with steric hindrance to rotation of the picryl ring around the Cα−Cipso linkage being observed at the probe temperature in the p-methoxy-substituted carbanion C-3c, at −40 °C for the unsubstituted carbanion C-3b, but not at all for the p-nitro carbanion C-3a. A major finding, however, is that the three carbanions undergo protonation at the p-nitro group of the picryl ring to form nitronic acids in acidic media. This behavior clearly shows that charge delocalization through the 2,4,6-trinitrophenyl moiety is predominant in the three systems, including C-3a.