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

The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a−c (X = NO 2, H, MeO) by a variety of bases have been measured in 50% H 2 O−50%Me 2 SO (v/v) at 25 °C. Comparison of Bronsted β B values for the ionization of each carbon acid by phenoxide and carboxylate bases and α CH 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 k 0 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 1 H and 13 C NMR study of the ionization of 3a−c confirms this proposal, with steric hindrance to rotation of the picryl ring around the C α −C ipso 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 .