TRANSITION-STATE IMBALANCES IN THE DEPROTONATION OF PICRYLACETOPHENONES BY CARBOXYLATE AND PHENOXIDE BASES

The kinetics of the reversible deprotonation of 4-X-substituted picryl acetophenones 3a-c (X = NO2, H, MeO) by a variety of bases have been m easured in 50% H(2)0-50%Me(2)SO (v/v) at 25 degrees C. Comparison of B ronsted PB values for the ionization of each carbon acid by phenoxide and carboxylate bases and alpha(CH) values for deprotonation of 3a-c b y individual buffers bases indicates that the reaction proceeds throug h strongly imbalanced transition states. The intrinsic reactivities of 3a-c, as determined from the Bronsted plots for phenoxide ion reactio ns, are typical for the formation of resonance-stabilized polynitroben zyl-type carbanions, but the intrinsic rate constant k(0) decreases re gularly on going from the less acidic p-methoxyacetophenone derivative 3c to the more acidic p-nitroacetophenone derivative 3a. This trend i s attributed to the fact that the contribution of the benzoyl moiety t o the resonance stabilization of the resulting carbanion C-3 is comple tely negligible for the p-methoxy compound but not for the p-nitro com pound. An extensive H-1 and C-13 NMR study of the ionization of 3a-c c onfirms this proposal, with steric hindrance to rotation of the picryl ring around the C-alpha-C-ipso linkage being observed at the probe te mperature in the p-methoxy-substituted carbanion C-3c, at -40 degrees 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 carbanion s undergo protonation at the p-nitro group of the picryl ring to form nitronic acids in acidic media. This behavior clearly shows that charg e delocalization through the 2,4,6-trinitrophenyl moiety is predominan t in the three systems, including C-3a.