Rates of deprotonation of picrylacetone, i e . , 2,4,&trinitrophenylacetone 4, by a variety of bases B (phenoxide and carboxylate ions, OH-) and of protonation of the resulting carbanion (C-4) by the conjugated acids BH+ have been measured in a 50%H2O-50%Me,SO (v/v) mixture at 25 "C. The intrinsic reactivity (in the Marcus sense) of 4, as determined from the Brensted plot for phenoxide reactions @*" = 0.38) is low: log = 2.37. This value is more typical for the formation of a strongly resonancestabilized benzyl-type carbanion than for that of an enolate-type carbanion and it may be regarded as a good measure of the high n-acceptor capability of a conjugated 2,4,6trinitrophenyl structure. Definitive evidence that the negative charge of C-4 is essentially delocalized through the picryl moiety comes from the observation that this carbanion undergoes instantaneous protonation at itsp-nitro group at low pH (pH < 6). The pKi value associated with the ionization of the resulting nitronic acid (C-4H) is z 5, which compares well with similar data previously reported in the literature.
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 B values for the ionization of each carbon acid by phenoxide and carboxylate bases and R 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 R -C ipso linkage being observed at the probe temperature in the p-methoxysubstituted 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.
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