THE LOW INTRINSIC REACTIVITY OF PICRYLACETONE - AN INDEX FOR THE PI-ACCEPTOR CAPABILITY OF A 2,4,6-TRINITROPHENYL STRUCTURE

Rates of deprotonation of picrylacetone, i.e., 2,4,6-trinitrophenylace tone 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(2)SO (v/v) mixture at 2 5 degrees C. The intrinsic reactivity (in the Marcus sense) of 4, as d etermined from the Bronsted plot for phenoxide reactions (beta(ArO) = 0.38) is low: log k(0)(ArO) = 2.37. This value is more typical for the formation of a strongly resonance-stabilized benzyl-type carbanion th an for that of an enolate-type carbanion and it may be regarded as a g ood measure of the high pi-acceptor capability of a conjugated 2,4,6-t rinitrophenyl 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 protonati on at its p-nitro group at low pH (pH < 6). The pK'(a) value associate d with the ionization of the resulting nitronic acid (C-4H) is approxi mate to 5, which compares well with similar data previously reported i n the literature.