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.

A study of the enolization of phenylacetylpyrazine (PzCOCH(2)Ph) catalyzed by acid, base and metal ions in aqueous solution shows, unusually, that metal ions are more effective catalysts than protons, e.g., for zinc k(Zn)/k(H) = 600. Such behavior contrasts with that of the structurally related phenacylpyridine (PyCH(2)COPh) for which k(Zn)/k(H) = 0.0065. To interpret this difference, equilibrium constants for the tautomerization of phenylacetylpyrazine and for binding of protons and metal ions to its keto tautomer and enolate anion have been measured or estimated and are compared with existing measurements for phenacylpyridine. A tautomeric constant, K(E) = 1.2 x 10(-3) (pK(E) = 2.9), is derived by combining forward and reverse rate constants for enolization measured, respectively, by iodination or bromination of the keto tautomer and relaxation of the less stable enol. For the keto tautomer, NMR measurements yield a pK(a) = -0.90 for N-protonation, and spectrophotometric measurements give pK(a) = 11.90 for ionization to an enolate anion. For the enol, pK(a) values of 0.44 and -4.80 for mono- and diprotonation are obtained from the pH profile for ketonization and absorbance measurements for the transient enol reactant. Binding constants for metal ions (Cu(2+), Ni(2+), Zn(2+), Co(2+), and Cd(2+)) are derived from the saturation of their catalysis of the ketonization reaction. It is found that ketonization is efficiently catalyzed by metal ions but inhibited by acid. These findings, and the striking difference from phenacylpyridine, are ascribed to differences in thermodynamic driving force arising from stronger binding of the proton to the more basic pyridine than pyrazine nitrogen atom in both the reactant keto tautomer and in the enaminone or zwitterion product of the rate-determining (proton transfer) step of the enolization.