TRANSITION-STATE IMBALANCE IN THE GENERAL-BASE CATALYSIS OF THE DEPROTONATION OF 4-PHENACYLPYRIDINIUM CATIONS

Second-order rate constants (k(B)) have been measured in aqueous solution at ionic strength 0.1 and 25-degrees-C for the general-base-catalyzed deprotonation of eight 1-methyl-4-(X-phenacyl)pyridinium cations (1), eight 1-(X-benzyl)-4-phenacylpyridinium cations (2), and five 1-benzyl-4-(X-phenacyl)pyridinium cations (3) by a variety of primary amines and also N-methyl- and N,N-dimethylbenzylamine. Bronsted plots for the deprotonation of 1a (X = H) by these amine bases show considerable scatter. However, there are close linear correlations between log k(B) for 1a (and 2a) and log k(B) for the deprotonation of the 1-methyl-4-(phenylacetyl)pyridinium cation (4) by these same amine bases. This observation indicates that the solvation phenomena that have previously been established as the reasons for scattered Bronsted plots for the deprotonation of 4 are general phenomena for the deprotonation of ketonic carbon acids by amine bases. Bronsted alpha-values for the deprotonation of 1-3 by individual amine bases indicate significant transition-state imbalances between the developing cationic charge on the ammonium ion conjugate acids and the delocalization of electron density in the developing enolate ion conjugate bases. Comparisons of Bronsted alpha-values for 1 and 2 reacting with the same amine base also indicate transition-state imbalance within the developing enolate ion conjugate base species. The Bronsted alpha-values for the reaction of 2 with benzylamine and its N-methyl and N,N-dimethyl derivatives are particularly small (alpha = 0.14-0.18) and suggest an interaction between the aromatic ring of these amine bases and the pyridinium ring of 2. This interaction results in delayed delocalization of electron density into the pyridinium ring in the transition-state species. All of these data can be analyzed in terms of Marcus theory by use of a variable intrinsic barrier, in a manner similar to that described previously for related base-catalyzed deprotonation reactions.