Theoretical Studies on the Base-Catalyzed Deprotonation of 4-Phenacylpyridinium Cations

Theoretical studies on the base-catalyzed deprotonation of 4-phenacylpyridinium cations, R1-CO-CH2-C5H4N-R2, I (R1=YC6H4 -and R2=CH3), and II (R1=C6H5 and R2=CH2C6H4Y) have been carried out with bases, NH3 and XC6H4NH2 using AM1 MO method. The Brψnsted α values are 0.20 and 0.22 and the βB values are 0.62 and 0.61, respectively for cations I and II. The negative Ⅰ (=α-βB) values obtained are in accord with the experimental results in aqueous solution, although the theoretical gas-phase α values for I are somewhat smaller than the experimental values in water due to neglect of solvation effect. It has been stressed that the Brψnsted α is distorted not only by the lag in the resonance and solvation development in the carbanion, but also by the difference in the distance between the anionic center and substituents in the TS and in the product anion.