Effect of Alkali Metal Ions on Alkaline Ethanolysis of 2-Pyridyl and 4-Pyridyl Benzoates in Anhydrous Ethanol

Pseudo-first-order rate constants (k(obsd)) have been measured for nucleophilic substitution reactions of 2-pyridyl benzoate 5 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol. The plots of k(obsd) vs. [EtOM](o) are curved upwardly but linear in the excess presence of 18-crown-6-ether (18C6) with significant decreased k(obsd) values in the reaction with EtOK. The kobsd value for the reaction of 5 with a given EtONa concentration decreases steeply upon addition of I 5-crown-5-ether (15C5) to the reaction medium up to ca. [15C5]/[KEtONa](o) = 1, and remains nearly constant thereafter, indicating that M+ ions catalyze the reaction in the absence of the complexing agents. Dissection k(obsd) into k(EtO)- and k(EtOm), i.e., the second-order rate constants for the reaction with the dissociated EtO- and the ion-paired EtOM, respectively has revealed that ion-paired EtOM is 3.2 - 4.6 times more reactive than dissociated EtO-. It has been concluded that M+ ions increase the electrophilicity of the reaction center through a 6-membered cyclic transition state. This idea has been examined from the corresponding reactions of 4-pyridyl benzoate 6, which cannot form such a 6-membered cyclic transition state.