Effect of alkali metal ions on alkaline ethanolysis of 2-pyridyl and 4-pyridyl benzoates in anhydrous ethanol

Abstract

Pseudo-first-order rate constants (kobsd) 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 15-crown-5-ether (15C5) to the reaction medium up to ca. [15C5]/[EtONa]o = 1, and remains nearly constant thereafter, indicating that M+ ions catalyze the reaction in the absence of the complexing agents. Dissection kobsd into kEtO- 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.