Metal ion catalysis in nucleophilic displacement reactions of 2-pyridyl X-substituted benzoates with potassium ethoxide in anhydrous ethanol

Abstract

A kinetic study on nucleophilic displacement reactions of 2-pyridyl X-substituted benzoates 1a-e with potassium ethoxide (EtOK) in anhydrous ethanol is reported. Plots of pseudo-first-order rate constants (kobsd) vs. [EtOK]° exhibit upward curvature. The kobsd value at a fixed [EtOK]° decreases steeply upon addition of 18-crown-6-ether (18C6) to the reaction mixture up to [18C6]/[EtOK]° = 1 and then remains nearly constant thereafter. In contrast, kobsd increases sharply upon addition of LiSCN or KSCN. Dissection of kobsd into kEtO- and k EtOM has revealed that ion-paired EtOK is more reactive than dissociated EtO , indicating that K+ ion acts as a Lewis acid catalyst. Hammett plots for the reactions of 1a-e with dissociated EtO and ion-paired EtOK result in excellent linear correlation with p values of 3.01 and 2.67, respectively. The kEtOK/kEtO ratio increases as the substituent X in the benzoyl moiety becomes a stronger electron-donating group. K+ ion has been concluded to catalyze the current reaction by stabilizing the transition state through formation of a 6-membered cyclic complex.