Metal ion catalysis in nucleophilic substitution reaction of 4-nitrophenyl picolinate with alkali metal ethoxides in anhydrous ethanol

Abstract

Pseudo-first-order rate constants (kobsd) were measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl picolinate (6) with alkali metal ethoxides (EtOM, M+ = K+, Na+ and Li+) in anhydrous ethanol at 25.0 ± 0.1 °C. The plot of kobsd vs. [EtOM] exhibits upward curvature regardless of the nature of M+ ions. However, the plot for the reaction of 6 with EtOK is linear with significantly decreased kobsd values when 18-crown-6-ether (18C6, a complexing agent for K+ ion) is added in the reaction medium. Dissection of kobsd into k EtO- and kEtOM (i.e., the second-order rate constant for the reaction with dissociated EtO- and ion-paired EtOM, respectively) has revealed that ion-paired EtOM is 3 ∼ 17 times more reactive than dissociated EtO-. The reaction has been proposed to proceed through a 5-membered cyclic transition state, in which M+ ion increases the electrophilicity of the reaction site. Interestingly, Na+ ion exhibits the largest catalytic effect. The presence of a nitrogen atom in the pyridine moiety of 6 has been suggested to be responsible for the high Na + ion selectivity.