Metal-Ion Catalysis in Alkaline Ethanolysis of 2-Pyridyl Thionobenzoate: Effects of Modification of Electrophilic Center from C=O to C=S

摘要

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2-pyridyl thionobenzoate (5b) with alkali-metal ethoxides (EtOM, M+ = Li+, Na+, K+, and 18-crown-6-ether complexed K+) in anhydrous ethanol at 25.0 ± 0.1 oC. The plots of kobsd vs. [EtOM]o curve upward regardless of the nature of the M+ ions, while those of kobsd/[EtO–]eq vs. [EtO–]eq are linear with a positive intercept. Dissection of kobsd into kEtO− and kEtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO– and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is more reactive than the dissociated EtO–, and M+ ions catalyze the reactions in the order K+ Na+ Li+ 18C6- complexed K+. The plot of log kEtOM vs. 1/rStokes results in an excellent linear correlation, indicating that the reactions are catalyzed by the solvated M+ ions but not by the bare M+ ions. The reactions of 5b with EtOM have been concluded to proceed through a six-membered cyclic TS, in which the solvated M+ ions increase the electrophilicity of the reaction center and the nucleofugality of the leaving group.