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 (k_(obsd)) 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-ethercomplexedK~+)inanhydrousethanolat25.0±0.1 °C. The plots of k_(obsd). [EtOM]0 curve upward regardless of the nature of the M~+ ions, while those of k_(obsd)/[EtO~-]_(eq) vs. [EtO~-]_(dq) are linear with a positive intercept. Dissection of k_(obsd) into k_(EtO)- and k_(EtOM) (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 k_(EtOM) vs. 1/r_(stokes) 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.