Kinetic Study on Aminolysis of O-2-Pyridyl Thionobenzoate in Acetonitrile: Effect of Changing Electrophilic Center from C=O to C=S on Reactivity and Reaction Mechanism

Pseudo-first-order rate constants (k(obsd)) for nucleophilic substitution reaction of O-2-pyridyl thionobenzoate (7) with a series of secondary amines in MeCN at 25.0 +/- 0.1 degrees C have been measured spectrophotometrically. The plots of k(obsd) vs. [amine] curve upward, indicating that the reaction proceeds through a stepwise mechanism with a zwitterionic tetrahedral intermediate (T-+/-), which decomposes to the products through uncatalyzed and catalyzed routes. It has been proposed that the uncatalyzed reaction proceeds through a six-membered cyclic transition state (TS), in which expulsion of the leaving group occurs in the rate-determining step. The catalyzed reaction from T-+/- proceeds through a concerted mechanism with a six-membered cyclic TS rather than via a stepwise pathway with an anionic intermediate T-. This is in contrast to the report that the corresponding reaction of 2-pyridyl benzoate (6, a CO analogue of 7) proceeds through a forced concerted mechanism. Comparison of the second-order rate constants for the uncatalyzed reaction of 7 with those reported previously for the corresponding reaction of 6 has revealed that 7 is much more reactive than 6. Factors that affect the reactivity and reaction mechanism are discussed in detail.