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

Abstract

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°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 CO 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.