Alkali-Metal Ion Catalysis in Alkaline Ethanolysis of 2-Pyridyl Benzoate and Benzyl 2-Pyridyl Carbonate: Effect of Modification of Nonleaving Group from Benzoyl to Benzyloxycarbonyl

摘要

A kinetic study is reported on nucleophilic displacement reactions of benzyl 2-pyridyl carbonate 6 with alkali-metal ethoxides, EtOM (M = Li, Na, and K), in anhydrous ethanol at 25.0 ±0.1 °C. The plots of pseudo-first-order rate constant k_(obsd) vs. [EtOM] curve upward, a typical phenomenon reported previously for alkaline ethanolysis of esters in which alkali-metal ions behave as a Lewis-acid catalyst. The k_(obsd) value for the reaction of 6 with a fixed EtOK concentration decreases rapidly upon addition of 18-crown-6-ether (18C6), a complexing agent for K~+ ion up to [18C6]/[EtOK] = 1.0 and then remains constant thereafter, indicating that the catalytic effect exerted by K~+ ion disappears in the presence of excess 18C6. The reactivity of EtOM towards 6 increases in the order EtO~- < EtOLi < EtONa < EtOK, which is contrasting to the reactivity order reported for the corresponding reactions of 2-pyridyl benzoate 4, i.e., EtO" < EtOK < EtONa < EtOLi. Besides, 6 is 1.7 and 3.5 times more reactive than 4 towards dissociated EtO~- and ion-paired EtOK, respectively. The reactivity difference and the contrasting metal-ion selectivity are discussed in terms of electronic effects and transition-state structures.