Kinetic Study on Aminolysis of 4-Nitrophenyl 2-Pyridyl Carbonate in Acetonitrile: Kinetic Evidence for a Stepwise Mechanism with Two Intermediates

Abstract

Nucleophilic substitution reactions of esters are an important class of reactions in biological processes and organic syntheses. Numerous studies on aminolysis of esters have been performed to investigate the reaction mechanism. Aminolysis of C=O centered esters has generally been reported to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate (T). However, stability of T has been suggested to be an important factor which governs the reaction mechanism, e.g., a forced concerted mechanism with an unstable T but a stepwise pathway with a stable T. We have recently reported that the reactions of 4-nitrophenyl 2-methoxybenzoate with a series of cyclic secondary amines in MeCN proceed through a stepwise mechanism with a stable T as modeled by I, in which the intramolecular H-bonding interactions provide a high stability to the cyclic intermediate. In contrast, the corresponding reactions of benzyl 2-pyridyl carbonate and t-butyl 2-pyridyl carbonate have been suggested to proceed through a forced concerted mechanism, although the reactions were predicted to proceed through a stepwise mechanism with a stable intermediate as modeled by II.