Kinetics and Reaction Mechanism of Aminolyses of Benzyl 2-Pyridyl Carbonate and t-Butyl 2-Pyridyl Carbonate: Effect of Nonleaving Group on Reactivity and Reaction Mechanism

摘要

Second-order rate constants (kN) have been measured spectrophotometrically for the reactions of benzyl 2- pyridyl carbonate 7 and t-butyl 2-pyridyl carbonate 8 with a series of alicyclic secondary amines in H2O at 25.0 oC. Substrate 8 is less reactive than 7. Steric hindrance exerted by the bulky t-Bu group of 8 has been suggested to be responsible for the decreased reactivity. The Brønsted-type plots for the reactions of 7 and 8 are linear with βnuc = 0.49 and 0.44, respectively, which is typical for reactions reported previously to proceed through a concerted mechanism. Aminolyses of 7 and 8 were expected to proceed through a zwitterionic tetrahedral intermediate T±, which would be stabilized through an intramolecular H-bonding interaction. However, the kinetic results suggest that the reactions proceed through a concerted mechanism. The H-bonding interaction in T± has been suggested to accelerate the rate of leaving-group expulsion from T±. Another factor that might accelerate expulsion of the leaving group is the “push” provided by the RO group in T± through resonance interactions. Thus, it has been concluded that the enhanced nucleofugality through the H-bonding interaction and the “push” provided by the RO group forces the reactions to proceed through a concerted mechanism.