Kinetics and Reaction Mechanism for Aminolysis of Benzyl 4-Pyridyl Carbonate in H2O: Effect of Modification of Nucleofuge from 2-Pyridyloxide to 4-Pyridyloxide on Reactivity and Reaction Mechanism

摘要

Pseudo-first-order rate constants kamine have been measured spectrophotometrically for the reactions of benzyl 4-pyridyl carbonate 6 with a series of alicyclic secondary amines in H2O at 25.0 oC. The plots of kamine vs. [amine] curve upward, indicating that the reactions proceed through a stepwise mechanism with two intermediates, a zwitterionic tetrahedral intermediate T± and its deprotonated form T–. This contrasts to the report that the corresponding reactions of benzyl 2-pyridyl carbonate 5 proceed through a forced concerted pathway. The kamine values for the reactions of 6 have been dissected into the second-order rate constant Kk2 and the thirdorder rate constant Kk3. The Brønsted-type plots are linear with βnuc = 0.94 and 1.18 for Kk2 and Kk3, respectively. The Kk2 for the reaction of 6 is smaller than the second-order rate constant kN for the corresponding reaction of 5, although 4-pyridyloxide in 6 is less basic and a better nucleofuge than 2-pyridyloxide in 5.