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

Pseudo-first-order rate constants k(amine) 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 degrees C. The plots of k(amine) 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 k(amine) values for the reactions of 6 have been dissected into the second-order rate constant Kk(2) and the thir-dorder rate constant Kk(3). The Bronsted-type plots are linear with beta(nuc) = 0.94 and 1.18 for Kk(2) and Kk(3), respectively. The Kk(2) for the reaction of 6 is smaller than the second-order rate constant k(N) for the corresponding reaction of 5, although 4-pyridyloxide in 6 is less basic and a better nucleofuge than 2-pyridyloxide in 5.