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

Abstract

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 H 2O at 25.0°C. 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 Kk 2 and the thirdorder rate constant Kk 3. The Brønsted-type plots are linear with β 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 kN for the corresponding reaction of 5, although 4-pyridyloxide in 6 is less basic and a better nucleofuge than 2-pyridyloxide in 5.