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 <TEX>$k_{amine}$</TEX> have been measured spectrophotometrically for the reactions of benzyl 4-pyridyl carbonate 6 with a series of alicyclic secondary amines in <TEX>$H_2O$</TEX> at <TEX>$25.0^{\circ}C$</TEX>. The plots of <TEX>$k_{amine}$</TEX> vs. [amine] curve upward, indicating that the reactions proceed through a stepwise mechanism with two intermediates, a zwitterionic tetrahedral intermediate <TEX>$T^{\pm}$</TEX> and its deprotonated form <TEX>$T^-$</TEX>. This contrasts to the report that the corresponding reactions of benzyl 2-pyridyl carbonate 5 proceed through a forced concerted pathway. The <TEX>$k_{amine}$</TEX> values for the reactions of 6 have been dissected into the second-order rate constant <TEX>$Kk_2$</TEX> and the thirdorder rate constant <TEX>$Kk_3$</TEX>. The Br<TEX>${\o}$</TEX>nsted-type plots are linear with <TEX>${\beta}_{nuc}=0.94$</TEX> and 1.18 for <TEX>$Kk_2$</TEX> and <TEX>$Kk_3$</TEX>, respectively. The <TEX>$Kk_2$</TEX> for the reaction of 6 is smaller than the second-order rate constant <TEX>$k_N$</TEX> for the corresponding reaction of 5, although 4-pyridyloxide in 6 is less basic and a better nucleofuge than 2-pyridyloxide in 5.