Aminolysis of benzyl 2-pyridyl thionocarbonate and t-butyl 2-pyridyl thionocarbonate: Effects of nonleaving groups on reactivity and reaction mechanism

Abstract

A kinetic study is reported for nucleophilic substitution reactions of benzyl 2-pyridyl thionocarbonate (5b) and t-butyl 2-pyridyl thionocarbonate (6b) with a series of alicyclic secondary amines in H2O at 25.0 °C. Generalbase catalysis, which has often been reported to occur for aminolysis of esters possessing a C=S electrophilic center, is absent for the reactions of 5b and 6b. The Brønsted-type plots for the reactions of 5b and 6b are linear with ßnuc = 0.29 and 0.43, respectively, indicating that the reactions of 5b proceed through a stepwise mechanism with formation of a zwitterionic tetrahedral intermediate (T±) being the rate-determining step while those of 6b proceed through a concerted mechanism. The reactivity of 5b and 6b is similar to that of their oxygen analogues (i.e., benzyl 2-pyridyl carbonate 5a and t-butyl 2-pyridyl carbonate 6a, respectively), indicating that the effect of modification of the electrophilic center from C=O to C=S (i.e., from 5a to 5b and from 6a to 6b) on reactivity is insignificant. In contrast, 6b is much less reactive than 5b, indicating that the replacement of the PhCH2 in 5b by the t-Bu in 6b results in a significant decrease in reactivity as well as a change in the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway). It has been concluded that the contrasting reactivity and reaction mechanism for the reactions of 5b and 6b are not due to the electronic effects of PhCH2 and t-Bu but are caused by the large steric hindrance exerted by the bulky t-Bu in 6b.