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A kinetic study is reported for the nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5k) with piperidine in 80 mol % /20 mol % DMSO at . The plots of vs. [piperidine] for the reactions of substrates possessing a strong electron-withdrawing group (EWG) in the leaving group (i.e., 5a-5i) are linear and pass through the origin. In contrast, the plots for the reactions of substrates bearing a weak EWG or no substituent (i.e., 5j or 5k) curve upward, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. Thus, it has been suggested that the reactions of 5a-5i proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate (i.e., ) while those of 5j and 5k proceed through a stepwise mechanism with two intermediates (i.e., and its deprotonated form ). The slope of the Brnsted-type plot for the second-order rate constants (i.e., or ) changes from -0.41 to -1.89 as the leaving-group basicity increases, indicating that a change in the rate-determining step (RDS) occurs. The reactions of 5a-5k with piperidine result in larger values than the corresponding reactions with ethylamine.

Leaving-Group Substituent Controls Reactivity and Reaction Mechanism in Aminolysis of Phenyl Y-Substituted-Phenyl Carbonates