A Kinetic Study on Ethylaminolysis of Phenyl Y-Substituted-Phenyl Carbonates: Effect of Leaving-Group Substituents on Reactivity and Reaction Mechanism

摘要

A kinetic study on nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5j) with ethylamine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C is reported. The plots of k_(obsd)vs. [amine] are linear for the reactions of substrates possessing a strong electron-withdrawing group (EWG) but curve upward for those of substrates bearing a weak EWG, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. The reactions have been concluded to proceed through a stepwise mechanism with one or two intermediates (a zwitterionic tetrahedral intermediate T~± and its deprotonated form T) depending on the nature of the substituent Y. Analysis of Bronsted-type plots and dissection of k_(obsd)into microscopic rate constants have revealed that the reactions of substrates possessing a strong EWG (e.g., 5a-5f) proceed through T~± with its formation being the rate-determining step, while those of substrates bearing a weak EWG (e.g., 5g-5j) proceed through T~± and T~-.