A kinetic study is reported for the nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5k) with piperidine in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 o C. The plots of k obsd 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., T ± ) while those of 5j and 5k proceed through a stepwise mechanism with two intermediates (i.e., T ± and its deprotonated form T -). The slope of the Brønsted-type plot for the second-order rate constants (i.e., k N or Kk 2 ) 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 k 1 values than the corresponding reactions with ethylamine.

Pseudo-first-order rate constants (k obsd ) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2-pyridyl thionobenzoate (5b) with alkali-metal ethoxides (EtOM, M + = Li ] eq are linear with a positive intercept. Dissection of k obsd into k EtO − and k EtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO -and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is more reactive than the dissociated EtO -, and M + ions catalyze the reactions in the order K + < NaThe plot of log k EtOM vs. 1/r Stokes results in an excellent linear correlation, indicating that the reactions are catalyzed by the solvated M + ions but not by the bare M + ions. The reactions of 5b with EtOM have been concluded to proceed through a six-membered cyclic TS, in which the solvated M + ions increase the electrophilicity of the reaction center and the nucleofugality of the leaving group.

A kinetic study on nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5j) with ethylamine in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 o 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 Brønsted-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 -.