Kinetics and Mechanism of Triethylamine Catalysed Michael Addition of Benzenethiol to 1-(2-Nitrovinyl)benzene in Acetonitrile

摘要

Nucleophilic addition reaction of benzenethiols (PhSH) to 1-(2-nitrovinyl) benzenes (モNS) in the presence of triethylamine (TEA) has been studied in acetonitrile at 25∩. The rate is first order with respect to [PhSH], [TEA] and [モNS]. The reaction is found to proceed with the formation of ion-pair between benzenethiol and TEA. A suitable mechanism with the formation of an adduct between ion-pair and substrate in a slow step followed by its rearrangement to 1,2-addition product in a fast step has been proposed and corresponding rate law derived. From the rate law, the rate constants for the interaction between ion-pair and モNS have been evaluated. Interestingly, in both para-substituted substrates and benzenethiols the rate increases with the electron-withdrawing power of the substituents. The positive sign of ヱ_x in benzenethiols has been explained. The magnitude of cross-interaction constant, ヱ_xy is small (0.08). The magnitude of the Hammett ヱ_x values is higher than that of the Bronsted, モ_x values for benzenethiols. The kinetic isotope effect, k_H/k_D, is found to be greater than unity. A suitable transition state with simultaneous formation of C_モ-H and C_メ-S bonds involving the ion-pair and モNS in a single concerted step has been proposed to account for these observations.