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

Abstract

Nucleophilic addition reaction of benzenethiols (PhSH) to 1-(2-nitrovinyl) benzenes (<TEX>$\beta$</TEX> NS) in the presence of triethylamine (TEA) has been studied in acetonitrile at 25 <TEX>${^{\circ}C}$</TEX>. The rate is first order with respect to [PhSH], [TEA] and [<TEX>$\beta$</TEX> 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 <TEX>$\beta$</TEX>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 <TEX>$\rho_x$</TEX> in benzenethiols has been explained. The magnitude of cross-interaction constant, <TEX>$\rho_{xy}$</TEX> is small (0.08). The magnitude of the Hammett <TEX>$\rho_x$</TEX> values is higher than that of the Bronsted, <TEX>$\beta_x$</TEX> values for benzenethiols. The kinetic isotope effect, <TEX>$k_H/k_D$</TEX>, is found to be greater than unity. A suitable transition state with simultaneous formation of <TEX>$C_\beta$</TEX> -H and <TEX>$B_\alpha$</TEX> -S bonds involving the ion-pair and <TEX>$\beta$</TEX>NS in a single concerted step has been proposed to account for these observations.