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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 4
BKCSDE 29(4)
April 20, 2008 

Kinetics and Mechanism of Triethylamine Catalysed Michael Addition of Benzenethiol to 1-(2-Nitrovinyl)benzene in Acetonitrile
P. A. Sarathi, C. Gnanasekaran, A. Shunmugasundaram*
Kinetics, Michael addition, 1-(2-Vinylnitro)benzene, Ion-pair and concerted addition
Nucleophilic addition reaction of benzenethiols (PhSH) to 1-(2-nitrovinyl) benzenes (βNS) in the presence of triethylamine (TEA) has been studied in acetonitrile at 25℃. The rate is first order with respect to [PhSH], [TEA] and [βNS]. 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 βNS 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 ρx in benzenethiols has been explained. The magnitude of cross-interaction constant, ρxy is small (0.08). The magnitude of the Hammett ρx values is higher than that of the Bronsted, βx values for benzenethiols. The kinetic isotope effect, kH/kD, is found to be greater than unity. A suitable transition state with simultaneous formation of Cβ-H and Cα-S bonds involving the ion-pair and βNS in a single concerted step has been proposed to account for these observations.
790 - 794
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