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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 23, Number 2
BKCSDE 23(2)
February 20, 2002 

Anomalous Behavior of the Ethyl Group in the Aminolysis of S-Phenyl Acetate with Benzylamine in Acetonitrile
Ikchoon Lee, Hai Whang Lee, Byung Choon Lee, Jin Heui Choi
Aminolysis of S-phenyl substituted-acetate series, Anomalous behaviors of the ethyl group, Natural bond orbital (NBO) analysis, Cross-interaction constant.
The rates of the aminolysis of S-phenyl substituted-acetate series (RC(=O)SC6H4Z, with R=Me, Et, i-Pr, t-Bu and Bn) with benzylamines (XC6H4CH2NH2) are not correlated simply with the Taft’s polar ( σ* ) and/or steric effect constants (Es) of the substituents due to abnormally enhanced rate of the substrate with R=Et. Furthermore, the cross-interaction constant, ρXz , is the largest with R=Et. These anomalous behaviors can only be explained by invoking the vicinal bond (σ)-antibond ( σ* ) charge transfer interaction between C-Cα and C-S bonds. In the tetrahedral zwitterionic intermediate, T± , formed with R=Et the vicinal σC-C*C-S delocalization is the strongest with an optimum antiperiplanar arrangement and a narrow energy gap, Δε= εσ*- εσ. Due to this charge transfer interaction, the stability of the intermediate increases (with the concomitant increase in the equilibrium constant K (= ka/k-a )) and also the leaving ability of the thiophenolate leaving group increases (and hence kb increases) so that the overall rate, kN = Kkb, is strongly enhanced. Theoretical support is provided by the natural bond orbital (NBO) analyses at the B3LYP/6-31+G* level. The anomaly exhibited by R=Et attests to the stepwise reaction mechanism in which the leaving group departure is rate limiting.
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