Anomalous Behavior of the Ethyl Group in the Aminolysis of S-Phenyl Acetate with Benzylamine in Acetonitrile

摘要

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 ( ヲ* ) and/or steric effect constants (E_s) of the substituents due to abnormally enhanced rate of the substrate with R=Et. Furthermore, the cross-interaction constant, ヱ_Xz , is the largest with R=Et. These anomalous behaviors can only be explained by invoking the vicinal bond (ヲ)-antibond ( ヲ* ) charge transfer interaction between C-Cメ and C-S bonds. In the tetrahedral zwitterionic intermediate, T【 , formed with R=Et the vicinal ヲ_C-C-ヲ*_C-S delocalization is the strongest with an optimum antiperiplanar arrangement and a narrow energy gap, ツュ= ュ_ヲ*- ュ_ヲ. Due to this charge transfer interaction, the stability of the intermediate increases (with the concomitant increase in the equilibrium constant K (= k_a/k_-a )) and also the leaving ability of the thiophenolate leaving group increases (and hence k_b increases) so that the overall rate, k_N = Kk_b, 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.