Origin of the ¥á-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization

Bulletin of the Korean chemical society

Se Won Min¹  Ik Hwan Um¹  Jin A Seo¹  Mi Sun Kim¹ 

关键词: The ¥á-effect;    Concerted mechanism;    Ground-state destabilization;    Solvent effect;    Transition-state stabilization;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

Second-order rate constants (k_Nu? ) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-i) with butane-2,3-dione monoximate (Ox?, an �?-nucleophile) and Z-substituted phenoxides in 80 mol% H2O/20 mol% DMSO at 25.0 �? 0.1 oC. Hammett plots correlated with ヲo and �?^? constants for reactions of 1a-h with Ox^? exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in a good linear correlation with ヱY = 2.20 and r = 0.45, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS). A stepwise mechanism with expulsion of the leaving-group being the RDS has been excluded, since Y-substituted phenoxides are less basic and better nucleofuges than Ox^?. Thus, the reactions have been concluded to proceed through a concerted mechanism. Ox^? is over 102 times more reactive than its reference nucleophile, 4-chlorophenoxide (4-ClPhO^?). One might suggest that stabilization of the transition-state (TS) through intramolecular general acid/base catalysis is responsible for the �?-effect since such general acid/base catalysis is not possible for the corresponding reactions with 4-ClPhO^?. However, destabilization of the ground-state (GS) of Ox^? has been concluded to be mainly responsible for the �?-effect found in this study on the basis of the fact that the magnitude of the �?-effect is independent of the nature of the substituent Y.

【 授权许可】

Unknown