Bulletin of the Korean chemical society | |
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 | |
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 (kNu? ) 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】
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