The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl X-Substituted-Cinnamates with Butane-2,3-dione Monoximate
作者:Min-Young Kim、Yu-Jin Son、Ik-Hwan Um
DOI:10.5012/bkcs.2013.34.10.2877
日期:2013.10.20
Second-order rate constants ($k_Ox^-}$) have been measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl X-substituted-cinnamates (7a-7e) and Y-substituted-phenyl cinnamates (8a-8e) with butane-2,3-dione monoximate ($Ox^-$) in 80 mol % $H_2O$/20 mol % DMSO at $25.0\pm}0.1^\circ}C$. The Hammett plot for the reactions of 7a-7e consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linearity with $\rho}_X$=0.85 and r=0.58, indicating that the nonlinear Hammett plot is not due to a change in the rate-determining step but is caused by resonance stabilization of the ground state (GS) of the substrate possessing an electron-donating group (EDG). The Br$\o}$nsted-type plot for the reactions of Y-substituted-phenyl cinnamates (8a-8e) is linear with $\beta}_lg}$ = -0.64, which is typical of reactions reported previously to proceed through a concerted mechanism. The $\alpha}$-nucleophile ($Ox^-$) is more reactive than the reference normal-nucleophile ($4-ClPhO^-$). The magnitude of the $\alpha}$-effect (i.e., the $k_Ox^-}/k_4-ClPhO^-}$ ratio) is independent of the electronic nature of the substituent X in the nonleaving group but increases linearly as the substituent Y in the leaving group becomes a weaker electron-withdrawing group (EWG). It has been concluded that the difference in solvation energy between $Ox^-$ and $4-ClPhO^-$ (i.e., GS effect) is not solely responsible for the $\alpha}$-effect but stabilization of transition state (TS) through a cyclic TS structure contributes also to the Y-dependent $\alpha}$-effect trend (i.e., TS effect).
二阶速率常数($k_Ox^-}$)通过分光光度法测定,用于4-硝基苯基X-取代肉桂酸酯(7a-7e)和Y-取代苯基肉桂酸酯(8a-8e)与丁烷-2,3-二酮单肟盐($Ox^-$)在$25.0\pm}0.1^\circ}C$下80 mol % $H_2O$/20 mol % DMSO中的亲核取代反应。7a-7e反应的Hammett图由两条相交直线组成,而Yukawa-Tsuno图显示出极佳的线性,$\rho}_X$=0.85,r=0.58,表明非线性Hammett图并非由于速率决定步骤变化所致,而是由具有供电子基团(EDG)的底物基态(GS)共振稳定化引起。Y-取代苯基肉桂酸酯(8a-8e)反应的Br$\o}$nsted型图线性,$\beta}_lg}$ = -0.64,这与先前报道的通过协同机制进行的反应一致。$\alpha}$-亲核试剂($Ox^-$)比参考正常亲核试剂($4-ClPhO^-$)更具反应活性。$\alpha}$-效应(即$k_Ox^-}/k_4-ClPhO^-}$比值)的大小与非离去基团中取代基X的电子性质无关,但随着离去基团中取代基Y变为更弱的吸电子基团(EWG)而线性增加。结论是,$Ox^-$与$4-ClPhO^-$之间的溶剂化能差异(即GS效应)并非$\alpha}$-效应的唯一原因,过渡态(TS)通过环状TS结构稳定化也对依赖于Y的$\alpha}$-效应趋势(即TS效应)有贡献。