Aminolysis of S-4-Nitrophenyl X-Substituted Thiobenzoates: Effect of Nonleaving-Group Substituents on Reactivity and Mechanism
作者:Li-Ra Im、Sang-Eun Jeon、Ik-Hwan Um
DOI:10.5012/bkcs.2011.32.4.1153
日期:2011.4.20
A kinetic study is reported for aminolysis of S-4-nitrophenyl X-substituted thiobenzoates 3a-g in 80 mol % $H_2O$/20 mol % DMSO at $25.0\pm}0.1^\circ}C$. Thiol esters 3a-g are 7.8-47.6 fold more reactive than the corresponding oxygen esters (i.e., 4-nitrophenyl X-substituted benzoates 1a-g). Such reactivity order appears to be in accordance with the expectation that 4-nitrothiophenoxide in 3a-g is a better nucleofuge than 4-nitrophenoxide in 1a-g since the former is 2.64 pKa units less basic than the latter. Hammett plot for the reactions of 3a-g exhibit poor correlation coefficients ($R^2$ = 0.977-0.986) with negative deviation by substrates possessing an electrondonating group (EDG), while the Yukawa-Tsuno plots result in excellent linear correlation ($R^2$ = 0.995-0.997) with $\rho}$ = 0.93-1.23 and r = 0.57-0.67, indicating that the negative deviation shown by substrates possessing an EDG is caused by ground-state stabilization through resonance interactions but not due to a change in ratedetermining step upon changing the nonleaving-group substituent X. The $\rho}$ value increases as the incoming amine becomes more basic and more reactive, indicating that the RSP is not operative in the current reactions.
报道了一项关于S-4-硝基苯基X取代硫代苯甲酸酯3a-g在80摩尔%水/20摩尔% DMSO中于25.0±0.1°C进行的氨基水解的动力学研究。硫酯3a-g的反应性是相应的氧酯(即4-硝基苯基X取代苯甲酸酯1a-g)的7.8-47.6倍。这种反应性顺序似乎符合预期,因为3a-g中的4-硝基硫苯氧负离子比1a-g中的4-硝基苯氧负离子更容易转移,前者的碱性比后者低2.64个pKa单位。3a-g的反应的Hammett图显示,具有电子给体基团(EDG)的底物的相关系数较差(R² = 0.977-0.986),且呈负偏差,而Yukawa-Tsuno图则显示出良好的线性相关性(R² = 0.995-0.997),并且ρ = 0.93-1.23,r = 0.57-0.67,这表明具有EDG的底表现出的负偏差是由共振相互作用造成的基态稳定化,而不是由于改变非离去基团取代基X时速率决定步骤的变化。随着入体胺的碱性和反应性增强,ρ值增加,表明在当前反应中,RSP机制并未起作用。