Gas-Phase Acidities of α- and α,α-SO<sub>2</sub>CF<sub>3</sub>-Substituted Toluenes. Varying Resonance Demand in the Electron-Rich System
作者:Min Zhang、Md. Mizanur Rahman Badal、Ilmar A. Koppel、Masaaki Mishima
DOI:10.1246/bcsj.20130052
日期:2013.7.15
The gas-phase acidities (GA) of aryl(trifluoromethylsulfonyl)methanes (ArCH2SO2CF3; 1) and arylbis(trifluoromethylsulfonyl)methanes (ArCH(SO2CF3)2; 2) were determined by measuring proton-transfer equilibria. Substituent effects for acidities of a series of ArCH(R1)R2 including 1 and 2 have been analyzed successfully in terms of the Yukawa–Tsuno equation. The resonance demand parameter r− value was found to decrease linearly with increasing acidity of the GA values of the unsubstituted parent carbon acids, and the change of the r− value was correlated with the acidifying effect of the phenyl group (R = Ph) in the RCH2(R1)R2. In addition, the geometric features and natural charges of the conjugate anions calculated at B3LYP/6-311+G(d,p) were found to be correlated linearly with the r− values. Such behavior of the resonance demand parameter in the electron-rich system, ArC−(R1)R2, is completely consistent with that observed for the electron-deficient system, ArC+(R1)R2, revealing that the resonance demand is contingent upon the structure of carbanions and carbocations. Furthermore, it was found that the ρ values also decreased with increasing acidity of the GA values of the unsubstituted parent carbon acids. This would be related to the distribution of the charge between the aromatic moiety and the C(R1)R2 moiety.
通过测量质子转移平衡,确定了芳基(三氟甲基磺酰基)甲烷(ArCH2SO2CF3;1)和芳基双(三氟甲基磺酰基)甲烷(ArCH(SO2CF3)2;2)的气相酸度(GA)。根据 Yukawa-Tsuno 方程,成功地分析了包括 1 和 2 在内的一系列 ArCH(R1)R2 酸性的取代基效应。研究发现,共振需求参数 r- 值随着未取代母体碳酸的 GA 值的酸度增加而线性降低,r- 值的变化与 RCH2(R1)R2 中苯基(R = Ph)的酸化效应相关。此外,B3LYP/6-311+G(d,p)计算得出的共轭阴离子的几何特征和自然电荷与 r- 值呈线性相关。在富电子体系 ArC-(R1)R2 中,共振需求参数的这种行为与在缺电子体系 ArC+(R1)R2 中观察到的行为完全一致,揭示了共振需求取决于碳离子和碳阳离子的结构。此外,研究还发现,随着未取代母体碳酸 GA 值酸度的增加,ρ 值也会降低。这与芳香族分子和 C(R1)R2 分子之间的电荷分布有关。