Substituent Effects and pH Profiles for Stability Constants of Arylboronic Acid Diol Esters
Substituent Effects and pH Profiles for Stability Constants of Arylboronic Acid Diol Esters
复制标题
DOI：
10.1021/jo400617j
复制
发表时间：
2013-05-17
影响因子：
3.6
通讯作者：
Yatsimirsky, Anatoly K.
中科院分区：
文献类型：
--
作者：
Martinez-Aguirre, Mayte A.;Villamil-Ramos, Raul;Yatsimirsky, Anatoly K.
用电位法测定了一系列间位、对位取代苯基硼酸和可变酸度的二醇在水溶液中的稳定常数。中性形式的反应物之间的反应常数β(11-1)符合Hammett方程，Rho取决于二醇的pK(A)，从葡萄糖的2.0到4-硝基邻苯二酚的1.29。用UV-Vis和荧光滴定测定了4，5-二羟基-1，3-苯二磺酸酯(TIron)在不同pH下的稳定常数(K-OBS)，结果与基于β(11-1)值的预测值基本一致，但K-OBS与pH曲线的直接拟合给出了硼酸和二醇的pK(A)值移动，这是拟合参数显著相互依赖的结果。研究了取代基对芳基硼酸铁酯吸收光谱和荧光光谱的影响。在相同条件下，用B-11核磁共振滴定法测定的TIron的K-OBS比用UV-Vis滴定法测定的K-OBS小约1个数量级。在建立β(11-1)的Hammett参数与白痴酸度的Bronsted型关联的基础上，提出了一个通用方程，它可以从任何一对硼酸和二元醇的pK(A)值估计其β(11-1)。该方程允许人们只根据组分的酸碱性质来计算预期的稳定常数，从而允许更严格地评估其他因素对酯稳定性的贡献，例如空间或电荷效应。
Stability constants of boronic acid diol esters in aqueous solution have been determined potentiometrically for a series of meta-, para-substituted phenylboronic acids and diols of variable acidity. The constants beta(11-1) for reactions between neutral forms of reactants producing the anionic ester plus proton follow the Hammett equation with rho depending on pK(a) of diol and varying from 2.0 for glucose to 1.29 for 4-nitrocatechol. Observed stability constants (K-obs) measured by UV-vis and fluorometric titrations at variable pH for esters of 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) generally agree with those expected on the basis of beta(11-1) values, but the direct fitting of K-obs vs pH profiles gives shifted pK(a) values both for boronic acids and diol as a result of significant interdependence of fitting parameters. The subsituent effects on absorption and fluorescence spectra of Tiron arylboronate esters are characterized. The K-obs for Tiron determined by B-11 NMR titrations are approximately 1 order of magnitude smaller than those determined by UV-vis titrations under identical conditions. A general equation, which makes possible an estimate of beta(11-1) for any pair of boronic acid and diol from their pK(a) values, is proposed on the basis of established Bronsted-type correlation of Hammett parameters for beta(11-1) with acidity of idiots. The equation allows one to calculate stability constants expected only on basis of acid-base properties of the components, thus permitting more strict evaluation of contributions of additional factors such as steric or charge effects to the ester stability.