Substituent Effects and pH Profiles for Stability Constants of Arylboronic Acid Diol Esters

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• 作者：Mayte A. Mart铆nez-Aguirre ; Raul Villamil-Ramos ; Jorge A. Guerrero-Alvarez ; Anatoly K. Yatsimirsky
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：May 17, 2013
• 年：2013
• 卷：78
• 期：10
• 页码：4674-4684
• 全文大小：602K
• 年卷期：v.78,no.10(May 17, 2013)
• ISSN：1520-6904

文摘

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 尾_11-1 for reactions between neutral forms of reactants producing the anionic ester plus proton follow the Hammett equation with 蟻 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鈥搗is 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 尾_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 ¹¹B NMR titrations are approximately 1 order of magnitude smaller than those determined by UV鈥搗is titrations under identical conditions. A general equation, which makes possible an estimate of 尾_11-1 for any pair of boronic acid and diol from their pK_a values, is proposed on the basis of established Br枚nsted-type correlation of Hammett parameters for 尾_11-1 with acidity of diols. The equation allows one to calculate stability constants expected only on basis of acid鈥揵ase properties of the components, thus permitting more strict evaluation of contributions of additional factors such as steric or charge effects to the ester stability.