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

Abstract

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 β(11-1) for reactions between neutral forms of reactants producing the anionic ester plus proton follow the Hammett equation with ρ depending on pKa 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 β(11-1) values, but the direct fitting of K(obs) vs pH profiles gives shifted pKa 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 (11)B 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 β(11-1) for any pair of boronic acid and diol from their pKa values, is proposed on the basis of established Brönsted-type correlation of Hammett parameters for β(11-1) with acidity of diols. 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.

Boron Macrocycles Based on Multicomponent Assemblies using (3‐Aminophenyl)boronic Acid and Pentaerythritol as Common Reagents; Molecular Receptors toward Lewis Bases

“…Boronic acids have been reported to rapidly and reversibly react with alcohols, forming boronates 1,2. Actually, the reaction with diols leads to the formation of highly stable cyclic esters.…”

Thermodynamic and structural study of complexation of phenylboronic acid with salicylhydroxamic acid and related ligands

“…The respective observed association constant defined in terms of total concentrations of components ( K obs ), which is the stability constant experimentally measured e.g. by spectroscopic titration at a given pH, depends on pH in accordance with theoretical equation , where K a1 L and K a2 L are the acid dissociation constants of the ligand and K a B is the dissociation constant of PBA (Scheme ). …”

Ratiometric Sensing of Hydrogen Peroxide Utilizing Conformational Change in Fluorescent Boronic Acid Polymers

“…In the absence of H 2 O 2 , the relative intensity of excimer emission was almost constant with the increase in pH and then decreased when pH exceeded 10. The decrease in the excimer intensity may be due the extension of the polymer chain that was induced by the ionization of the boronic acid groups into negatively charged boronate groups [27] by which electrostatic repulsion was created within the polymer chain. This idea is supported by the study made by Chandar et al on the conformations of a pyrene-labeled poly(acrylic acid) using fluorescence of the excimer/monomer emissions.…”