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

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.