Phenyl boronic acid complexes of diols and hydroxyacids

Abstract

Cumulative formation constants for the interaction of phenyl boronic acids with 1,2-diols and structurally related α-hydroxy carboxylic acids were determined by potentiometric titration in aqueous solution. Although there is a significant electronic effect on the acidity of phenyl boronic acid (ρ = 2.1), there is no marked electronic effect on the stability of the complexes. Rather, the complexes are significantly destabilised by adjacent anionic groups, by steric interactions across the face of the cyclic boronate ester and by angle strain within the boronate ester ring. Binding that is nearly independent of pH is observed for some favourably constituted α-hydroxy acid complexes as a result of the relatively high acidity of the acids, which in turn allows tetrahedral boronate complexes to persist in acidic solution (pH < 3). Boronate complex stability is remarkably sensitive to steric and electrostatic factors.