Fluorinated Boronic Acids: Acidity and Hydrolytic Stability of Fluorinated Phenylboronic Acids

The acidity constants and hydrolytic stability of all isomers of fluoro-substituted phenylboronic acids (F-1-F-5) have been determined by both spectrophotometric and potentiometric methods. The introduction of fluorine into the aromatic ring enhances the Lewis acidity of boronic acids, depending on the position and number of fluorine substituents. Results of both methods show good agreement in most of the cases. To explain the observed discrepancies for several compounds, stability studies have been carried out by spectrophotometry. The observed change of maximum absorption intensity is dependent on pH and temperature, but there is no simple correlation between the pK(a) and the decomposition rate. The less-stable compounds are those with two fluorine atoms at the ortho positions. It was found that the pK(a) values for mono- and difluoro-substituted boronic acids show good linear correlation with the values for the corresponding benzoic acids.