Equilibria in the 3-(trifluoromethyl)phenylboronic acid - boroxine system. Solubility of the acid and its cyclic esters in organic solvents

Abstract

Solubilities of 3-(trifluoromethyl)phenylboronic acid, its pinacol ester and azaester in organic solvents have been determined experimentally by a dynamic method. The disappearance of turbidity was determined by measuring light intensity using a luminance probe. The title acid easily dehydrates with the formation of a considerable amount of the corresponding boroxine. The dehydration proceeds both in the solid state as well as in solutions. The process was investigated using various techniques (NMR, Raman, differential scanning calorimetry, thermogravimetric analysis and powder X-ray diffraction). The possibilities of using above methods to describe the acid-boroxine equilibria was assessed. The title acid displays high solubility in 3-pentanone, moderate in chloroform and very low in methylcyclohexane. Its pinacol ester and azaester derivatives show better solubility than the parent acid in all tested solvents. For pinacol ester, differences between particular solvents are small, while for azaester they are significant. In this case the highest solubility is observed in chloroform and the lowest in methylcyclohexane. The solubilities of pinacol ester and azaester have been correlated by some GE-models.