17O NMR studies of boronic acids and their derivatives

Abstract

The group of 155 substituted phenylboronic acids and their derivatives: esters, boroxines and benzoxaboroles, were investigated by ¹⁷O NMR spectroscopy. The influence of substituents of phenylboronic acids on the ¹⁷O chemical shift was evaluated. For the compounds with substituents at the para position, excellent correlation with the Hammett constant was obtained. For the ortho-substituted compounds use of the Charton equation gave a good correlation of results. Surprisingly, meta-substituted compounds show none of the above-mentioned correlations. A series of boronic esters of alcohols, diols and hydroxyacids were synthesized. The influence of the structure of parent hydroxy compounds on the ¹⁷O chemical shift was discussed. Selected benzoxaboroles and boroxines were also investigated, and their chemical shifts were compared with those of boronic acids. ¹⁷O NMR spectroscopy was also used to investigate the equilibria in solution. Equilibrium of the reaction of selected boronic acids with hydroxyl anions in acetonitrile was determined by ¹⁷O and ¹¹B titration experiments. Monomer–dimer equilibria were also studied. The experimental chemical shifts were compared with the results of theoretical calculations. Very good correlations were obtained for the ortho- and para-substituents, but not for the meta ones.