Elucidation of the Mechanism of the Reaction between Phenylboronic Acid and a Model Diol, Alizarin Red S

In this work, the reaction between phenylboronic acid and the diol-containing, fluorescent dye Alizarin Red S (ARS) was probed. Fluorescence titrations, 11B NMR measurements, and both pre- and steady-state kinetic experiments were used for the characterization of this reaction over a large pH range (4–10.5). It was shown that ARS preferentially reacted with the boronic (neutral, trigonal) form of phenylboronic acid; however, the boronate (anionic, tetrahedral) form was also reactive. All in all, four reactant species were implicated in the formation of four different adduct species. The rate of a given adduct formation depended on the combination of the solution pH and the pKa's of both ARS and the arylboronic acid. The reaction was found to proceed in two distinct kinetic steps with the products and starting materials in facile exchange. In addition, the elucidation of the mechanism indicated the presence of two fluorescent products with the structure of the major contributor differing from what had been cited in the literature.