The Boroxine-Boronic Acid Equilibrium

The equilibrium between boroxines and boronic acids is a major component of their physicochemical properties, key to many of their applications, and a complicating factor in the analysis of their purity and stoichiometry. While the equilibrium is well-known and extensively applied, there is a dearth of information on the mechanism and kinetics of the multistep process that underpins the overarching hydrolytic sequence. The kinetics of the interconversion of arylboronic acids and arylboroxines in aqueous THF have now been studied in detail by ¹H/¹⁹F NMR and UV-vis spectroscopy, including titrations, stopped-flow kinetics, magnetization transfer, ¹H/²H solvent kinetic isotope effects, numerical simulations of reaction kinetics and equilibria, and DFT calculations using explicit solvation approaches. The study has identified two acyclic anhydride intermediates in a three-step interconversion of boroxines and boronic acids. Water acts as a reagent and as a catalyst in each of these steps, and the hydrogen-bond acceptor properties of the solvent strongly affect the overarching hydrolytic equilibrium. A spreadsheet for estimating boroxine-boronic equilibrium speciation is provided. In addition, competing equilibria of boroxines with boric acid generate hydroxyboroxines, and direct associative transfer of Ar-B units occurs between boroxines and boronic acids, bypassing the hydrolysis. These findings have implications for the use of boronic acids and boroxines as reagents, catalysts, sensors, and pharmaceuticals, and in the design and application of boroxine-based materials, polymers, and dynamic architectures.