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.