Connecting the dynamics and reactivity of arylboronic acids to emergent and stimuli-responsive material properties.

Over the past two decades, arylboronic acid-functionalized materials have been used in a variety of sensing and stimuli-responsive scaffolds. Their diverse applications result from the various modes of reactivities of arylboronic acids. Arylboronate ester-crosslinked hydrogels are self-healing because the boronate ester bond is dynamic covalent. The hydrogels degrade in acidic environments because of pH-sensitive boronate ester degradation, in the presence of diols because of reversible boronate ester formation, and in the presence of reactive oxygen species (ROS) because of arylboronic acid oxidation. Connecting small-molecule reactivities and dynamics to mechanical and stimuli-responsive properties enables a better understanding of material properties and informs next-generation material design. Here, we highlight recent advances in arylboronic acid-based networks and nanomaterials and how the fundamental chemistry of arylboronic acids can enhance an understanding of the emergent material properties and improve the rational design of stimuli-responsive materials.

Diazaborines oxidize slowly with H2O2 but rapidly with peroxynitrite in aqueous buffer.

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and peroxynitrite (ONOO-) oxidize arylboronic acids to their corresponding phenols. When used in molecular imaging probes and in ROS-responsive molecules, however, simple arylboronic acids struggle to discriminate between H2O2 and ONOO- because of their fast rate of reaction with both ROS. Here, we show that diazaborines (DABs) react slowly with H2O2 but rapidly with peroxynitrite in an aqueous buffer. In addition to their slow reaction with H2O2, the immediate product of DAB oxidation with H2O2 and ONOO- can yield a kinetically trapped CN Z-isomer that slowly equilibrates with its E-isomer. Taken together, our work shows that diazaborines exhibit enhanced kinetic discrimination between H2O2 and ONOO- compared to arylboronic acids, opening up new opportunities for diazaborine-based tools in chemical biology.

Tuning the exchange dynamics of boronic acid hydrazones and oximes with pH and redox control.

Dynamic bonds continually form and dissociate at equilibrium. Carbonyl compounds with proximal boronic acids, including 2-formylphenylboronic acid (2-FPBA), have been reported to form highly dynamic covalent hydrazone and oxime bonds in physiological conditions, but strategies to tune the dynamics have not yet been reported. Here, we characterize the dynamics of 2-FPBA-derived hydrazones and oximes and account for both the rapid rate of formation (∼102-103 M-1 s-1) and the relatively fast rate of hydrolysis (∼10-4 s-1) at physiological pH. We further show that these substrates undergo exchange with α-nucleophiles, which can be reversibly paused and restarted with pH control. Finally, we show that oxidation of the arylboronic acid effectively abolishes the rapid dynamics, which slows the forward reaction by more than 30 000 times and increases the hydrolytic half-life from 50 minutes to 6 months at physiological pH. These results set the stage to explore these linkages in dynamic combinatorial libraries, reversible bioconjugation, and self-healing materials.

Covalent assembly of heterosequenced macrocycles and molecular cages through orthogonal dynamic covalent chemistry (ODCC).

Shape-persistent heterosequenced 2-D macrocycles and 3-D molecular cages have been prepared in one pot from two or three different monomers, through orthogonal dynamic covalent chemistry using dynamic imine and olefin metathesis.