Switchable selectivity within a series of boronate esters for dynamic covalent exchange in nonaqueous solvents

The reversible condensation-hydrolysis reactions of boronic acids have proven to be a highly useful class of thermodynamically controlled dynamic covalent process, enabling the construction of sugar sensors, stimuli-responsive materials, and complex covalent architectures. Yet, the common diol or diphenol coupling partners tend to produce relatively unstable condensation products, exhibit oxidative sensitivity, or offer limited options for expanding structural diversity. To address these drawbacks, we explore a series of coupling partners including non-diol salicylate and salicylamide derivatives, in combination with two boronic acids. In nonaqueous solvents, the condensation-hydrolysis equilibria are sensitive to the nature and concentration of Lewis bases, with equilibrium constants that can be tuned across at least five orders of magnitude. Furthermore, differential responses to base concentration can be exploited to create a switchable dynamic covalent system in which a boronic acid can be cycled between expressing each of two condensation products with high fidelity, in response to a simple chemical stimulus.