The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

ortho -Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o -aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the p K a of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o -aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids. ortho -Aminomethylphenylboronic acids are routinely used in sensors for carbohydrates, but the function of the o -aminomethyl group in enhancing binding affinity and modulating the emission of appended fluorophores has been the matter of some debate. This Review presents a unified picture of the structural features, mechanisms of sugar complexation and photophysics of these kinds of sensors.