Sulfonyl Diazaborine 'Click' Chemistry Enables Rapid and Efficient Bioorthogonal Labeling**.

Finding an ideal bioorthogonal reaction that responds to a wide range of biological queries and applications is of great interest in biomedical applications. Rapid diazaborine (DAB) formation in water by the reactions of ortho‐carbonyl phenylboronic acid with α‐nucleophiles is an attractive conjugation module. Nevertheless, these conjugation reactions demand to satisfy stringent criteria for bioorthogonal applications. Here we show that widely used sulfonyl hydrazide (SHz) offers a stable DAB conjugate by combining with ortho‐carbonyl phenylboronic acid at physiological pH, competent for an optimal biorthogonal reaction. Remarkably, the reaction conversion is quantitative and rapid (k2>103 M−1 s−1) at low micromolar concentrations, and it preserves comparable efficacy in a complex biological milieu. DFT calculations support that SHz facilitates DAB formation via the most stable hydrazone intermediate and the lowest energy transition state compared to other biocompatible α‐nucleophiles. This conjugation is extremely efficient on living cell surfaces, enabling compelling pretargeted imaging and peptide delivery. We anticipate this work will permit addressing a wide range of cell biology queries and drug discovery platforms exploiting commercially available sulfonyl hydrazide fluorophores and derivatives. [ABSTRACT FROM AUTHOR]