Selective on-line detection of boronic acids and derivatives in high-performance liquid chromatography eluates by post-column reaction with alizarin

•This method enables selective pinpointing of boronic acids in complex mixtures.•All tested boronic acids and derivatives were detected.•The minimal detectable amount of phenylboronic acid was 1.2 ng.•Boron-free compounds were not detected, unless they were autofluorescent.•Boronic acids were detected in the monitoring of a novel thiol–ene reaction.

An on-line high-performance liquid chromatography (HPLC) method for the rapid and selective detection of boronic acids in complex mixtures was developed. After optimization experiments at an HPLC flow rate of 0.40 mL/min, the HPLC-separated analytes were mixed post-column with a solution of 75 μM alizarin and 0.1% triethylamine in acetonitrile, which was delivered at a flow rate of 0.60 mL/min. The reaction between alizarin and boronic acids occurred in a reaction coil of dimensions of 3.5 m × 0.25 mm at a temperature of 50 °C, resulting in fluorescent complexes that were detected as positive peaks by a fluorescence detector (λexc 469 nm and λem 610 nm). The method enabled the selective detection of various boronic acids and derivatives, with a limit of detection of phenylboronic acid of 1.2 ng or 1 μM. It could successfully monitor the progress of two organic reactions involving boronic acid-containing compounds, and provided useful insights into the course of the reactions.