We report here a distinct reaction, which generates aryl radicals from aryl pinacol boronates under mild aqueous conditions and can be used for peptide and protein modifications. The strategy leverages the versatile reactivity of aryl pinacol boronates to form aryl radicals in presence of ammonium persulfate (APS). The formed aryl radicals insert readily into peptide disulfide bonds while tolerating other functionalities. On the protein level, the reactivity extends beyond the disulfide bonds. The methodology benefits from the accessibility of starting aryl pinacol boronates, as well as biocompatible conditions. In contrast to conventional methods used for aryl radical generation, the strategy is metal‐free, does not require photoinduction and can be readily performed under aqueous conditions. The mechanism of the reactions was investigated by radical‐trapping experiments, spectroscopic analysis and oxygen scavenging. The presented approach broadens the application of aryl pinacol boronate esters in radical reactions.
