Verdazyl Radical Organocatalysis Overcomes Chain-Propagation Limitations in Borylation of Arenediazonium Salts.

We report an organocatalytic borylation of arenediazonium salts enabled by the bench-stable, nitrogen-centered persistent radical 1,3,5-triphenylverdazyl (TPV). Although this transformation is well precedented, limitations associated with its underlying chain-transfer mechanism have remained largely overlooked. We demonstrate that TPV functions as an efficient redox catalyst that outcompetes rapid radical chain propagation steps while also enabling catalytic chain repair, thereby mitigating inhibition and chain-termination processes that limit purely chain-driven reactions. Comparative studies with common organic electron donors reveal the unique efficacy of electron-rich verdazyl radicals, and contrast with the poor performance of conventional organic electron donors, which are prone to self-inhibition. DFT and Marcus theory calculations indicate lower activation barriers for electron transfer in the TPV-catalyzed pathway, particularly for challenging electron-rich arenediazonium substrates. The reaction proceeds under mild conditions without the need for visible light or mechanoredox mediators, is compatible with a broad range of functional groups, and enables late-stage borylation of aniline-containing drug molecules as well as one-pot telescoped applications of arylpinacolboronate products.