Photocatalytic generation of ligated boryl radicals from tertiary amine-borane complexes: An emerging tool in organic synthesis

Photocatalysis has recently given impetus to the use of ligated boryl radicals (LBRs) in synthesis, thanks to the mild conditions required for their generation based on the use of visible light. LBRs are B-centered radicals wherein the boron atom is coordinated with a suitable Lewis base (e.g., amine, phosphine, or N-heterocyclic carbene [NHC]) and can be conveniently accessed from the corresponding ligated boranes through the cleavage of a B-H bond. While NHC-boranes featuring a rather labile B-H bond are routinely used in photocatalytic strategies, this perspective highlights the recent adoption of more challenging tertiary amine-boranes, which unlocked unprecedented reaction manifolds. The highlighted applications include the Minisci-type borylation of azines and the implementation of polarity-reversal catalysis for the generation of electrophilic C-centered radicals via hydrogen-atom transfer (HAT). The possibility to devise an analogous strategy based on halogen-atom transfer (XAT) is also discussed.