1,4-Hydroboration Reactions of Electron-Poor Aromatic Rings by N‑Heterocyclic Carbene Boranes

Reactions of N-heterocyclic
carbene boranes (NHC-boranes) with
electron-poor aromatic rings under photoredox conditions provide dearomatized
3-NHC-boryl-1,5-cycohexadienes, which are formally products of 1,4-hydroboration
reactions. When regioisomers are possible, the more crowded (doubly
ortho-substituted) product is formed preferably or exclusively. The
mechanism may involve oxidation of the NHC-borane to an NHC-boryl
radical, reduction of the electron-poor aromatic ring to a radical
anion, coupling of the radical and the radical anion to give a cyclohexadienyl
anion, and finally regioselective protonation.