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.