Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6 F5 )3 Cannot Do!

The transition-metal-free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3 ) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C6 F5 )3 ) is reluctant to react. Unlike B(C6 F5 )3 , BArF3 is found to engage in substituent redistribution with HBpin, resulting in the formation of ArF Bpin and the electron-deficient diboranes [H2 BArF ]2 and [(ArF )(H)B(μ-H)2 BArF2 ]. These in situ-generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin-derived anti-Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.