Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BAr3F Does What B(C6F5)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(C6F5)(3)) is reluctant to react. Unlike B(C6F5)(3), BAr3F is found to engage in substituent redistribution with HBpin, resulting in the formation of ArFBpin and the electron- deficient diboranes [H2BArF] 2 and [(ArF)(H) B(mH) 2BAr(2)(F)]. 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.