BAr^F₃‑Catalyzed Imine Hydroboration with Pinacolborane Not Requiring the Assistance of an Additional Lewis Base

The rarely used boron Lewis acid tris­[3,5-bis­(trifluoromethyl)­phenyl]­borane (BAr^F₃) is found to be an excellent catalyst for metal-free hydroboration of imines. In the presence of 1.0 mol % of BAr^F₃, several ketimines and aldimines undergo hydroboration with pinacolborane (HBpin) at room temperature without the aid of an external Lewis base. BAr^F₃ is more reactive than other Lewis acidic boranes, including the often-used tris­(pentafluorophenyl)­borane (B­(C₆F₅)₃). The steric hindrance imparted by the six fluorine atoms ortho to the boron center in B­(C₆F₅)₃ accounts for this. Mechanistic control experiments indicate conventional Lewis acid catalysis involving imine activation and hydride transfer from HBpin.