Taking the F out of FLP: Simple Lewis Acid-Base Pairs for Mild Reductions with Neutral Boranes via Borenium Ion Catalysis

Discrete three-coordinate borenium salts 1c and 1d are accessed by cooperative Lewis acid-base pair-mediated heterolytic splitting of the B-H bond in pinacolborane by B(C6F5)(3)center dot DABCO and Ph3C+/DABCO, respectively. The resulting salts are competent catalysts in the reduction of a broad range of imines and can be generated in situ. Moreover, a mechanistic framework for borenium catalysis based on experimental evidence is proposed. The reaction is suggested to proceed by borenium activation of the imine substrate followed by counterintuitive hydride delivery from HBPin (with the assistance of DABCO) rather than from the HB(C6F5)(3)(-) anion, contrary to typical mechanisms of reduction in FLP systems.