Experimental Analysis of the Catalytic Cycle of the Borane-Promoted Imine Reduction with Hydrosilanes: Spectroscopic Detection of Unexpected Intermediates and a Refined Mechanism

The discovery of intermediates that had not been seen before in imine reduction involving borane-mediated Si–H bond activation provided new insight into the mechanism, eventually leading to a refined catalytic cycle that also bears relevance to asymmetric variants. The catalysis proceeds through an ion pair composed of a silyliminium ion and a borohydride that subsequently reacts to yield an N-silylated amine and the borane catalyst. The latter step is enantioselectivity-determining when using a chiral borane. It was now found that there are additional intermediates that profoundly influence the outcome of such enantioselective transformations. Significant amounts of the corresponding free amine and N-silylated enamine are present in equimolar ratio during the catalysis. The free amine emerges from a borohydride reduction of an iminium ion (protonated imine) that is, in turn, generated in the enamine formation step. The unexpected alternative pathway adds another enantioselectivity-determining hydride transfer to reactions employing chiral boranes. The experiments were done with an axially chiral borane that was introduced by us a few years ago, and the refined mechanistic picture helps to understand previously observed inconsistencies in the level of enantioinduction in reductions catalyzed by this borane. Our findings are general because the archetypical electron-deficient borane B­(C₆F₅)₃ shows the same reaction pattern. This must have been overlooked in the past because B­(C₆F₅)₃ is substantially more reactive than our chiral borane with just one C₆F₅ group. Reactions with B­(C₆F₅)₃ must be performed at low catalyst loading to allow for detection of these fundamental intermediates by NMR spectroscopy.