Autoinduced Catalysis and Inverse Equilibrium Isotope Effect in the Frustrated Lewis Pair Catalyzed Hydrogenation of Imines

The frustrated Lewis pair (FLP)‐catalyzed hydrogenation and deuteration of N‐benzylidene‐tert‐butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6‐F3‐C6H2)3 (4), and B(2,6‐F2‐C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol−1) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect. The FLP‐catalyzed reaction of N‐benzylidene‐tert‐butylamine with H2 and D2 was kinetically investigated. The free activation energy for the H2 activation by the FLP consisting of an imine and a less Lewis‐acidic borane is 2 kcal mol−1 higher than for B(C6F5)3 , resulting in autoinduced catalysis. For the first time, the free activation enthalpies for the H2 activation by FLPs were experimentally determined and an inverse kinetic isotope effect was observed.