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(C 6 F 5 ) 3 ( 1 ), B(2,4,6‐F 3 ‐C 6 H 2 ) 3 ( 4 ), and B(2,6‐F 2 ‐C 6 H 3 ) 3 ( 5 ) and the free activation energies for the H 2 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 H 2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D 2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.