A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis
This manuscript probes the steric and electronic attributes that lead to “frustrated Lewis pair” (FLP)-type catalysis of imine hydrogenation by borenium ions. Hydride abstraction from (ItBu)HB(C6F5)22 prompts intramolecular C–H bond activation to give (CHN)2(tBu) (CMe2CH2)CB(C6F5)23, defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene–borane (NHC–borane) adducts ((R′CNR)2C)(HBC8H14) (R′ = H, R = dipp 4a, Mes 5a, Me 8a; R = Me R′ = Me 9a, Cl, 10a) and ((HC)2(NMe)(NR)C)(HBC8H14) (R = tBu, 6a, Ph 7a) are prepared and converted to corresponding borenium salts. These species are evaluated as catalysts for metal-free imine hydrogenation at room temperature. Systematic tuning of the carbene donor for the hydrogenation of archetypal substrate N-benzylidene-tert-butylamine achieves the highest reported turn-over frequencies for FLP-catalyzed hydrogenation at amongst the lowest reported catalyst loadings. The most active NHC–borenium catalyst of this series, derived from 10a, is readily isolable, crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional group-containing imines and N-heterocycles.