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)(2) 2 prompts intramolecular C-H bond activation to give (CHN)(2)(tBu) (CMe2CH2)CB(C6F5)(2) 3, defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene-borane (NHC-borane) adducts ((R'CNR)(2)C)(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.