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 (IBu)HB(CF) prompts intramolecular C-H bond activation to give (CHN)(Bu) (CMeCH)CB(CF), defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene-borane (NHC-borane) adducts ((R'CNR)C)(HBCH) (R' = H, R = dipp , Mes , Me ; R = Me R' = Me , Cl, ) and ((HC)(NMe)(NR)C)(HBCH) (R = Bu, , Ph ) 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 -benzylidene--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 , is readily isolable, crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional group-containing imines and N-heterocycles.