A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis† †Electronic supplementary information (ESI) available: Synthetic and spectral details are deposited. CCDC 1035566–1035574. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc03675a

• Published in: Chemical science (Cambridge) 2015-01, Vol.6 (3), p.2010-2015
• Main Authors: Farrell, Jeffrey M., Posaratnanathan, Roy T., Stephan, Douglas W.
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c4sc03675a

Room-temperature metal-free 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 (I t Bu)HB(C 6 F 5 ) 2 2 prompts intramolecular C–H bond activation to give (CHN) 2 ( t Bu) (CMe 2 CH 2 )CB(C 6 F 5 ) 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)(HBC 8 H 14 ) (R′ = H, R = dipp 4a , Mes 5a , Me 8a ; R = Me R′ = Me 9a , Cl, 10a ) and ((HC) 2 (NMe)(NR)C)(HBC 8 H 14 ) (R = t Bu, 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.