Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenationsElectronic supplementary information (ESI) available: Spectroscopy details. CCDC 1482172-1482179. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt02202b

The carbene derived from (1 R ,3 S )-camphoric acid was used to prepare the borane adduct with Piers' borane 7 . Subsequent hydride abstraction gave the borenium cation 8 . Adducts with 9-BBN and the corresponding (1 R ,3 S )-camphoric acid-derived carbene bearing increasingly sterically demanding N -substituents (R = Me 9 , Et 10 , i-Pr 11 ) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17 . The IBOX-carbene precursors 18 and 19 derived from amino alcohols ( S )-valinol and ( S )- tert -leucinol (R = i -Pr, t -Bu) were used to prepare borane adducts 20-23 . Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1- tert -butyl-3-methylimidazol-2-ylidene (I t BuMe) with diisopinocampheylborane (Ipc 2 BH) gave chiral adducts: (IMe)(Ipc 2 BH) 24 , (IPr)(Ipc 2 BH) 25 , (IBnMe)(Ipc 2 BH) 26 , (IPhMe)(Ipc 2 BH) 27 , and (I t BuMe)(Ipc 2 BH) 28 . Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1 H -1,2,3-triazolium, rac- 29 and the 9-BBN derivative of ( S )-2-amino-2′-methoxy-1,1′-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered. A series of chiral borenium cations are prepared characterized and evaluated in metal-free asymmetric hydrogenations of imines.