Chiral carbene–borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers’ borane. Subsequent hydride abstraction gave a borenium cation. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me, Et, i-Pr) and the corresponding borenium cations were also prepared. These cations were not active as catalysts in hydrogenation, although were shown to undergo carbene ring expansion reactions at 50 °C to give species. The IBOX-carbene precursors derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts . 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 (ItBuMe) with diisopinocampheylborane (Ipc2BH) gave chiral adducts: (IMe)(Ipc2BH), (IPr)(Ipc2BH), (IBnMe)(Ipc2BH), (IPhMe)(Ipc2BH), and (ItBuMe)(Ipc2BH). Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4- triphenyl-1H-1,2,3-triazolium, and the 9-BBN derivative of (S)-2-amino-2’- methoxy-1,1’-binaphthalene-1,2,3-triazolium 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.

The supporting information, including figures of the NMR spectra for the compounds and reactions in the aper, are included as well as tables from the results of catalytic studies. X-ray data are included here which can be visualized using software for X-ray crystal structure determination/visualization which was used to characterise the compounds synthesised.

Results based upon these data are published at http://doi.org/10.1039/c6dt02202b