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

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