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 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R, 3S)-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 degrees 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 (ItBuMe) 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 (ItBuMe)(Ipc(2)BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2] decane adduct of 1,3,4-triphenyl-1H-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 enantio-selectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.