|Title:||Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations||Authors:||Lam, Jolie
Guenther, Benjamin A. R.
Farrell, Jeffrey M.
Bestvater, Brian P.
Newman, Paul D.
Melen, Rebecca L.
Crudden, Cathleen M.
Stephan, Douglas W.
JEFFREY M. FARRELL
|Issue Date:||2016||Journal Volume:||45||Journal Issue:||39||Start page/Pages:||15303-15316||Source:||DALTON TRANSACTIONS||Abstract:||
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 °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 (Ipc2BH) gave chiral adducts: (IMe)(Ipc2BH) 24, (IPr)(Ipc2BH) 25, (IBnMe)(Ipc2BH) 26, (IPhMe)(Ipc2BH) 27, and (ItBuMe)(Ipc2BH) 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 enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered. ? 2016 The Royal Society of Chemistry.
|DOI:||10.1039/c6dt02202b||metadata.dc.subject.other:||Catalysts; Hydrogenation; Positive ions; Binaphthalenes; Carbenes; Catalyst designs; Hydride abstraction; Methoxy; Ring-expansion reaction; Sterically demanding; Organic compounds|
|Appears in Collections:||化學系|
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