Chiral carbene–borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations. Issue 39 (7th July 2016)

• Record Type: Journal Article
• Authors: Lam, Jolie; Günther, Benjamin A. R.; Farrell, Jeffrey M.; Eisenberger, Patrick; Bestvater, Brian P.; Newman, Paul D.; Melen, Rebecca L.; Crudden, Cathleen M.; Stephan, Douglas W.
• Is Part Of: Dalton transactions. Volume 45:Issue 39(2016)
• Page Start: 15303
• Page End: 15316
• Publication Date: 2016-07-07
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c6dt02202b
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : A series of chiral borenium cations are prepared characterized and evaluated in metal-free asymmetric hydrogenations of imines. Abstract : The carbene derived from (1 R, 3 S )-camphoric acid was used to prepare the borane adduct with Piers' borane7 . Subsequent hydride abstraction gave the borenium cation8 . Adducts with 9-BBN and the corresponding (1 R, 3 S )-camphoric acid-derived carbene bearing increasingly sterically demanding N -substituents (R = Me9, Et10, i-Pr11 ) and the corresponding borenium cations12–14 were also prepared. These cations were not active as catalysts in hydrogenation, although9–11 were shown to undergo carbene ring expansion reactions at 50 °C to give species15–17 . The IBOX-carbene precursors18 and19 derived from amino alcohols ( S )-valinol and ( S )- tert -leucinol (R = i -Pr, t -Bu) were used to prepare borane adducts20–23 . 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 (Ipc2 BH) gave chiral adducts: (IMe)(Ipc2 BH)24, (IPr)(Ipc2 BH)25, (IBnMe)(Ipc2 BH)26, (IPhMe)(Ipc2 BH)27, and (I t BuMe)(Ipc2 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-triazolium34a/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.