Preparation of chiral imidazolin-2-imine ligands and their application in ruthenium-catalyzed transfer hydrogenation

2-Chloro-1,3-diisopropyl-4,5-dimethylimidazolium tetrafluoroborate (1) serves as a convenient starting material for the preparation of mono-and bis(imidazolin-2-imine) ligands. Thus, the reaction of two equivalents of 1 with 1,2-ethylenediamine in the presence of potassium fluoride afforded the bis(2-aminoimidazolium) salt [(BLH2)-H-iPr][BF4](2) (2), from which the achiral bis(imidazolin-2-imine) ligand N,N'-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BLiPr) can be obtained by deprotonation. Likewise, the reaction of 1 with (1R,2R)-(-)-1,2-diaminocyclohexane (DACH) gave [DACH(Im(iPr)H)(2)][BF4](2) (3) and its deprotonation the chiral, C-2-symmetric diimine DACH(Im(iPr))(2) (4). Under similar conditions, chiral, C-1-symmetric mono(imidazolin-2-imines) were obtained from the reaction of 1 with one equivalent of (1R,2R)-(-)-1,2-diaminocyclohexane (DACH) or (1S,2S)-(-)-1,2-diphenylethylenediamine (DPEN), which afforded the 2-aminoimidazolium salts [DACH(Im(iPr)H)NH2][BF4] (5) and [DPEN(Im(iPr)H)NH2][BF4] (6), respectively. The reaction of 4 with [(C6H6)RuCl2](2) gave ruthenium complex [(C6H6)Ru{DACH(Im(iPr))(2)}]Cl-2, [7] Cl-2, which was treated with KPF6 to form [7][PF6](2). The ligand precursors 5 and 6 were deprotonated in the presence of [(C6H6)RuCl2](2), which resulted in the formation of complexes [(C6H6)Ru{DACH(Im(iPr))NH2}Cl]Cl [8] Cl and [(C6H6)Ru{DPEN(Im(iPr))NH2}Cl]Cl [9] Cl. Complexes [7][PF6](2), [8] Cl and [9] Cl were investigated for their ability to catalyze the transfer hydrogenation of acetophenone in isopropanol. Complex [8] Cl proved to be the most active system, while complex [9] Cl produced the highest enantioselectivity, albeit of only 27% ee. The molecular structures of [7][(C6H6)RuCl3](2)center dot CH2Cl2, formed as a side product, and of [8] Cl center dot acetone were determined by X-ray diffraction analyses. (C) 2014 Elsevier B. V. All rights reserved.