Quaternary ammonium salts ionic liquids for immobilization of chiral Ru-BINAP complexes in asymmetric hydrogenation of β-ketoesters

Chiral catalytic complex (R)-[RuCl(binap)(p-cymene)]Cl was used in asymmetric hydrogenation of methyl acetoacetate to methyl-3-hydroxybutyrate in the mixed methanol-ionic liquid phase. Quaternary ammonium salts ionic liquids, namely n-alkyl-triethylammonium bis(trifluoromethane sulfonyl) imides (NR222Tf2N, R = 6, 7, 8,10,12, 14), were prepared and employed in this transformation. Enough evidence was provided that only a small amount of this type of IL in a conventional solvent was necessary to accommodate the catalytic complex and that the reaction could be carried out with very high enantioselectivity. Similarly it was proved that under optimized conditions the catalytic complex immobilized in this manner could be used repeatedly. A part of the work was focused on the role of the alkyl chain length in the NR222Tf2N ionic liquid which was found as very essential. Role of the reaction impurities with origin in the synthesis of the employed ILs was also investigated. (C) 2009 Elsevier B.V. All rights reserved.