New benzimidazolium-based chiral ionic liquids: synthesis and application in enantioselective sodium borohydride reductions in water

New chiral ionic liquids based on benzimidazolium salts have been synthesized from the reaction of (−)-menthol or (−)-borneol, chloroacetic acid and 1-methylbenzimidazole. The structures of these chiral ionic liquids have been confirmed from 1H, 13C NMR and mass spectral studies. The application of these chiral ionic liquids in enantioselective sodium borohydride reductions of prochiral ketones to produce optically active alcohols has been studied. The effect of the solvent on the enantioselectivity and reusability of the chiral catalyst has also been studied.

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3-(2-(((1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl)oxy)-2-oxoethyl)-1-methyl-1H-benzo[d]imidazol-3-ium chlorideC20H29ClN2O2[α]D25 = −35.7 (c 0.25, EtOH)Source of chirality: (−)-mentholAbsolute configuration: (1R,2S,5R)

1-Methyl-3-(2-oxo-2-(((1S,2R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl)oxy)ethyl)-1H-benzo[d]imidazol-3-ium chlorideC20H27ClN2O2[α]D25 = −22.5 (c 0.25, EtOH)Source of chirality: (−)-borneolAbsolute configuration: (1S,2R,4R)