Potential of ionic liquids as co-modifiers in asymmetric hydrogenation on platinum

The enantioselective hydrogenation of methyl benzoylformate on Pt/Al2O3 has been studied in the presence of various ionic liquids (ILs) and three structurally related chiral modifiers: cinchonidine (CD), O-phenyl-cinchonidine (PhOCD), and O-naphthyl-cinchonidine (NaphOCD). Addition of ca. 1% IL to a polar organic solvent – particularly alcohols – improved the enantioselectivity by up to 12% (to 92–93%) in the presence of CD. On the other hand, ILs diminished the reaction rate sometimes dramatically, by two orders of magnitude, and the poisoning effect was even stronger in cyclohexene hydrogenation under identical conditions. Comparative studies revealed that only ILs possessing a heteroaromatic cation interact strongly with Pt or the O-aryl function of PhOCD and NaphOCD. A tentative explanation is the strong π-bonding interaction of ILs with the Pt surface (leading to site blocking and deactivation) and with the O-aryl function of the modifier not involved in the adsorption onto Pt (resulting in a shift in enantioselectivity).