%0 Journal Article %A Huck, W.-R. %A Bürgi, T. %A Mallat, T. %A Baiker, A. %T Enantioselective hydrogenation on cinchona-modified metal catalysts: Mechanistic implication of acid additive %J Journal of Catalysis %@ 0021-9517 %V 205 %N 1 %P 213-216 %D 2002 %R 10.1006/jcat.2001.3436 %U http://dx.doi.org/10.1006/jcat.2001.3436 %U http://archive-ouverte.unige.ch/unige:14710 %X The enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone in the presence of acetic acid and trifluoroacetic acid has been studied on cinchonidine-modified Pd/TiO2. Catalytic experiments and theoretical calculations indicate the formation of a cinchonidine–trifluoroacetic acid cyclic ion pair. We propose that this is the actual modifier, which interacts with 4-hydroxy-6-methyl-2-pyrone in the enantiodifferentiating step. The new mechanistic model is assumed to be valid also for other reactions over cinchona-modified Pt or Pd, in the presence of trifluoroacetic acid.