School of Engineering and Science - Jacobs University

Fig. 1-21. The optimized structures of the proposed transition state formed on
interaction of cinchonidine’s protonated nitrogen and the pyruvate ester [88].
It has to be noted, that in the transition states the α-ketoesters are stabilized in their
half-hydrogenated form [88] and cinchonidine (in the flat adsorption mode) is
represented in open 3 conformation in these transition states as was suggested from the
investigation of obtained ee with conformation analysis.
Today it is practically accepted that enantioselective hydrogenation proceeds through
the “two-cycle” mechanism as presented in Fig. 1-22. It is assumed, that the active
chiral sites associated with adsorbed cinchonidine [CD] ad on Pt surface. Ethyl pyruvate
from the liquid phase adsorb reversibly on these sites in its two enantiofacial
configurations forming the diastereomeric intermediate complexes [EP-CD] R ad and
[EP-CD] S ad, which upon hydrogenation afford the (R)- and (S)- ethyl lactate,
respectively. It has been suggested that the adsorbed cinchonidine interacts with the
adsorbed ethyl pyruvate via hydrogen bonding between quinuclidine N and O atom of
the α-carbonyl moiety [73, 84]. Blaser et al. [89] suggested that for hydrogenation on
unmodified sites, leading to the racemic products, addition of the first hydrogen is ratedetermining
whereas for hydrogenation on modified sites, leading to the major
enantiomer, the rate determining step is the addition of the second hydrogen.
Fig. 1-22. Two-cycle mechanism suggested for enantiomeric hydrogenation of ethyl
pyruvate [EP] over Pt modified by cinchonidine. The [CD] ad present an active chiral
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