| Abstract
| - Chirally modified platinum surfaces as used in enantioselective hydrogenation have been generated byadsorption of O-phenyl derivatives of cinchonidine on Pt/Al2O3 films. The adsorption behavior as well as thestructure of the resulting chiral solid−liquid interfaces has been investigated using attenuated total reflectioninfrared (ATR-IR) spectroscopy. In particular O-phenyl-cinchonidine, O-(3,5-dimethylphenyl)-cinchonidine,and O-[(3,5-bis(trifluoromethyl)phenyl]-cinchonidine have been analyzed, and their behavior has been comparedto that of the parent alkaloid. The ATR-IR spectroscopic investigation provided insight into submolecularstructural details of the conformation of the adsorbates under conditions close to those existing during thecatalytic enantioselective hydrogenation. As a complement to the spectroscopic observations, electronic structurecalculations were performed using density functional theory (DFT). The result of the conformational study ofeach adsorbed modifier revealed a correlation between the spatial orientation of the substituted phenyl ringsand the enantioselectivity of the catalyst observed in the asymmetric hydrogenation of ketopantolactone andother α-activated ketones. Experiments and calculations support an interpretation according to which theorientation in space of the phenyl ring reshapes the surface chiral space formed by the adsorbed modifiers,thus generating surfaces (and catalysts) having different, in some cases even opposite, enantioselective propertieswithout altering the absolute configuration of the modifier. Competitive adsorption experiments betweenmodifiers were also carried out in order to determine their affinity toward the metal and allow assignment ofa relative adsorption strength scale.
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