| Abstract
| - Density functional theory (DFT) at the B3LYP/T(ON)DZP level was used to model one-to-one reactant−modifier interactions relevant to the enantioselective hydrogenation of 1-phenyl-1,2-propanedione and methylpyruvate over platinum catalysts. Two protonated modifiers, cinchonidine and 9-methoxycinchonidine, inthe Open(3) and Open(5) conformations, were considered. So-called bifurcated and cyclic hydrogen-bondedcomplexes were investigated. The effects of a flat Pt(111) surface on the complexes were taken into accountusing molecular mechanics with the COMPASS force field. Only the bifurcated reactant−modifier(Open3)complexes were suggested to contribute to the enantioselectivity of the hydrogenation reaction due to theirthermodynamic stability. The stabilization of the π and π* orbitals of the reactants' keto carbonyl moieties,that is, the kinetic factor, indicated that the substitution of cinchonidine's hydroxyl group with a methoxygroup does not have any notable effect on the enantiomeric excess of (R)-methyl lactate but decreases theenantiomeric excess of (R)-1-hydroxy-1-phenylpropanone. These results are well in accord with theexperimentally observed enantiomeric excesses, thus supporting the validity of the studied reactant−modifierinteraction model. The DFT calculations at the RI-BP86/SV(P) level indicated that protonated cinchonidineand 10,11-dihydrocinchonidine are more stable on Pt when adopting the so-called QA-Open(4) conformationrather than the Open(3) conformation that dominates in solution. The QA-Open(4) conformation of a modifieris adsorbed on the surface via both its quinoline and quinuclidine moieties, and a reactant may interactsimultaneously with the protonated quinuclidine nitrogen and the functional group at the C(9) position of themodifier.
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