The Role of Modifier Structure in Heterogeneous Enantioselective Hydrogenation: One-to-One Interactions of 1-Phenyl-1,2-propanedione and Methyl Pyruvate with Modifiers on the Pt(111) Surface

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 methyl pyruvate over platinum catalysts. Two protonated modifiers, cinchonidine and 9-methoxycinchonidine, in the Open(3) and Open(5) conformations, were considered. So-called bifurcated and cyclic hydrogen-bonded complexes were investigated. The effects of a flat Pt(111) surface on the complexes were taken into account using 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 their thermodynamic 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 methoxy group does not have any notable effect on the enantiomeric excess of ( R )-methyl lactate but decreases the enantiomeric excess of ( R )-1-hydroxy-1-phenylpropanone. These results are well in accord with the experimentally observed enantiomeric excesses, thus supporting the validity of the studied reactant−modifier interaction model. The DFT calculations at the RI-BP86/SV(P) level indicated that protonated cinchonidine and 10,11-dihydrocinchonidine are more stable on Pt when adopting the so-called QA-Open(4) conformation rather than the Open(3) conformation that dominates in solution. The QA-Open(4) conformation of a modifier is adsorbed on the surface via both its quinoline and quinuclidine moieties, and a reactant may interact simultaneously with the protonated quinuclidine nitrogen and the functional group at the C(9) position of the modifier.