Interaction of Cinchonidine and 1-Phenyl-1,2-Propanedione on the Surface of a Chirally Modified Pt/Al₂O₃ Hydrogenation Catalyst

Enantioselective hydrogenation of 1-phenyl-1,2-propanedione (PPD) over cinchona-modified oxide-supported platinum catalysts represents an extension to the well-known Orito reaction with methyl and ethyl pyruvates (EtPy) as typical substrates for the asymmetric reduction. In this paper, the adsorption of PPD and cinchonidine on a Pt catalyst was studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalyst preparation and the adsorption of the reactant and modifier were carried out according to the procedure generally used in catalysis under actual reaction conditions. The catalysts with adsorbed species were analyzed ex situ. The selective deposition of cinchonidine and PPD on the metal surface was confirmed by parallel experiments with alumina samples and by analyzing the platinum adsorption sites with carbon monoxide as a probe molecule. Among the IR visible adsorbed species, η(O2)-configuration of PPD was found as the most populated on Pt surface under the conditions of ex situ measurements. New data on the mutual interactions among cinchonidine, PPD, and the Pt catalyst are presented. Tilted π-bonded cinchonidine was detected on the surface under the modifier concentration profile used for obtaining high enantioselectivity in the hydrogenation of PPD. Hydrogen bonding between adsorbed cinchonidine and PPD was also detected.