The Fate of Ethyl Pyruvate during Adsorption on Platinum Chirally Modified by Cinchonidine Studied by ATR−IR Spectroscopy

Abstract

The behavior of ethyl pyruvate during adsorption on vapor deposited alumina-supported platinum films and on a commercial 5 wt % Pt/Al2O3 catalyst has been studied in the absence and presence of coadsorbed cinchonidine, which is usually applied as a chiral modifier in the platinum-catalyzed enantioselective hydrogenation of α-ketoesters. The in situ ATR−IR measurements, performed at room temperature using hydrogen-saturated CH2Cl2 as solvent, revealed that upon adsorption on the platinum some of the ethyl pyruvate decomposes leading to strongly adsorbed CO and other fragmentation products. The CO originating from decomposition of ethyl pyruvate reached approximately 14% of the amount of adsorbable CO on the free platinum surface and is proposed to be adsorbed preferentially on energetically favored sites such as edges and corners. The presence of cinchonidine (10-4 M) lead to a drastic decrease of the decomposition rate of ethyl pyruvate by a factor of about 60 under the conditions used. Competitive adsorption experiments of CO and cinchonidine in the presence of hydrogen indicated that cinchonidine can displace the adsorbed CO, confirming the strong anchoring of cinchonidine on the platinum surface, which is a prerequisite for its action as a chiral modifier. The findings of the adsorption studies provide a plausible explanation for the earlier made observation that the sequence of admission of α-ketoester, chiral modifier, and hydrogen affects the catalytic performance of platinum-catalyzed enantioselective hydrogenation. The decomposition is likely to occur also with other α-ketoesters and may have a bearing on the initial transient period, typically observed during hydrogenation of such compounds on cinchona-modified platinum catalysts.