Enantioselective Hydrogenation of Ethyl Pyruvate over Pt/Alumina Modified by (R)-1-(1-Naphthyl)ethylamine Derivatives

Abstract

A new chiral modifier, (R)-1-(1-naphthyl)ethylamine, has been tested in the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate over 5 wt% Pt/Al2O3. The influence of catalyst (2–28 g liter−1) and modifier concentration (0–20 mM), temperature (282–333 K), pressure (1–75 bar), and solvents was studied in a slurry reactor. The 82% enantiomeric excess (ee) at full conversion was achieved in acetic acid after optimizing the reaction parameters. Under mild conditions the new modifier affords ee's better than that achieved with cinchona alkaloids. A drop in ee at pressures higher than 10 bar and temperatures above 288 K is attributed to partial hydrogenation of the naphthalene ring, which hinders the adsorption of the modifier parallel to a flat Pt surface. Maximum rate acceleration by a factor of 12, compared to the racemic reaction, was observed after thermal treatment of the catalyst in flowing hydrogen at 673 K, followed by aerobic treatment at 273–298 K in acetic acid. It is shown that naphthylethylamine is only a precursor of the actual modifier, which is a secondary amine formedin situfrom naphthylethylamine and ethyl pyruvate by condensation to the corresponding imine and subsequent reduction of the C=N bond. Several other derivatives of naphthylethylamine were prepared by reductive alkylation and tested as modifiers. The results indicate that the presence of an oxygen function such as a hydroxy or methoxy group, as in previously used modifiers, is not an indispensable requirement for obtaining high ee in the hydrogenation of α-ketoesters.