Enantioselective hydrogenation of ethyl pyruvate and 1-phenyl-1,2-propanedione on catalysts prepared by impregnation of colloidal platinum on SiO2

Abstract

The enantioselective hydrogenation of ethyl pyruvate and 1-phenyl-1,2-propanedione was studied at 298 K and 40 bar of H2 over colloidal Pt stabilized with cinchonidine (CD) and supported on silica. The catalysts were prepared by impregnation of colloidal Pt on SiO2, leading to a metal load close to 1 wt.%. The colloid was prepared from an aqueous solution of H2PtCl6 and stabilized with different quantities of CD. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, XPS, XRD and TEM techniques. The reactions were carried out in a stainless steel batch reactor using cyclohexane as solvent and cinchonidine as chiral modifier. The addition of CD to the catalysts allows the stabilization and control of the platinum particle size and also affects the enantiomeric excess (ee) of the system, affording a higher proportion of the (R)-product. The relationship between the enantioselectivity and the CD concentration (added in situ) exhibits a bell type curve for both reactions, this being indicative of the importance of a competitive adsorption of the modifier and substrate on the catalyst surface.