NATURE OF INITIAL TRANSIENT PERIOD DURING ENANTIOSELECTIVE HYDROGENATION ON PT AND PD

The enantioselective hydrogenation of ethyl pyruvate, trifluoroacetoph enone, and tiglic acid has been studied over cinchonidine-modified Pt/ alumina or Pd/alumina catalysts. In situ measurement of the catalyst p otential during reaction, cyclic voltammetric, and FTIR studies were u sed to elucidate the nature of the initial transient period. The studi es demonstrate that the recently reported surprising change of enantio meric excess tee) with the conversion of ethyl pyruvate cannot be cons idered as an intrinsic feature of the catalyst system, because it is m ainly caused by impurities in ethyl pyruvate (0.6-1.9% racemic ethyl l actate even after distillation) and irreversible (destructive) adsorpt ion of ethanol or propanol used as solvents. The initial adsorption of ethyl pyruvate on Pt was also found to be irreversible, producing con siderable contamination of the Pt surface by linearly bonded CO and Cx HyOz type organic residues. Astonishingly, this side reaction suppress ed the initial ee only by a few percent in the presence of hydrogen an d acetic acid as solvent. These findings provide a plausible explanati on for the positive influence of high surface hydrogen concentration ( high pressure, efficient mixing) on the initial and final ee in ethyl pyruvate hydrogenation and for the outstanding enantioselectivity obta ined in acetic acid solvent. The observed contamination renders the va lue of any kinetic or mechanistic analysis of enantioselective hydroge nation reactions over Pt in alcoholic solvents questionable. The hydro genation of the carbonyl group of trifluoroacetophenone with cinchonid ine-modified Pt/alumina and that of the C=C double bond of tiglic acid with cinchonidine-modified Pd/alumina provide further examples on the increase of ee during the initial transient period and on the strong influence of impurities. (C) 1997 Academic Press.