Nature of initial transient period during enantioselective hydrogenation on Pt and Pd

The enantioselective hydrogenation of ethyl pyruvate, trifluoroacetophenone, and tiglic acid has been studied over cinchonidine-modified Pt/alumina or Pd/alumina catalysts. In situ measurement of the catalyst potential during reaction, cyclic voltammetric, and FTIR studies were used to elucidate the nature of the initial transient period. The studies demonstrate that the recently reported surprising change of enantiomeric excess tee) with the conversion of ethyl pyruvate cannot be considered as an intrinsic feature of the catalyst system, because it is mainly caused by impurities in ethyl pyruvate (0.6-1.9% racemic ethyl lactate even after distillation) and irreversible (destructive) adsorption of ethanol or propanol used as solvents. The initial adsorption of ethyl pyruvate on Pt was also found to be irreversible, producing considerable contamination of the Pt surface by linearly bonded CO and CxHyOz type organic residues. Astonishingly, this side reaction suppressed the initial ee only by a few percent in the presence of hydrogen and acetic acid as solvent. These findings provide a plausible explanation 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 obtained in acetic acid solvent. The observed contamination renders the value of any kinetic or mechanistic analysis of enantioselective hydrogenation reactions over Pt in alcoholic solvents questionable. The hydrogenation of the carbonyl group of trifluoroacetophenone with cinchonidine-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.