ENANTIOSELECTIVE HETEROGENEOUS CATALYSIS .3. EFFECT OF OXYGEN ON CATALYST ACTIVITY AND SELECTIVITY IN THE ENANTIOSELECTIVE HYDROGENATION OFPYRUVATES

Data are presented which are in agreement with previous reports concer ning the enhanced activity and enantioselectivity of freshly re-reduce d platinum catalysts when used for the chiral hydrogenation of alpha-k etoesters. Catalyst deactivation was observed on heating the platinum catalyst in either hydrogen or helium at 350 degrees C for two hours a nd then using it without exposure to oxygen. The presence or absence o f cinchonidine had no effect on the rates of pyruvate hydrogenations r un using these catalysts. These data establish that the deactivation o f the catalyst was not caused by a strong adsorption of the alkaloid o n the platinum surface. The activity and enantioselectivity of these c atalysts were increased on addition of air to the reaction mixture. Th e reported enhanced activity and enantioselectivity shown by freshly r e-reduced catalysts depend, then, not only on the re-reduction step bu t also on a brief exposure of this catalyst to air. It appears from pu blished experimental details that the oxygen present in the solvent us ed in the hydrogenation is probably sufficient for this purpose.