Enantioselective heterogeneous catalysis .3. Effect of oxygen on catalyst activity and selectivity in the enantioselective hydrogenation of pyruvates

Data are presented which are in agreement with previous reports concerning the enhanced activity and enantioselectivity of freshly re-reduced platinum catalysts when used for the chiral hydrogenation of alpha-ketoesters. Catalyst deactivation was observed on heating the platinum catalyst in either hydrogen or helium at 350 degrees C for two hours and then using it without exposure to oxygen. The presence or absence of cinchonidine had no effect on the rates of pyruvate hydrogenations run using these catalysts. These data establish that the deactivation of the catalyst was not caused by a strong adsorption of the alkaloid on the platinum surface. The activity and enantioselectivity of these catalysts were increased on addition of air to the reaction mixture. The reported enhanced activity and enantioselectivity shown by freshly re-reduced catalysts depend, then, not only on the re-reduction step but also on a brief exposure of this catalyst to air. It appears from published experimental details that the oxygen present in the solvent used in the hydrogenation is probably sufficient for this purpose.