ENANTIOSELECTIVE HETEROGENEOUS CATALYSIS .1. A WORKING MODEL FOR THE CATALYST-MODIFIER-SUBSTRATE INTERACTIONS IN CHIRAL PYRUVATE HYDROGENATIONS

The room temperature and atmospheric pressure hydrogenation of ethyl pyruvate over Pt/Al2O3 catalysts modified by varying amounts of dihydrocinchonidine was examined. Data were obtained which showed that the hydrogenation occurred on the comer atoms and adatoms on the Pt crystallites in the catalyst. The formation of (S) ethyl lactate was observed when very low concentrations of the alkaloid modifier were used while at higher modifier concentrations the (R) lactate was produced. The formation of the (R) lactate was accompanied by an increase in the hydrogenation rate. A working hypothesis was formulated to explain these results and to serve as a model for the design of future experiments. This model suggested that the initial adsorption of the dihydrocinchonidine takes place on the face atoms adjacent to the comer atoms on the metal crystallite. This will place the chiral portion of the alkaloid close to the comer atom active site. Enantioselective pyruvate adsorption would be facilitated by a hydrogen bond between the C-9 OH of the alkaloid and the ethoxy oxygen of the pyruvate and hydrogenation of the keto group will lead to (S) lactate formation. In order to place the chiral portion of the modifier near an adatom it is proposed that the alkaloid is adsorbed in an edge-on manner on the face near the adatom active site. In this way the modifier can interact directly with the pyruvate to change its adsorption characteristics. This will lead to an increase in reaction rate and the formation of the (R) lactate.