ENANTIOSELECTIVITY OF NICKEL CATALYSTS MODIFIED WITH TARTARIC ACID OR NICKEL TARTRATE COMPLEXES

The enantioselective hydrogenation of methylacetoacetate to optically active methyl hydroxybutyrate has been studied in a static reactor at initial hydrogen pressures of 100 bar with nickel catalysts modified with S,S-(-)-tartaric acid or with complexes derived from it. Enantioselectivity was found to be comparable for SiO2-supported, unsupported, and Raney Ni catalysts, but significantly lower for γ-Al2O3-supported Ni. Separate tests showed that no racemization takes place on γ-Al2O3. Upon modifying catalysts with tartaric acid in a defined atmosphere, it was found that enantioselectivity increases with the modification temperature and that it is much higher after modification under air than under hydrogen. These results suggest that formation of the enantioselective site is an activated process, viz. a corrosive chemisorption where an Ni atom is pulled out of the metal surface and an Ni-tartrate complex is formed. This hypothesis was, indeed, confirmed by modifying Ni-metal with discrete Ni-tartrate or Na-Ni-tartrate complexes. Under our conditions none of these complexes displayed hydrogenation activity in the absence of metallic Ni and hydrogenation was nonselective for Ni modified with the corresponding Cu-tartrate complexes. Having established that both the Ni-tartrate complex and the Ni-metal are essential we propose a mechanism in which the Ni atom in the Ni-tartrate complex is the enantioselective site, while H atoms generated at the Ni-metal surface by dissociative chemisorption of H2 reach this site by migrating over the oxygen atoms of the tartrate ligands. These ligands thus serve both as transport routes for H atoms and as templates favoring the formation of one enantiomer. © 1979.