The intrinsic stereoselectivity of asymmetric sites on heterogeneous catalysts

Ni/SiO₂ catalysts, which have been 'modified' with (R,R)-(+)-tartaric acid (TA) are used for the enantioselective hydrogenation of methyl acetoacetate to (R)-(-)-methyl 3-hydroxybutyrate. Auger Electron Emission spectroscopy has been employed to assess the uniformity of the modification through the catalyst pellet. Conditions have been identified to achieve high uniformity. The fraction of the metal surface left bare after modification is estimated by the rates of 1-hexene hydrogenation at low temperature before and after modification. Even when this fraction is very low, the enantioselectivity in the absence of co-modifiers remains near 50%. It is suggested that this value reflects the intrinsic selectivity of the asymmetric sites. Higher selectivities achieved by adding a co-modifier are due to an enhanced intrinsic selectivity of such sites, not to a blockage of bare nickel.