Mechanism of enantioselective catalysis by nickel surfaces coordinated with optically active hydroxy acids
Enantioselective hydrogenation of methyl acetoacetate (MAA) on nickel surfaces coordinated with optically active hydroxy acids has been investigated in the gas phase under the conditions of in situ preparation and modification of catalysts. The catalyst modified with tartaric acid in vacuo showed a high enantioface-differentiating ability of 48 % in enantiomer excess, whereas the modification of catalysts in air and oxygen atmosphere decreased the selectivity to 68–60 % of that value. Activities of several modifier molecules such as (S)-malic acid, (S)-3-hydroxybutyric acid and (S)-lactic acid were compared with that of (2R,3R)-tartaric acid and the adsorbed states of modifiers were examined by means of X-ray photoelectron spectroscopy. The mechanism of enantioface differentiation is discussed on the basis of these results, a proposed reaction pathway and the results of hydrogenation of related compounds. A model of enantioselective control is proposed on the assumption of MAA oriented by a pair of weak bonds with the modifier through the hydroxy and carbonyl groups of the hydroxy acid: the advantage of a dicarboxylic acid modifier is pointed out for the asymmetric hydrogenation of diketone compounds.