Asymmetric Hydrogenation with Modified Raney Nickel. VI

Abstract The asymmetric hydrogenation catalyst has been prepared by the modification of Raney nickel with solutions of (−)-2-methyltartaric acid (A), (+)-2-methyltartaric acid (B) and (+)-2,3-dimethyltartaric acid, and its asymmetric hydrogenation activity has been measured by the hydrogenation of methyl acetoacetate to optically active methyl 3-hydroxybutyrate. The asymmetric activity of the catalyst is much influenced by the pH and the temperature of the modifying solution. Generally, (+)-2-methyltartaric acid (B) and (+)-2,3-dimethyltartaric acid have a high asymmetric activity in reaction to Raney nickel. The highest asymmetric activity has been obtained by modification with (+)-2-methyltartaric acid (B) and with it the specific rotation of the hydrogenation product has been found to be [α]D20 +11.72° (without a solvent) under the optimum conditions. This value means that the product is 78% composed of the dextro- and 22% of the levorotatory form. On the other hand, the asymmetric activity of the catalyst modified with (−)-2-methyltartaric acid (A) is very low and quite different from that of the catalyst modified with (+)-2-methyltartaric acid (B). It has been supposed that this difference depends on the stereoisomerism of the threo-erythro form of the 2-methyltartaric acid molecule.