Asymmetric Catalysis by Chiral Rhodium Complexes in Hydrogenation and Hydrosilylation Reactions

The preparation of optically active molecules needs a chiral auxiliary. It is important to use the minimum amount of this auxiliary, and from that point of view asymmetric catalysis is much more advantageous than stoichiometric asymmetric synthesis. Some homogeneous catalysts prepared from chiral complexes have become during the past few years a useful tool in asymmetric synthesis. The complexes L2RhC1 where L2 is a family of chiral diphosphines were prepared and used in asymmetric reduction. DIOP is a readily available ligand prepared from tartaric acid. Many of its derivatives were obtained as well as other types of chiral phosphines. General syntheses of optically active x-amino acids, amines or acids are described. Optical yields as high as 90 per cent could be attained. The same complexes can catalyse the hydrosilylation of ketones and imines, giving rise after hydrolysis to optically active alcohols and amines. To improve the usefulness of asymmetric catalysis a supported chiral catalyst was prepared starting from a Merrifield resin. It was used both in reduction and in hydrosilylation. The mechanism of the reactions and the origin of the asymmetric induction will be discussed.