Homogeneous Rhodium-Chiral Phosphine Catalyst Systems

Homogeneous Rhodium-Chiral Phosphine Catalyst Systems 

At about the time of Wilkinson s discovery, new schemes were developed by others for the preparation and configurational correlation of chiral phosphines (lOa-e). The combination of advances in homogeneous catalysis and chiral phosphine technology prompted research on chiral phosphine complexes. Horner et al. (11) were the first to hypothesize in print that rhodium complexes containing optically active tertiary phosphine ligands should effect the asymmetric hydrogenation of unsymmetrically substituted olefins. 

The first examples of asymmetric hydrogenation based on this principle were reported by Knowles and co-workers (the Monsanto group) in 1968 (12). Rhodium complexes of the type RhL3Cl3 (where L was a chiral phosphine) were used in the hydrogenation of a-phenylacrylic acid (atropic acid) and itaconic acid under the conditions indicated in Fig. 3. When L was (R)-( )-methylphenyl-n-propylphosphine,3 15% optically pure (S)-(+)-a-phenyl-pro-pionic acid and 3% optically pure methylsuccinic acid (configuration unreported) were obtained. 

2However, the second hydrogen transfer may follow the first so closely in many instances that the distinction between a two-step and simultaneous process is fragile. 

3The phosphine ligand (PPhMePr ) had an optical purity of about 70%. In the original publication (12) there is a misprint that shows the chiral phosphine as PPhMePr1 rather than PPhMePr . 

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II. Homogeneous Rhodium-Chiral Phosphine Catalyst Systems. 85... 

Wilkinson s (I) discovery that the soluble rhodium(I) phosphine complex, [Rh(PPh3)3Cl], was capable of homogeneous catalytic hydrogenation of olefins immediately set off efforts at modifying the system for asymmetric synthesis. This was made possible by the parallel development of synthetic methods for obtaining chiral tertiary phosphines by Horner (2) and Mislow (3,4, 5). Almost simultaneously, Knowles (6) and Horner (7) published their results on the reduction of atropic acid (6), itaconic acid (6), a-ethylstyrene (7) and a-methoxystyrene (7). Both used chiral methylphenyl-n-propyl-phosphine coordinated to rhodium(I) as the catalyst. The optical yields were modest, ranging from 3 to 15%. 

This chapter summarizes some of the most characteristic results obtained with the use of mainly homogeneous metal complex eatalysts either in the industry or in processes recommended for practical use. These are large seale processes of asymmetric synthesis of the herbicide metolachlor, synthesis of optically pure menthol with the use of chiral iridium and rhodium phosphine complexes, consideration of the synthesis of ethyl 2-hydroxybutyrate as a monomer for the preparation of biodegradable polyesters with use of heterogeneous ehiral modified nickel catalyst, the manufacturing of (fJ)-pantolactone by means of a possible eata-IjTic systems for enantioselective hydrogenation of ketopantolactone, and catalytic systems for the preparation of other pharmaceuticals. 

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