Hydrogenation with surfactant phosphines

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

One of the most successful asymmetric catalytic reactions is the asymmetric hydrogenation of amino acid precursors by means of optically active rho-dium(I)phosphine or phosphinite complexes [55]. Usually, the reaction is carried out in methanol as solvent. When water is used the activity and enantiose-lectivity decrease significantly [16], but the addition of micelle forming surfactants leads to a solubilization of catalyst and substrate and increases activity and enantioselectivity. The results are somewhat better than the ones obtained with methanol as solvent [56]. Table 2 shows the effect with different types of surfactants. 

Dror and Manassen hydrogenated a- and cyclic olefins such as 1-octene, 1 -dodecene and cyclohexene in micellar systems using rhodium catalysts modified with water-insoluble carboxylated tenside phosphines 45 (Table 3 n=3,5,7,9,l 1) in the presence of conventional tensides such as sodium dodecylsulfate (SDS) or cetyltrimethylammonium bromide (CTAB) and cosolvents e.g. dimethyl sulfoxide. Linear olefins were more reactive than cyclic olefins. Maximum efficiency was observed in the presence of the anionic surfactant SDS when 45 contained a chain of 5-7 carbon atoms. In contrast, using the cationic tenside CTAB the ligand 45 with 5 C atoms was almost inactive but became active again with dodecyl trimethylammonium bromide. 

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