The Effect of Assembled Amphiphiles on Catalytic Reactions in Aqueous Media

Micelle-forming amphiphiles can promote catalyzed reactions in aqueous media. An example is the asymmetric hydrogenation of amino acid precursors. In comparison to the most common solvent methanol, the hydrogenation in water leads to a decrease of activity and a decrease of enantioselectivity, but, by adding a small amount of a surfactant (20 mol %), the activity increased to a value similar to methanol and the enantioselectivity increased sometimes to values higher than in methanol. A disadvantagous consequence of the use of surfactants is the more difficult phase separation within the work up of the reaction mixture because of the detergent effect of the amphiphile. Another problem is the recycling of the catalyst. To solve both these problems, we developed two methods: (i) the use of polymerized and polymeric amphiphiles in a membrane reactor and (ii) an immobilization of amphiphiles on an inorganic support. The precondition for the realization of these concepts is a leaching-free embedding of the catalytic rhodium complex in the macromolecular amphiphilic layer. Advantages and disadvantages of these methods will be discussed.