Examples of Micellar-Promoted Reactions

Reactions in micelles and in the double layers of vesicles are analogous to natural membrane systems and in special cases are comparable with the function of an enzyme (cf. Section 3.2.1). In contrast to macromolecular enzymes, amphiphile aggregates with linked catalytic centers have a less rigid supramolecular structure. The preorganization of an enzyme is highly selective and as a consequence enzyme catalysis is much more effective than micellar catalysis, a type of artifical mimic [6]. 

Micellar catalysis enables water to be employed as a reaction medium, to gain not only an enhancement in the rate of reaction but also improvements in selectivity and sometimes even to allow the catalyst to be recycled in a simple manner (cf. Section 3.1.1.1). A micellar system is a multiphase system in a colloidal dimension and allows the microheterogenization of a catalyst under special conditions [7]. 

Typically reported topics are solvolytic reactions, oxidations, reductions, and C-C coupling reactions. The saponification of activated esters in aqueous micelles is a typical model for an enzyme mimetic reaction. The influence of the micellar medium on the reaction rate has been investigated, as well as the alteration of the stereoselectivity. Models of metalloenzymes were developed with the ligands 1-3 [8]. 

Typical metal ions for complexation are Ni , Cu , Zn and Co . The amphiphilic ligands which were embedded in micelles or vesicles enhanced the reaction 

All types of surfactants promote the reaction but only the hydrogensulfate was successful in the case of cationic amphiphiles. Sometimes the enantioselectivity surpassed the values observed when methanol was used as a solvent. This method was even used successfully for the enantioselective hydrogenation of a-aminopho-sphonic [14] and a-aminophosphinic acid precursors [15]. The concentration of the surfactant leads to an efficient reaction only above the CMC. 

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