Some Examples of Reactions in Reverse Micelles and Microemulsions

Some Examples of Reactions in Reverse Micelles and Microemulsions 

Reverse micelles are formed by association of polar headgroups of amphiphiles with colloidal drops of water in an organic medium. A favored surfactant seems to be AOT (sodium-di[2-ethylhexyl]sulfosuccinnate) but SDS and tetraalkylammoni-um salts have also proved to be useful. Like aqueous micelles, reverse micelles exist in highly diluted systems. 

A microemulsion is a multicomponent (3-4 components) system, e.g., water in hydrocarbon (water/oil) or hydrocarbon in water (oil/water), surfactant, and cosurfactant, and generally it exists only in small concentration ranges. Nevertheless, the capacity for reactants and variability of solubilization properties are high and of practical interest [76]. On the basis of microemulsions Menger and co-workers developed a method for an economical environmental cleanup of chemical warfare contamination [77]. As an example of organometallic catalysis in a microemulsion, Beletskaya [78] performed palladium-catalyzed C—C coupling reactions in aqueous medium with a very high content of surfactant. 

An acceleration was observed in the hydrogen-transfer reaction from 1,2-cyclo-hexanedimethanol to ( )-4-phenyl-3-butene-2-one in toluene catalyzed by [RuCl((S)BINAP)(benzene)]Cl (2 mol% BINAP = 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) and SDS (6mol%) [79]. The surfactant is essentia] for the enhancement of catalytic activity and the authors proposed the existence of reverse micelles. 

It was shown by Buriak and Osborn [80] that non-micelle-forming anions improved the enantioselectivity of an imine hydrogenation catalyzed by rhodium complexes in the same way as reverse micelles. Complexation of the sulfate or sulfonate anion with the catalyst appears to be responsible for the enhancement of the enantioselectivity. The very strong dependence of the product chirality on the structure of the anion is discussed. Finally, a long-chain ephedrinium salt 17 as surfactant, should be mentioned. 

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