Organic Reactions in Microemulsions

Abstract

AbstractMicroemulsions, i.e., thermodynamically stable, one‐phase mixtures of oil, water and surfactants, are useful as reaction media as a way to overcome the reactant incompatibility problem that one frequently encounters in organic synthesis. The use of a microemulsion can be seen as an alternative to phase‐transfer catalysis, i.e., use of a two‐phase system with added phase‐transfer agent. The microemulsion approach and phase‐transfer catalysis can also be combined, i.e., the reaction can be performed in a microemulsion in the presence of a small amount of a phase‐transfer agent. A very high reaction rate may then be obtained. The reaction rate in a microemulsion is often influenced by the charge at the interface and this charge depends on the surfactant used. For instance, reactions involving negatively charged species will be accelerated by the use of a cationic surfactant. This effect is analogous to micellar catalysis and may be termed microemulsion catalysis. The acceleration is more pronounced when the surfactant counterion is small and weakly polarizable. Microemulsions can also be used to induce regiospecificity of organic reactions. Organic molecules with one more polar and one less polar end will accumulate at the oil‐water interface of microemulsions. They will orient at the interface so that the polar part of the molecule extends into the water domain and the nonpolar part extends into the hydrocarbon domain. A water‐soluble reagent will react from the “water side”, i.e., attack the polar part of the amphiphilic molecule, and a reagent soluble in hydrocarbon will react at the other end of the amphiphilic molecule. This review demonstrates that microemulsions can be a useful for (i) overcoming reactant incompatibility, (ii) speeding up reactions of one polar and one apolar reactant (microemulsion catalysis), and (iii) inducing regiospecificity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)