Reactions in micellar systems

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

Table 4. Sc(OTf)3-catalyzed aldol reactions in micellar systems.

Several examples of Sc(OTf)3-catalyzed aldol reactions in micellar systems are shown in Table 4. Not only aromatic, but also aliphatic and a,j8-unsaturated aldehydes react with silyl enol ethers to afford the corresponding aldol adducts in high yields. Aqueous formaldehyde solution also worked well. Even the ketene silyl acetals, which readily hydrolyze in the presence of a small amount of water, reacted with aldehydes... 

Table 14-12. Sc(OTf)rCatalyzed allylation reactions in micellar systems.

K. Takuma, T. Sakamoto, T. Nagamura, and T. Matsuo, Novel properties ofthe self-assembling amphiphatic viologen system. 1. A study of electron-exchange reactions in micellar systems, J. Phys. Chem. 85, 619-621 (1981). 

Surfactants by definition self-organise in water giving rise to micelles of varying size and shape. The core of micelles is non-polar and can solubilise reactants that are insoluble in water. Thus, a simple surfactant-water system at a surfactant concentration well above the critical micelle concentration can be used to overcome the problem of reactant incompatibility the polar reagent will be situated in the bulk aqueous domain, the non-polar reagent will be present in the micelles, and the reaction will occur at the micelle boundary. Organic reactions in micellar systems have been reported more than 40 years ago [1,2]. 

Some other C—C bond coupling reactions in micellar systems should be mentioned here. Monflier et al. [72] described, in both papers and patents, the telome-rization of 1,3-butadiene into octadienol in a micellar system by means of a palladium-phosphine catalyst. Water-soluble and amphiphilic phosphines have been used and the surfactants were widely varied. The authors have shown that the promoting effect of surfactants appeared above the CMCs of the surfactants, and they conclude that micellar aggregates were present in the reaction mixture. Cationic, anionic, and nonionic surfactants gave this micellar effect but the combination of the highly water-soluble TPPTS and the surfactant dodecyldimethylamine hydrocarbonate was found to be best. A speculation about the location of reactants shows that the reaction probably occurs in the interface between the micellar pseudophase and water. 

Thomas, J.K., 1977. Effect of structure and charge on radiation-induced reaction in micellar systems. Acc. Chem. Res., 10 133—138. 

A wide range of chemical reactions in micellar systems depend mainly upon the difference of properties of the micellar phase and the bulk phase. One should only distinguish the solubilized and non-solubilized reactant molecules, the micelles altogether being considered as a pseudophase. For second-order reactions, the intermicellar distribution of reactant molecules should be taken into consideration as discussed above. 

It should be noted that the reactions were successfully carried out in water without using any organic solvents. Use of the reusable scandium catalyst and water as a solvent would result in clean and environmentally friendly systems. Further studies to develop other synthetic reactions in micellar systems and also to clarify the precise mechanism in these reactions are now actively in progress in our laboratories. 

An important characteristic of reactions in micellar systems is that the micellar concentration can be varied to some extent. In a ususal solvent it is almost impossible to avoid side reactions. In a micellar system, on the other hand, side reactions can be avoided by adjusting the concentrations of reactants and micelles so that most micelles contain just one reactant molecule. The distribution of reactant molecules among micelles thus has a crucial influence on reaction in micellar assemblies, as discussed in Chapter 9. 

One of the most important characteristics of ionic micelles is their electrostatic potential (up to hundreds of millivolts at the micellar surface) and their resulting ability to select specific counterion species. The potential also depends on the counterion, so the above two quantities are not necessarily independent. They have a crucial effect on electron transfer reactions in micellar systems. 

Other examples of electron transfer reactions in surfactant assemblies are those between pyrene and dimethylaniline in micelles, between viologen derivative and zinc porphyrin as an electron relay, and between chlorophyll a and methylviologen in microemulsions the photoinduced reduction of duroquinone by zinc porphyrin in micellar solution the photoinduced redox reaction of proflavine in aqueous and micellar solutions retardation of back reactions in micellar systems light-driven electron transfer from tetrathiafulvalene to porphyrin and tris a, a -bipyridine)... 

Dwats T, Paetzold E, Oehme G. Reactions in micellar systems. Angeur Chem Int Ed. 2005 44 7174-7199. 

---