Microemulsions organic synthesis

Today microemulsions are used in catalysis, preparation of submicron particles, solar energy conversion, extraction of minerals and protein, detergency and lubrication [58]. Most studies in the field of basic research have dealt with the physical chemistry of the systems themselves and only recently have microemulsions been used as a reaction medium in organic synthesis. The reactions investigated to date include nucleophilic substitution and additions [59], oxidations [59-61], alkylation [62], synthesis of trialkylamines [63], coupling of aryl halides [64], nitration of phenols [65], photoamidation of fluoroolefins [66] and some Diels-Alder reactions. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

One advantage of using a cleavable acetal surfactant instead of a conventional amphiphile has been elegantly demonstrated in a work by Bieniecki and WUk [51]. A cationic 1,3-dioxolane derivative was used as surfactant in a microemulsion formulation that was employed as a reaction medium for an organic synthesis. When the reaction was complete, the surfactant was decomposed by addition of acid and the reaction product easily recovered from the resulting two phase system. Through this procedure the problems of foaming and emulsion formation, frequently encountered with conventional surfactants, could be avoided. 

Exploitation of liquid-liquid microreactor in organic synthesis offers attractive advantages, including the reduction of diffusion path lengths to maximize the rate of mass transfer and reaction rates. Despite the advantages, interest in liquid-liquid micro reactors did not take off until recently, perhaps because of the complication of flow pattern manipulation combined with the limited numbers of liquid-liquid reactions. Initial interest focused on the control of parameters responsible for variation in flow patterns to engineer microemulsions or droplets. However, it was soon realized that liquid-liquid microdevices are more than just a tool for controlling flow patterns and further interest developed. 

Keywords. Microemulsion, Surfactant, Organic synthesis, Catalysis, Nucleophilic substitution... 

The use of a two-phase system with added phase transfer catalyst and the use of a microemulsion are two alternative approaches to overcome reagent incompatibility problems in organic synthesis. Both routes have proved useful but on entirely different accounts. In phase transfer catalysis the nucleophilic reagent is carried into the organic phase where it becomes highly reactive. In the microemulsion approach there is no transfer of reagent from one environment to another the success of the method relies on the very large oil-water interface at which the reaction occurs. 

Jacobsen GB, Lee CT Jr, Johnston KP. Organic synthesis in water/carbon dioxide microemulsions. J Org Chem 1999 64 1201-1206. 

True micellar systems have low capacity for dissolving non-polar reactants, however. They are therefore of limited preparative value. Microemulsions, which contain not only surfactant and water but also an oil component, can dissolve appreciable amounts of both a polar and a non-polar reactant and are therefore much more practically useful as media for organic synthesis. There has been considerable interest in the use of microemulsions as media for organic reactions in recent years [7—11]. Not only can such a formulation be a way to overcome compatibility problems, the capability of microemulsions to compartmentalise and concentrate reactants can also lead to considerable rate enhancement compared to one-phase systems. A third aspect of interest for preparative organic synthesis is that the large oil-water interface of the system can be used as a template to induce regioselectivity. These aspects will be dealt with in this chapter. 

At higher surfactant concentration liquid crystalline phases may be formed. Surfactant liquid crystals can also solubilise appreciable amounts of oil into the non-polar regions made up of the surfactant tails. Thus, both binary surfactant-water systems and ternary systems with oil included can be formulated into liquid crystals. Such systems can also be used as media for organic synthesis. In fact, a reaction in a surfactant liquid crystal often runs very rapidly, considerably faster than in a microemulsion based on the same surfactant [19]. Figure 5.1 shows the reaction profiles of a typical substitution reaction of... 

Thus the microemulsion field continues to be a very active field both scientifically and in applications, as is amply shown by the different contributions in this timely book. Here, several important novel aspects are discussed in depth, like effects of polymers on microemulsions and the use of microemulsions as reaction media for organic synthesis and for the preparation of nanomaterials. That microemulsions constitute just one type of self-assembled surfactant systems continues to be an important consideration. As illustrated... 

We will describe the current state-of-the-art of the microemulsion method for the preparation of metal-based catalysts. First, some general considerations concerning the nature of a microemulsion and its relation to the preparation of particles will be given. Then, both the preparation of solid oxides and metal-supported catalysts by microemulsion will be detailed. When possible, the properties of the solids prepared by microemulsion will be compared with those of their counterparts prepared by traditional techniques. Particular attention will be paid to the description of the catalytic properties of these solids. There is a large body of work in the field of organic synthesis, and enzyme catalysis in which microemulsion techniques play an important role. However this topic is not included in this paper, for that purpose several reviews are available, see for example those by Holmberg and Lawrence Rees... 

A microemulsion of low water content has been found to be an excellent medium for synthesis of long-chain lactones [97]. These compounds, which are important perfume ingredients, are not easily made by conventional organic synthesis because intermolecular esterification dominates over intramolecular esterification. In the microemulsion, the molecular arrangement at the oil/water interface seems to favor the cyclization reaction (Fig. 15). 

Chhatre, A.S., Joshi, R.A., and Kulkarni, B.D., Microemulsions as media for organic synthesis selective nitration of phenol to ortAo-nitrophenol using dilute nitric acid, J. Colloid Interface Set, 158, 183, 1993. 

Polystyrene Nanoparticles. Pol3mier-based nanoparticles are used in several areas of life science, for example drug delivery (56), and one such poljoner matrix commonly used is poly(L-lactic acid) (57). However, nanoparticles with a polystyrene-DVB matrix have been also synthetized (58,59) and used as novel solid supports for organic synthesis (60). The method commonly used to generate these monodisperse particles is Microemulsion Polymerization (qv) (58). However, nanoparticles have also been prepared by precipitation polymerization (61). A microemulsion of styrene and DVB, with an amphiphilic comonomer, in water (three component oil-in-water) (62) allows the preparation of nanobeads with a hard core of polystyrene and the amphiphihc comonomer dispersed on the surface with diameters around 50 and 300 nm that can be precipitated by the addition of methanol. Functionalization of the nanoparticles surface, can be easily achieved using the functionalized amphiphihc comonomer (Fig. 9) (60). 

IL-based microemulsion as a green solvent has been extensively investigated in the fields of material synthesis, polymerization, biocatalysis, organic synthesis, drug release, protein extraction, and capillary electrophoresis. 

Organic Synthesis in Microemulsions An Alternative or a Complement to Phase Transfer Catalysis... 

Microemulsions or reverse micelles are composed of enzyme-containing, surfactant-stabiHzed aqueous microdroplets in a continuous organic phase. Such systems may be considered as a kind of immobilization in enzymatic synthesis reactions. 

Hager M, Currie F, Holmberg K (2003) Organic Reactions in Microemulsions. 227 53-74 Hausler H, Stiitz AE (2001) d-Xylose (d-Glucose) Isomerase and Related Enzymes in Carbohydrate Synthesis. 215 77-114... 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

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