Micellar system

Quite recently, it has been found that scandium triflate (Sc(OTf)rcatalyzed aldol reactions of silyl enol ethers with aldehydes can be successfully carried out in micellar systems [24], While the reactions proceeded sluggishly in pure water (without organic solvents), remarkable enhancement of the reactivity was observed in the presence of a small amount of a surfactant (cf. Section 4.5). 

Lewis acid catalysis in micellar systems was first found in the model reaction of the silyl enol ether of propiophenone with benzaldehyde. Although the reaction proceeded sluggishly in the presence of 0.2 equiv. Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 0.2 equiv. Yb(OTf)3 in an aqueous solution of sodium dodecylsul-fate (SDS, 0.2 equiv., 35 mM), and the corresponding aldol adduct was obtained in a 50% yield. In the absence of the Lewis acid and in surfactant (water-promoted conditions) [11], only 20% yield of the aldol adduct was isolated after 48 h, while a 33% yield of the aldol adduct was obtained after 48 h in the absence of the Lewis acid in an aqueous solution of SDS. The amounts of the surfactant also influenced the reactivity, and the yield was improved when Sc(OTf)3 was used as a Lewis acid catalyst. Judging from the critical micelle concentration, micelles would be formed in these reactions, and it is noteworthy that the Lewis acid-catalyzed reactions proceeded smoothly in micellar systems [25]. 

Additional silyl enolate (1.5 equiv.) was charged after 6 h. 

Several examples of Sc(OTf)3-catalyzed aldol reactions in micellar systems are shown in Table 1 and Eq. (5). Not only aromatic, but also aliphatic and ,/l-unsat-urated aldehydes, react with silyl enol ethers to afford the corresponding aldol adducts in high yields. Formaldehyde/water solution also works well. It is exciting that ketene silyl acetal 2, which is known to hydrolyze very easily even in the presence of a small amount of water, reacts with an aldehyde in the present micellar system to afford the corresponding aldol adduct in high yields. 

It was also found that the allylation reactions of aldehydes (cf. Section 6.8) with tet-ra-allyltin proceeded smoothly in micellar systems using Sc(OTf)3 as a catalyst 

Examples of such surfactants are cetyltrimethylammonium bromide (CTAB, 9.2 m), polyoxyethylene(22)hexadecanol (Brij 58, 7.7 10 m), and sodium 

Because of the relatively high regularity in the micellar core some asymmetric reactions occur with an enhanced stereoselectivity. Other typical effects of organized surfactants are the alteration of chemical and photophysical pathways, of quantum efficiency and ionization potentials, of oxidation and reduction properties, and finally of products and charges [6]. The morphology of micelles can change to rod-like structures at increasing concentrations of surfactants. The suitability of micellar 

Depending on the structure of the amphiphiles and the composition of the solvent mixture, other types of aggregates are possible, such as vesicles, reverse micelles, and microemulsions [8]. Because of the presence and proximity of polar and apolar regions in the interior of surfactant assemblies, there are some similarities to en2ymes and natural membranes. Supramolecular aggregates of surfactants have a membrane-mimetic chemistry [9], 

Important topics include solvolyhc reachons, oxidahons, reductions, and C-C coupling reachons. The saponificahon of achvated esters in aqueous micelles is a model for an enzyme-mimehc reachon. The influence of the micellar medium on the reachon rate and on the stereoselechvity has been investigated. Models of metalloen2ymes were developed with the ligands 1-3 and metal ions like Ni , Cu , 

and Co [10]. The amphiphilic ligands which were embedded in micelles enhanced the reachon rate in comparison with a dispersion in surfactant-free water. 

The main question here is whether or not polymerisation preserves, alters or destroys initial micelle structure. The answer is still a subject of debate. 

Polymerisation kinetics and the structure of the resulting polymer can be influenced by three effects closely related to micelle formation. 

Concentration effect. This refers to the intramicellar concentration of polymerisable groups. It has a value of about 5 M, independently of the total surfactant concentration in solution. The polymerisation rate will therefore be much higher in the micellar medium than in the homogeneous medium. 

Medium effect. This arises because polymerisable groups are in a different environment when surfactants are in the micelle state and when they are dispersed in solution. Their environment will affect both polymerisation rate, and structure and properties of the final polymer. 

Topochemical effect. This is a consequence of the organisation of surfactant molecules induced by the micellar state. Although micelles are relatively labile aggregates and less organised than vesicles, topochemical effects may nevertheless influence the polymerisation process. 

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Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

Levstein P R and van Willigen H 1991 Photoinduced electron transfer from porphyrins to quinones in micellar systems an FT-EPR study Chem. Phys. Lett. 187 415-22... 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

Richetti P and Ke kicheff P 1992 Direct measurement of depletion and structural forces in a micellar system Phys. Rev. Lett. 68 1951-4... 

Barzykin A V, Barzykina N S and Fox M A 1992 Electronic excitation transport and trapping in micellar systems— Monte-Carlo simulations and density expansion approximation Chem. Rhys. 163 1-12... 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Colloidal suspensions are systems of small mesoscopic solid particles suspended in an atomic liquid [1,2]. We will use the term colloid a little loosely, in the sense of colloidal particle. The particles may be irregularly or regularly shaped (Fig. 1). Among the regular shapes are tiny spherical balls, but also cylindrical rods or flat platelets. As the particles are solid, fluctuations of their form do not occur as they do in micellar systems. Not all particles in a suspension will, in general, have the same form. This is an intrinsic effect of the mesoscopic physics. Of course in an atomic system, say silicon, all atoms are precisely similar. One is often interested in the con-... 

Fig. 2. CD-spectra of (L-Lys HBr) in reversed micellar systems of AOT/octane/H20 at different w0-values 65). T 20 °C...

As mentioned above, water structure in reversed micelles deviates considerably from the structure in the bulk-phase. Therefore, the hydration shell of macromolecules entrapped in reversed micellar systems should be changed and thus also their conformation. According to the results of several authors this is indeed the case. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

In this review, we will describe the model studies of hydrolytic metalloenzymes of the second category, emphasizing those of micellar systems. Here it should be mentioned that such micellar models have been studied only little in the past so that the major,part of this review bases on our own work. 

Enantioselective deacylation of esters in micellar reactions has been extensively studied in order to understand enzyme stereospedficity, and some micellar systems... 

The above enantioselectivities are obviously complex functions of many factors, perhaps even more complex than in natural enzymes. Complexity is partly due to the present co-micellar system in which it is difficult to analyze separately the interaction of the substrate with the achiral micelle, and that of the substrate with the catalyst complex. 

The study on micellar models is still at the beginning. An amphiphilic ligand which can form micelles by itself has not yet been prepared. It is necessary to obtain complexes of higher stability in order to activate the hydroxyl group strong enough in the reactions of inactive esters or amides. Enantioselectivity must reach higher specifity. Nevertheless it seems to be clear that many features or some important clues have already been disclosed for further refinements of this micellar systems. More details about the present micellar reactions will be reported elsewhere in near future. 

Retardation of back ET was also observed with phenanthrene solubilized in the SDS micelle (kb = 6.8 x 107 M-1 s-1) (see Fig. 13) [75]. However, as can be seen from Fig. 13, the transient yield of SPV- for the micellar system is extremely low, presumably because only a small fraction of SPV- can escape from the geminate ion pair. This finding implies that SPV preferably resides inside the micelle and that the electron transfer mainly takes place in the micelle, not across the charged surface. 

Very large solvent effects arc also observed for systems where the monomers can aggregate either with themselves or another species. For example, the apparent kp for polymerizable surfactants, such as certain vinyl pyridinium salts and alkyl salts of dimethylaminoalkyl methacrylates, in aqueous solution above the critical micelle concentration (cmc) are dramatically higher than they are below the cmc in water or in non-aqueous media.77 This docs not mean that the value for the kp is higher. The heterogeneity of the medium needs to be considered. In the micellar system, the effective concentration of double bonds in the vicinity of the... 

Criado, S. Escalada, J. P. Pajares, A. Garcia, N. A. (2008). Singlet molecular oxygen [02(lAg)]-mediated photodegradation of tyrosine derivatives in the presence of cationic and neutral micellar systems. Amino Acids, Vol. 35, No. 1, (June 2008), 201-208, ISSN 0939-4451... 

Investigations of the solubilization of water and aqueous NaCl solutions in mixed reverse micellar systems formed with AOT and nonionic surfactants in hydrocarbons emphasized the presence of a maximum solubilization capacity of water, occurring at a certain concentration of NaCl, which is significantly influenced by the solvent used [132],... 

In addition to solubilization, entrapment of polymers inside reversed micelles can be achieved by performing in situ suitable polymerization reactions. This methodology has some specific peculiarities, such as easy control of the polymerization degree and synthesis of a distinct variety of polymeric structures. The size and shape of polymers could be modulated by the appropriate selection of the reversed micellar system and of synthesis conditions [31,191]. This kind of control of polymerization could model and/or mimic some aspects of that occurring in biological systems. 

Mortensen, K Brown, W Norden, B, Inverse Melting Transition and Evidence of Three-Dimensional Cubatic Structure in a Block-Copolymer Micellar System, Physical Review Letters 68, 2340, 1992. 

Size Exclusion Chromatography of Some Reversed Micellar Systems... 

It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C o clusters have been extensively reported because of the interesting properties of C o clusters [25,26]. The M F Es on the decay of the radical pair between a Cgo cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the Cgo cluster in mixed solvents have not yet been studied. 

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