Micelles elimination reactions

Me R = Br, R = H) by various donors in viscous soJventa are decreased by a factor of <2 in a 0.34 T magnetic field and pressure is reported to accelerate the photoreduction of p-benzoquinone in SOS and CTAB micelles in Aerosol OT reversed micelles, the reaction is retarded.The yield of anthraquinone-2-sulphonate radical anion (AQS) generated by irradiation of AQS in aqueous acetonitrile containing propan-2-oJ has been found to depend on the water concentration and to follow Perrin s equation.Complexation of aryl alkyl ketones with /3-cyclodextrin leads to changes in the ratio of the products of elimination and cyclisation which result from reaction of the... 

Polymerized Microemulsion Systems. A microemulsion of styrene and divinylbenzene with CTAB + hexanol may readily be made, and subsequently polymerized to form a polymerized microemulsion (5,6,7). This system exhibits two sites of solubilisation for photosystems such as pyrene, one in the surfactant skin layer, and the other in the polymerized styrene-divinylbenzene core. Photochemical reactions induced in the surfactant skin are very similar to those observed in micelles and are not immediately of concern to us at this stage. However, photochemical reactions induced in the rigid polymerized core are of interest, as they essentially confine reactants to a small region of space where movement is restricted as compared to a fluid non-polymerised microemulsion or a micelle. Thus, diffusion is minimised, and it may be possible to investigate reactions which occur over a distance rather than reactions which occur by diffusion. In order to eliminate reactions in the surfactant skin a microemulsion can be constructed which contains cetyl pyridinium chloride in place of CTAB. The pyrene that resides in the surfactant skin layer is immediately quenched by the pyridinium group following excitation. 

Hawker and co-workers reported the synthesis of unimolecular inverse micelles based on benzyl ether dendrimers with hydroxyl groups in its interior (142). They showed that this dendrimer was able to catalyze the El elimination reaction of 2-iodo-2-methyIheptane. The tridendron of G4 seems to be particularly effective in catalyzing this reaction. The conversions to products were found to be greater than 90%. The regioselectivity of the elimination was moderate. Turnover... 

Other notable micellar catalysed reactions are the attack of oxygen on an enolate anion, elimination reactions that have more carbanion-like transition states compared with non-micellar conditions and the selective hydroxymercuration of non-conjugated dienes. There has been considerable interest in the effect of micelles on photochemical and electron-transfer reactions. Micelles can increase yields in photoinduced electron transfer and lower the ionization threshold in photoionization. ... 

The simplest example of a functional micelle is (49), previously demonstrated to be more effective than its trimethylammonium analogue in both esterolysis and bimolecular elimination reactions. It has now been demonstrated that micelles of (49) are more effective catalysts for the hydrolysis of p-nitrobenzoyl phosphate dianion at high pH than non-functional surfactants. " 2,4-Dinitrochloro- and fluoro-benzene react with micelles of (49) at high pH 10" times faster than with hydroxide ion at a comparable external pH. The initial product is (50) and this in turn is hydrolysed in micelles 2.6 x 10 times faster than is 2,4-dinitrophenyl 2-(trimethylammonium)ethyl ether in water at pH 12. Acyl transfer between p-nitrophenyl acetate and (49) gives an intermediate whose hydrolysis is not micelle catalysed. In contrast to the rate acceleration observed in that case, hydrolysis of p-nitrophenyl acetate is inhibited by micelles of (51) since the phenoxide nucleophile is weak and at the reaction pH its micelles are zwitterionic, not cationic. Synthesis of functional choline-type micelles is facilitated by the use of sulphonate (52), which is reactive towards thiophenoxide in aqueous micelles, but its water-insoluble trifluoromethanesulphonate reacts with a range of anions under phase-transfer conditions. " ... 

The reaction involves the nucleophilic attack of a peracid anion on the unionized peracid giving a tetrahedral diperoxy intermediate that then eliminates oxygen giving the parent acids. The observed rate of the reaction depends on the initial concentration of the peracid as expected in a second-order process. The reaction also depends on the stmcture of the peracid (specifically whether the peracid can micellize) (4). MiceUization increases the effective second-order concentration of the peracid because of the proximity of one peracid to another. This effect can be mitigated by the addition of an appropriate surfactant, which when incorporated into the peracid micelle, effectively dilutes the peracid, reducing the rate of decomposition (4,90). 

It is truly amazing under what conditions, using completely different methods, amino acids can be linked up with elimination of water. Thus, it has been known for years that micelles (see Sect. 10.2) can catalyse various types of reactions in aqueous media, e.g., hydrolysis or aminolysis. Micelles are aggregates which form in aqueous solution from tensides under certain conditions (temperature, concentration). Cetyltrimethylammonium bromide (Cl llr J micelles (this... 

In the reaction between l-oximino-2-oxononane (59) and phosphoro- or phos-phono-fluoridates, the rate-controlling step is the liberation of F , which is only marginally increased in the presence of micelle-forming substances. On the other hand, the rate of liberation of CN is increased many times under the same conditions, possibly not only by straightforward elimination from the phosphorylated oxime but also by interaction of the starting material with acyl cyanide (Scheme 14) (see Organophosphorus Chemistry , Vol. 4, p. 124). 

Many of the early studies of rate effects of aqueous micelles were on reactions in which OH acted as a nucleophile, e.g., in ester saponification, but cationic micelles also speeded bimolecular eliminations in which OH is a base [53-55], and ester hydrolyses by the ElcB mechanism in which the first step of reaction is an equilibrium deprotonation [56]. 

A potentially important appUcation of micelles or similar species is the extent to which they can control product composition. The abiUty of cationic micelles to speed E2 eliminations and inhibit S l reactions has been cited [53-55]. But in addition micelles can control isomer distribution to some extent, as in the halogenation of phenolic ethers [179]. In another example Sukenik and coworkers have used micelles to mediate the isomeric composition of the products of oxymercuration of alkenes [180],... 

The ability of micellized surfactants to catalyze, or inhibit, reactions and to control stereochemistry and product composition, suggests that these agents could have a useful role in organic synthesis. A micelle can speed a desired reaction and inhibit an undesired one, and, for example, cationic micelles can control the ratio of unimolec-ular, Sf l, substitution to bimolecular, E2, elimination [54,55]. Micellization is of great importance in emulsion polymerization, but little use has been made of aqueous micelles in synthesis. 

The grafting is performed by direct reaction of 10 on the solid obtained after elimination of the template. The direct synthesis is a procedure which implies the presenee of 10 as a reagent during the preparation of the SiOa matrix. In this case 10 has a spacer with lipophilic properties, compatible with the core of the micelle. The C-SiOi.5 bond is attached to the matrix during the hydrolytic polycondensation, which permits the formation of Si02. 

Depending on the relative rates of the chemical and diffusion steps, the reaction can proceed in the kinetic, diffusion, or mixed regime, the entire process being controlled by the rate of the chemical step, a diffusion process, or by both kinetics and diffusion. Thus, under very good hydrodynamic conditions, e.g., upon vigorous agitation, the influence of the diffusion can be substantially eliminated and the kinetic results can be used to discuss the reaction mechanism. This conclusion is not always true, and the use of typical surfactant micellar aqueous solutions with extractants dissolved (solubilized) in micellar pseudophase (micelles) and inorganic species dissolved in aqueous pseudophase mimic the extraction systems effectively and the diffusion processes are totally eliminated. 

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