Micellar chiral

Phase Chirality of Micellar Lyotropic Liquid Crystals 477 MODEL OF INTRA-MICELLAR CHIRALITY Disc-like micelle local orientational order... 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

In the short term, we do not expect chiral membranes to find large-scale application. Therefore, membrane-assisted enantioselective processes are more likely to be applied. The two processes described in more detail (liquid-membrane fractionation and micellar-enhanced ultrafiltration) rely on established membrane processes and make use of chiral interactions outside the membrane. The major advantages of these... 

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

The Diels-Alder reaction of nonyl acrylate with cyclopentadiene was used to investigate the effect of homochiral surfactant 114 (Figure 4.5) on the enantioselectivity of the reaction [77]. Performing the reaction at room temperature in aqueous medium at pH 3 and in the presence of lithium chloride, a 2.2 1 mixture of endo/exo adducts was obtained with 75% yield. Only 15% of ee was observed, which compares well with the results quoted for Diels-Alder reactions in cyclodextrins [65d]. Only the endo addition was enantioselective and the R enantiomer was prevalent. This is the first reported aqueous chiral micellar catalysis of a Diels-Alder reaction. 

Clothier, Jr., J. G. and Tomellini, S. A., Chiral separation of veraprimil and related componds using micellar electrokinetic capillary chromatography with mixed micells of bile salt and polyoxyethylene ethers,. Chromatogr. A, 712, 179,1996. 

Chiral surfactants have been used in the aqueous chiral micellar catalysis of a Diels-Alder reaction using an (5)-leucine-derived surfactant (Figure 12.4) to catalyze the reaction between cyclopentadiene and nonyl acrylate.65... 

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

Micellar control of stereochemistry has also been realized in SN1 hydrolyses of chiral sulfonic esters (Okamoto et al., 1975 Sukenik and Bergman, 1976). 

The degree of stereoselectivity is usually not large in these reactions and appears to be due to transition-state rather than initial-state interactions. In other words the diastereomeric transition states derived from the enantiomeric substrates have different free energies in the micelle. To this extent the situation is essentially no different to the stereoselectivity which is often observed in non-micellar reactions involving reactions of enantiomeric substrates with a chiral reagent. In some cases it is possible to identify the noncovalent interactions which are responsible for the stereoselectivity (Brown et al., 1981). 

Several natural10 and synthetic (e.g., polyisocyanates11) polymers form lyotropic cholesterics with the appropriate solvent also micellar systems formed by amphiphilic molecules and water, if chirality is introduced by either using a chiral amphiphile or adding a chiral dopant, can give cholesteric phases.12... 

In this report, we have summarized some attempts to correlate molecular to cholesteric chirality both in thermotropic and lyotropic fields. In the former case, we have described only cholesteric induction, while in the lyotropic field, only cholesterics formed by helical polymers have been dealt with, as chiral micellar systems, for the moment, are too complex. 

An example of the use of PGSE NMR spectroscopy can be found in the studies of Selke et al. [33], who investigated the dependence of enantioselectivity on the distribution of a chiral hydrogenation catalyst between aqueous and micellar phases. When a compound is incorporated into a micelle, its mobility is much lower compared to its mobility in solution. This effect is exactly what is probed with PGSE NMR. The calculated diffusion coefficient is a time-averaged value of the lower diffusion coefficient of the catalyst incorporated into the micelles, and of the diffusion coefficient of the free catalyst. An increased amount of micelle-embedded catalyst was found to lead to an increased enantioselectivity. 

The effectiveness of micellar control on the rate and stereochemical course of hydrolysis at a saturated carbon atom was found to be fairly striking. Chiral 1-methylheptyl trifluoromethanesulfonate [45] undergoes hydrolysis via alkyl-oxygen bond fission, and the hydrolysis rate was only 1/300 (for CTAB) or 1/350 (for SDS) as fast as the rate in pure water (Okamoto et al., 1975). Interestingly, the 2-octanol formed shows net inversion (70%) in a nonmicellar... 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

Lewis-acid catalysis of Diels-Alder reactions involving bidentate dienophiles in water is possible also if the beneficial effect of water on the catalyzed reaction is reduced relative to pure water. There are no additional effects on endo-exo selectivity. As expected, catalysis by Cu ions is much more efficient than specific-acid catalysis.Using a-amino acids as chiral ligands, Lewis-acid enan-tioselectivity is enhanced in water compared to organic solvents. Micelles, in the absence of Lewis acids, are poor catalysts, but combining Lewis-acid catalysis and micellar catalysis leads to a rate accelaration that is enzyme-like. 

Numerous applications of chiral separations using CDs can be found in the literature. Some examples, either reviewing chiral separations in CE or presenting the separation of several compounds, can be found in References 97,101,107,121-124. Several studies related to the chiral separation of amino-acid derivatives by CE and micellar electrokinetic capillary chromatography with different types of CDs have also been reported 102,103,114,125-128... 

When micelles are used, the CE technique becomes a micellar elec-trokinetic chromatography (MEKC) one. Natural surfactants, such as bile salts, digitonin and saponins, optically active synthetic surfactants, e.g., amino-acid derived ones, alkylglycoside-, tartaric acid- and steroidal glucoside-based surfactants, and high-molecular mass or polymerized surfactants, have been used as chiral selectors in In the lat-... 

Newly used chiral surfactants often have a low critical micellar concentration, are highly soluble and can be synthesized both in L- and D-forms. This last feature makes it possible to easily change the migration order of the optical isomers, which is very interesting for the determination of the optical pnrity of drugs, where for quantification purposes it is favorable that the chiral impurity migrates before the main component. 

Micellar electric capillary chromatography (MECC) Chiral CE (CCE)... 

CE has been applied extensively for the separation of chiral compounds in chemical and pharmaceutical analysis.First chiral separations were reported by Gozel et al. who separated the enantiomers of some dansylated amino acids by using diastereomeric complex formation with Cu " -aspartame. Later, Tran et al. demonstrated that such a separation was also possible by derivatization of amino acids with L-Marfey s reagent. Nishi et al. were able to separate some chiral pharmaceutical compounds by using bile salts as chiral selectors and as micellar surfactants. However, it was not until Fanali first showed the utilization of cyclodextrins as chiral selectors that a boom in the number of applications was noted. Cyclodextrins are added to the buffer electrolyte and a chiral recognition may... 

Shamsi, S. A. (2001). Micellar electrokinetic chromatography-mass spectrometry using a polymerized chiral surfactant. Anal. Chem. 73, 5103-5108. 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

The PGSE methodology has also been applied to study the dependence of enan-tioselectivity on the distribution of the chiral Rh-hydrogenation catalyst 137 between an aqueous and micellar phase. The observed increase in enantioselectivity when amphiphiles are added to the water is associated with an aggregation of the catalyst to the micelles [309]. 

---