Surfactants optically activ

Novel chiral. separations using enzymes and chiral surfactants as carriers have been realized using facilitated transport membranes. Japanese workers have reported the synthesis of a novel norbornadiene polymeric membrane with optically active pendent groups that show enantio.selectivity, which has shown promi.se in the. separation of propronalol. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

The partial racemization of isolated 2-octanol suggests that the hydrolysis may proceed via ionization of optically active substrates as in the Sjjl hydrolysis in homogeneous solution. The hydrolysis via ionization may be suppressed in media with low dielectric constant like micelles (Okamoto and Kinoshita, 1972), resulting in net retention. The ineffectiveness of the stereochemical influence of the CTAB micelle may be interpreted as a consequence of the mutual repulsion of the positively charged head groups of [46] and CTAB, so there is need for molecules of solvent to be incorporated between surfactant head groups (Sukenik et al., 1975). An appreciable increase in retention was also observed in a reversed micellar system (Kinoshita and Okamoto, 1977). 

The first irrefutable observation of a discriminating interaction between enantiomeric surfactants in monolayers was most probably made by Filippus Johannes Zeelen (84) in a detailed study concerning the synthesis, monolayer behavior, and photochemistry of a series of A -stearoylamino acid derivatives that were employed to model the conformation and photochemical decomposition of proteins. Although Zeelen was able to demonstrate significant differences in the force-area curves obtained from racemic and optically active forms of several of these derivatives, publication of this work in 1956 was... 

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-... 

Composition of the subphase is of paramount importance for monolayer formation. Electrolytes, by virtue of their ability to salt-out organic molecules, often render surfactants insoluble and, thus, permit the formation of a well-behaved monolayer. A case in point is the previously cited example of optically active (jV-(a-methylbenzyl)stearamides [114]. Stable monolayers could not be formed with these molecules on water. Stable monolayers readily formed, however, on strong aqueous acid solutions [114],... 

Finely powdered monomer crystals were dispersed in distilled water containing a few drops of surfactant and irradiated, with vigorous stirring, by a 500-W super-high-pressure mercury lamp set outside of the flask. Optically active 2 was obtained by the irradiation of crystals 1 with X >365 nm, and by successive purification by preparative TLC. 

The chiral selectors most commonly used as additives in the buffer can be divided into three main categories inclusion systems [e.g., cyclodextrins (CDs) or crown ethers], enantioselective metal-ion complexes [e.g. cop-per(II)-L-histidine or copper(II)-aspartame], and optically active surfactants (e.g., chiral mixed micelles or bile acids). Cyclodextrins are the most widely reported, and they are used in low-pH buffers for the resolution of... 

Pc-based nanoparticles have also been prepared employing a solution-casted method from an optically active metal-free Pc substituted with two binaphthyl units. Surprisingly, when the same process was carried out in the presence of a surfactant, the formation of hollow-sphere nanostructures was observed as revealed by TEM and SEM. Low angle XRD and electronic absorption spectroscopy revealed that these nanoscale hollow spheres are formed by stacked Pc molecules in a face-to-face configuration [213],... 

Acrylanilides also undergo this photoinduced process the reaction can occur either in the solid state or in aqueous suspension. In this way, optically active photoproducts can be obtained by irradiating prochiral substrates in the presence of optically active partners (1 1 complexes) (see Scheme 9.27). The substrate is highly oriented and immobilized in the chiral environment. Although photocyclization in the solid state takes a long time (close to 150 h), the photoreaction of a powdered complex in a water suspension containing sodium alkylsulfate as a surfactant proceeds more efficiently (less than 50h) [40],... 

CMCs of these polyfluorinated surfactants are of the order of 10 5 m.47 Plots of the observed H chemical shifts versus surfactant concentration of cetyl trimethyl ammonium chloride, cetyl pyridinium chloride, cetyl dimethyl phenyl ammonium chloride, cetyl dimethyl benzyl ammonium chloride, cetyl dimethy 1-2-phenyl ethyl ammonium chloride, and cetyl dimethyl-3-phenyl propyl ammonium chloride, are sigmoidal and were fitted to a model based on the mass action. The H chemical shift-based CMC values are in excellent agreement with those determined by the surface tension method.48 The micellization processes of dodecyl trimethyl ammonium halides (chloride and bromide) studied by calorimetric titration show different behaviors at 298 K. However, these disappear at 313 K, while the results measured by the chemical shift versus surfactant concentration do not show this difference.49 The CMC of 3-aminopropyl triethoxy silane in toluene is ca. 0.47m, measured by H and l3C chemical shifts.50 The CMC of optically active potassium A -n-dodecanoyl alaminate measured by H and l3C chemical shifts is lower (11-15 him) in D20 than that in a mixed solvent of 1,4-dioxane and D20 (19mM).-51 The H chemical shift shows that the CMC of resorcinol-type calix[4] phosphoric esters having four alkyl side-chains, [4]Ar 5P-R-n, is insensitive to the length of the side-chains, n.52 The CMC values of a family of surfactants, the sodium cyclohexyl alkanoates, with different lengths of the alkanoate side-chains, were obtained from 13C chemical-shift measurements. The results show that these amphiphiles have high CMCs (0.12-1.02 m).-53... 

A suspension of powdered 1 1 inclusion complex of 88a with 2c (3.21 g) in water (120 ml) containing sodium alkylsulfate as a surfactant was irradiated under stirring at room temperature for 12 h. The reaction mixture was filtered to give optically active 89a in the optical and chemical yields indicated in Table 15-21 [49]. From aqueous solution, additional (+)-89a was isolated (Table 15-21). By the same procedure, 88b and 88c gave optically active 89b and 89c, respectively (Table 15-21) [49]. However, 88d was inert to irradiation in its inclusion crystal with 2c. 

On the other hand, when optically active 2-octyl triflate (197) was hydrolyzed in the solubilized state with surfactant concentrations greater than their CMC, the stereochemical course changed to net retention (ca. 48% for CTAB, and ca. 27% for NaLS)193 The rate constants, very much lower than those in the absence of micelles, indicate that the reaction proceeded in the micelles. The different behavior of (196) and (197), particularly in the presence of cationic micelles (CTAB), may be due to the greater efficiency of triflates in producing carbocations. The comparison of (196) and (197) then suggests that the solvation model of micellar stereochemical control applies to unassisted solvolyses whereas the double inversion mechanism operates with leaving groups which invite participation of a nucleophile. 

One of the most successful asymmetric catalytic reactions is the asymmetric hydrogenation of amino acid precursors by means of optically active rho-dium(I)phosphine or phosphinite complexes [55]. Usually, the reaction is carried out in methanol as solvent. When water is used the activity and enantiose-lectivity decrease significantly [16], but the addition of micelle forming surfactants leads to a solubilization of catalyst and substrate and increases activity and enantioselectivity. The results are somewhat better than the ones obtained with methanol as solvent [56]. Table 2 shows the effect with different types of surfactants. 

Rhodium complexes modified with the tenside chiral phosphine 5 were used as catalysts in the hydroformylation of styrene, according to Eq. 2, in an aqueous/organic two-phase system [19, 26], The TOFs achieved with the surfactant catalyst Rh/5 were higher (245 h-1) compared with the Rh/TPPTS system (100 Ir1) [26]. The n/iso ratios of the aldehydes were about 0.6 with Rh/TPPTS and ca. 0.4 with the Rh/5 system. Although the phosphine 5 is chiral, virtually no optical activity was observed in the phenylisopropanal product. HQ... 

Tab. 3 Chiral induction by optically active surfactants (for conditions see Table 2).

It is difficult to determine the location of the reactants in micelles. Both substrate and catalyst have to be located very close. Chiral induction by optically active surfactants, with the catalyst optically inactive, might indicate the location. Table 3 summarizes selected results with different types of chiral surfactants. 

The low chiral induction due to optically active surfactants in micelles could be a result of the low kinetic stability of micelles. It could be shown that micelle-analogous dendrimers as models for stabilized micelles gave significant higher inductions (> 50% ee) in the asymmetric reduction of ketones compared to supramolec-ular aqueous micelles (< 10% ee) [61]. 

Perrin and Idsvoog have found that optical activity can be induced into a symmetrica molecule by an optically active surfactant in micellar form. L-and D-N-decyl-N,N-dimethylaIanine hydrobromides (betaines) were used as surfactants, and sulfoethidole as the opticolly inoctive molecule. The use of optically active surfactant monomers, such as p-D-octyl glucoside, or of solubilized optically active molecules, offers the possibility of employing optical activity as a probe for studying properties of interfaces and of understanding the effect of the interface composition on the optical activity itself. ... 

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