Enantioselective micelle

However, when complexation experiments are performed with both d- and L-enan-tiomers (Fig. 5-18), this leads to selectivity values between 1.4 and 1.9. It was shown that complexation by enantioselective micelles can effectively be described using straightforward multicomponent Langmuir isotherms [74]. 

The two technqiues mentioned above were directed toward removing metallic ions from an aqueous solution by complexation with the constituents of a micelle. The next technique involves resolution of enantiomers using enantioselective micelles, as long as the micelles are retained by an appropriate UF membrane. 

The intrinsic enantioselectivity of the micelles has been established based on single-component binding isotherms [73], resulting in a remarkably high value of 7.7. 

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

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. 

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... 

Deacylation of p-nitrophenyl esters of amino acids Histidine-functionalized micelles and synthetic vesicles. Rate enhancements and enantioselectivity observed Murakami et al., 1981 b... 

Deacylation or hydrolysis of chiral carbamates, carbonates and alkanoates Micelles and comicelles of N-hexadecyl-N-methylephedrinium bromide or N -myristoyl-histidine with CTABr. Rate effects and enantioselectivities examined Fomasier and Tonellato, 1984... 

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. 

A more substantial example of stereoselective catalysis was reported by Brown and Bunton (1974). Hydrolysis of (R)- and (5)-[37] was promoted by the micelle of chiral surfactant [381 in an enantioselective manner. The... 

Further efforts to substantiate enantioselective micellar catalysis is now being continued. Yamada et al. (1979a,b) and Ihara (1978) found an enantiomeric rate ratio of 1.4 2.8 (l/d) for the combination of the surfactant [40] and D- and l- substrates [41]. Koga et al. (1977) also found slight rate differences between hydrolyses of D- and l- [42] catalyzed by micelles of [43]. 

Although similar efforts have been devoted to related polymer systems (Overberger and Cho, 1968 Overberger and Dixon, 1977 Okamoto, 1978), large enantioselectivity has not been observed. Goldberg et al. (1978) conducted borohydride reduction of phenyl ketones in micelles of the chiral surfactant [44]. The result was disappointing, since the maximal enantioselectivity was only 1.66% for phenyl propyl ketone. A much better optical yield was reported when this reaction was carried out under phase-transfer conditions (Masse and Parayre, 1976). The cholic acid micelle and bovine serum albumin exhibited the relatively high enantioselectivity in the reduction of trifluoroacetophenone (Baba ef al., 1978). 

It turned out that for all the polymeric amphiphiles of the (EO) -(PO)m-(EO) type there was an increase in enantioselectivity compared with the reaction without amphiphile. Moreover, the ratio of the length of the (PO) block compared with the (EO) block seemed to determine enantioselectivity and activity and not the cmc (critical micelle concentration). A (PO) block length of 56 units works best with different length of the (EO)n block in this type of hydrogenation [30]. for the work-up of the experiments, G. Oehme et al. used the extraction method, but initial experiments failed and the catalyst could not be recycled that way. To solve this problem the authors applied a membrane reactor in combination with the amphiphile (EO)37-(PO)5g-(EO)37 (Tab. 6.1, entry 9) [31]. By doing so, the poly-mer/Rh-catalyst was retained and could be reused several times without loss of activity and enantioselectivity by more than 99%. 

In methanol as solvent, the results are comparable with respect to activity and enantioselectivity (Tab. 6.2, entry (1)) whereas in water the complex of the amphiphilic ligand shows a significant increase in activity and enantioselectivity compared with the BPPM complex (entry (2)). The values obtained for mixed micelles... 

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]. 

In the one-phase reaction, complete conversion and ee values of about 72% were reached. In the biphasic system, the rhodium complex of the surfactant ligand 12 showed considerably higher activity than in the one-phase system, while retaining enantioselectivity (96). These results agree with results of earlier work that micelle-forming ligands enhance the solubility of lipophilic reactants in water. 

Recently, carbohydrate amphiphiles have been tested in the asymmetric hydrogenation of (Z)-methyl a-acetamidocinnamate in water (98). With a rhodium(I)-BPPM complex, 50% of the reactant was converted in 5 min, and enantioselectivities up to 96% were observed. A comparison of amphiphiles with alkyl chains of different lengths showed that micelle-forming properties, hydrophilic-lipophilic balance, and the structure caused by hydrogen bonding in the head group may be responsible for these effects. 

Hence, in this work, we report the heterogeneization of this new chiral macrocycle onto micelle-templated silicate (MTS) surface by substitution of chlorine atom of previously grafted 3-chloropropyl chain. After A-alkylation of the tetraazamacrocycle with propylene oxide and metalation with Mn(lI)Cl2, the catalytic performance of the corresponding hybrid materials was evaluated in the heterogeneous enantioselective olefin epoxidation. 

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... 

Some degree of success in supported enantioselective catalysis was accomplished by using functionalisation of mineral support. Due to their unique textural and surface properties, mesoporous micelle-templated silicas are able to bring new interesting properties for the preparation of optically active solids. Many successfully examples have been reported for enantioselective hydrogenation, epoxidation and alkylation. However, the stability of the immobilised catalysts still deserves efforts to allow industrial development of such attractive materials. 

In this work, the synthesis of high surface densities of chlororopropyl groups covalently grafted on mesoporous micelle templated aluminosilicates (Al-MTS) of various initial pore diameters is presented. The hybrid chiral materials resulting from halogen substitution are applied in the enantioselective addition of diethylzinc to benzaldehyde. 

The application of MEKC for chiral separation is primarily used when the enantiomers of interest are neutral. In conventional CE without micelles, neutral enantiomers will be swept along with the electro-osmotic flow (EOF) because they carry no ionic charge. If a neutral CD is present and forms a complex with these CDs, they will still move with the EOF. Thus, it is necessary for neutral enantiomers to create the potential for differential migration so that the overall complexes and free enantiomers will not just be swept along with the EOF. For this reason, the use of MEKC which utilizes ionic micelles for differential migration (through hydrophobic interaction) modified with CDs for enantioselectivity was applied [9]. The mechanism is as outhned for MEKC however, because there are hydrophobic micelles inherently present in the electrolyte, additional interactions between the enantiomers and the micelle over those with just a CD may, of course, occur which will normally influence any observed separation. 

Kumar, A. Oehme, G. Roque, J. R. Schwarze, M. Selke, R., Increase in the Enantioselectivity of Asymmetric Hydrogenation in Water Influenced by Surfactants or Polymerized Micelles. ... 

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