Surfactants chiral induction

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]. 

Another attempt to apply the oxo reaction to the synthesis of fine chemicals was made by the hydroformylation of styrene. The chiral catalyst, a rhodium complex of a surfactant phosphane 12, gave no optical induction (76). This result agrees with the known poor enantioselection ability of other complexes with monodentate chiral phosphanes. 

Asymmetric induction during the reduction of 4-(48) was observed when a surface-modified carbon cathode was used.70 Optical yields were low but the effect of the chiral amino acid bound to the carbon surface was proved to be a true surface phenomenon. Induction of chirality by homogeneous rather than surface-bound agents has also been studied.71 All the isomeric acetylpyridines (48) were reduced in the presence of three different chiral alkaloids. Both carbinol products 2- and 4-(49) were shown to possess induced chirality, but the 3-carbinol (49) had none under any of the conditions tried. More rapid protonation of the intermediate was proposed to account for the lack of induced chirality. Optimization of optical yields was done.72 The pinacols (50) formed along with 49 were found to have no induced chirality. Optical yields have been as high as 50%.73 The role of electroabsorption was found to be important in the reduction of 2-(48).74 Product distributions were noted as a function of surfactant present in the electrolyte, carbinol 49 being favored... 

Arguably one of the most valuable exploitations of an amination reaction is in the formation of amino acids. Wu and Zhang have employed chiral reverse micelles as asymmetric microenvironments for enantioselective synthesis of 2-phthalimides-esters 72.33 Hydrazinolysis and hydrolysis of the esters then lead to a-amino acids. It was found that asymmetric induction varied according to the reaction temperature, the alkyl chain of the surfactants, and the structure of the surfactants. 

Li), the lamellar Ld), and the nematic one (No), which in this case consists of disk-like micelles. One outstanding feature of the system is the extremely wide range of stability of the lamellar as well as of the nematic phase. Moreover, the capability to perform phase transitions lamellar/nematic/ isotropic by temperature variation makes the handling of the samples easy. The short, perfluorinated chains of the surfactant are responsible for the unusual properties of this lyotropic liquid crystal which in some aspects behaves very similar to a thermotropic phase. Both of the briefly introduced achiral lyotropics have been used extensively as host phases for the induction of phase chirality by means of chiral dopants. 

Asymmetric epoxidation of terminal alkenes with hydrogen peroxide was optimized with electron-poor chiral Pt(II) complexes bearing a pentafluorophenyl residue, as described in Section 23.3.1.6. The same catal3rtic system was made more sustainable by the employment of water as the solvent under micellar conditions. Surfactant optimization revealed the preferential use of neutral species like Triton-XIOO to solubihze both the catalyst and substrates. In several cases an increase of the asymmetric induction was observed (Scheme 23.43). The use of an aqueous phase and the strong affinity of the catalyst for the micelle allowed the recycling of the catalytic system by means of phase separation and extraction of the reaction products using an apolar solvent (hexane). The aqueous phase containing the catalyst was reused for up to three cycles with no loss of activity or selectivity. 

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