Amphiphilic reactant

The solubilization of enzymes and proteins in water-containing reversed micelles has attracted a great deal of interest for their selective separation, purification, and efficient refolding and for bioreactions involving a wide class of polar, apolar, and amphiphilic reactants and products [13,44,162-164]. 

Iminium-Activated Epoxidations Similar to ylide 153 and bromomalo-nates 157, it was proposed that hydrogen peroxide could also be used as an amphiphilic reactant for [2-1-1] reactions of a,P-unsaturated aldehydes to furnish epoxidation products. Inspired by this hypothesis, Jprgensen s group developed an organocatalytic asymmetric epoxidation system of a,P-unsaturated aldehydes with HjOj as the oxidant (Scheme 1.63) [ 106]. The reactions take place under mild conditions in good to high yields and enantio- and diastereoselectivities. 

As illustrated in Scheme 1.40, in addition to the amphiphilic reactants bearing nucleophilic and electrophilic sites, compatible separated nucleophiles and electrophiles can also be employed in iminium-activated cascades in multicomponent reactions. 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

A microemulsion (p.E) is a thermodynamically stable, transparent (in the visible) droplet type dispersion of water (W) and oil (O a saturated or unsaturated hydrocarbon) stabilized by a surfactant (S) and a cosurfactant (CoS a short amphiphile compound such as an alcohol or an amine) [67]. Sometimes the oil is a water-insoluble organic compound which is also a reactant and the water may contain mineral acids or salts. Because of the small dispersion size, a large amount of surfactant is required to stabilize microemulsions. The droplets are very small (about 100-1000 A [68]), about 100 times smaller than those of a typical emulsion. The existence of giant microemulsions (dispersion size about 6000 A) has been demonstrated [58]. 

The importance of the material exchange process can hardly be overemphasized since it is the mechanism whereby the equUibrium miceUar size and polydispersity are reached and maintained, the reversed micelles of ionic surfactants become charged, polar and amphiphilic solubilizates are transported, and hydrophilic reactants can come in... 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

Fig. 13. Reaction mechanism of 33-residue amphiphilic helix catalyzed chemoselective ligation of hehcal peptides. The fimction of the catalyst is to organize the amphiphihc peptide reactants by hydrophobic forces on the smdace of the folded helix...

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. 

The catalytic effect for reactions involving an ionic reactant usually shows a strong dependence on the total amphiphile concentration. The maximal effective rate constant is attained at concentrations just over the CMC. Romsted284 showed that this occurs due to the competition between the ion binding of the reactive ions (OH- in the example above) and the counterions of the amphiphile. Recently, Diekman and Frahm285 286 showed that it is possible to rationalize the kinetic data by describing the ion distribution through a solution of the Poisson-Boltzman equation. (See Fig. 5.1). 

The dilemma posed by these considerations was resolved for the DHP-organized system when it was noted that upon illumination the initially compartmented MV + partially leaked out of the vesicles [108]. Thus, the observed reaction actually occurred between reactants adsorbed on the same vesicle surface. The mechanism of this unprecedented light-induced scrambling of MV + is still not understood. Likewise, subsequent investigations of the PC-organized system provided evidence for transmembrane leakage of a small amount of the compartmented (C7)2V + ion, which then facilitated transmembrane electron transport [109]. Specifically, the reaction characteristics were duplicated when the amphiphilic Ru(bpy)3 + analog was bound only to the opposite side of the membrane as the oxidative quencher... 

Methylated cyclodextrins promote the hydroformylation of higher olefins, too. Molecular dynamics simulations show that the reaction takes place right at the interface and that cyclodextrins act as both surfactants and receptors that favour the meeting of the catalyst and the olefin. The methylated cyclodextrin adopts specific amphiphilic orientations at the interface, with the wide rim pointing towards the water phase. This orientation makes easier the formation of inclusion complexes with the reactant (1-decene), the key reaction intermediate [Rh(H)CO(TPPTS)2-decene)] and the reaction product (undecanal). ... 

In water the hydrogenation rate and the enantioselectivities were considerably lower. However, the addition of amphiphiles as proposed by G. Oehme mediates the dispersion of the relatively hydrophobic reactants and effects an enormous increase in hydrogenation activity and selectivity [17, 24, 25]. Fig. 10 depicts the results of the model hydrogenation reaction of methyl 2-acetamidoacrylate lc (see... 

Some other C—C bond coupling reactions in micellar systems should be mentioned here. Monflier et al. [72] described, in both papers and patents, the telome-rization of 1,3-butadiene into octadienol in a micellar system by means of a palladium-phosphine catalyst. Water-soluble and amphiphilic phosphines have been used and the surfactants were widely varied. The authors have shown that the promoting effect of surfactants appeared above the CMCs of the surfactants, and they conclude that micellar aggregates were present in the reaction mixture. Cationic, anionic, and nonionic surfactants gave this micellar effect but the combination of the highly water-soluble TPPTS and the surfactant dodecyldimethylamine hydrocarbonate was found to be best. A speculation about the location of reactants shows that the reaction probably occurs in the interface between the micellar pseudophase and water. 

Instead of simple methylviologen (MV ), Brugger and Gratzel use the amphiphilic derivative Ci4MV as an electron acceptor. The comparatively stable charge separation is brought about by organization of the reactants at molecular level and interplay of hydrophobic and electrostatic forces. 

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