Reverse phase transfer, aqueous organic solvents

Photochemical butterfly-like E — Z photoisomerization of a bis(crown ether) azobenzene derivative 354 was found to be thermally reversible and the stereoisomers exhibit unique contrasting behaviour in the presence of metal ions.1108 The concentration of the Z-isomer in the photostationary state was noticeably enhanced by the addition of K+, Rb + or Cs +, because the corresponding Z-complex achieved a stable sandwich geometry (Scheme 6.162). As a result, the cations could be selectively extracted by the Z-derivative from an aqueous phase to an organic solvent (o-dichlorobenzene), whereas no complexation (i.e. no transfer) took place in the case of the E-isomer. 

Alkylation reactions of ketones, active methylene compoimds, alcohols, and phenols with alkyl halides occurred in quite good yields in aqueous NaOH solution without added organic solvent in the presence of water-soluble calix[ ]arene (n = 4,6, or 8) containing trimethy-lammoniomethyl groups acting as a reverse phase-transfer catalyst (Scheme 8.20). ° ... 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

Another important argument for the use of the organic solvent is the reverse hydrolytic reactions that become feasible [61,75]. The inhibition of the biocatalyst can be reduced, since the substrate is initially concentrated in the organic phase and inhibitory products can be removed from the aqueous phase. This transfer can shift the apparent reaction equilibrium [28,62] and facilitates the product recovery from the organic phase [20,29,33]. A wide range of organic solvents can be used in bioreactors, such as alkanes, alkenes, esters, alcohols, ethers, perfluorocarbons, etc. (Table 1). 

The bonded hydrocarbon packings...are very hydrophobic...Therefore, in reversed phase separations...it is desirable to use aqueous mobile phases containing > 10% of a miscible organic solvent...to improve wetting characteristics. Mobile phases with no or low concentrations of organic solvent produce broad peaks because of the slow equilibrium resulting from the resistance to solute mass transfer across the interface of the two very unlike phases."... 

These show that alkyl chains play an important role in the protein extraction ability. Bulky two-tailed surfactants are favourable in the protein transfer from an aqueous phase into an organic phase, while the other surfactants, having saturated-straight alkyl chains, dioctyl phosphoric acid (DOPA) and dioctyl sulfosuccinate (AOC), or a single alkyl chain, monooleyl phosphoric acid (MOLPA), are limited by their solubility into an organic solvent or their ability to form reverse micelles. A tridecyl group in di-tridecyl phosphoric acid (DTDPA) and di-tridecyl sulfosuccinate (ATR) comes from the tridecyl alcohol which includes several isomers, and therefore these surfactants also have bulky tails. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

Kihara et al. [188] have introduced a series of redox reactions suitable for studying the electron transfer (8). These systems involved ferrocene or tetrathiafulvalene as electron donor or tetracyanoquinodimethane as electron acceptor in the organic solvent phase, and Fe(CN), Ce, Fe ", or Cr207 as electron acceptor, or Fe(CN)6 or hydroquinone as electron donor, in the aqueous phase. Since all these systems showed reversible behavior under the conditions of current-scan polarogra-phy, no kinetic- data have been reported. 

From the list in Table 3.9, cellulose and amylose-based phases are by far the most often used in preparative and, especially, SMB applications. These adsorbents offer good productivities because of their high loadabilities (Fig. 3.22). In addition, the four most commonly used CSP of this type separate a broad range of different race-mates. The major problem of these adsorbents is their limited solvent stability, especially towards medium-polar solvents such as acetone, ethyl acetate or dioxane. In the past their use in conjunction with aqueous mobile phases was not recommended by the manufacturer as well. However, this limitation was successfully overcome by recent studies, in which amylose- and cellulose-based CSPs are transferred to the reversed phase (RP) mode with aqueous mobile phases. The first results for the use of polysaccharide-type phases with aqueous solvents were reported by Ishikawa and Shibata (1993) and McCarthy (1994). The stability of the adsorbent after switching to RP conditions has been reported by Kummer et al. (1996) to be at least 11 months and by Ning (1998) to be 3 years. No peak deviation is observed after switching to RP mode. Novel developments have led to polysaccharide-based adsorbents dedicated to use with nearly all organic solvents (Cox and Amoss, 2004). 

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