Hydrophobic reactants

Biphasic systems are composed of two continuous liquid phases the organic and the aqueous phases [13,25,27,31,36]. An interface generally separates the aqueous phase containing the biocatalyst from the organic phase containing the substrate. Moreover, the reaction product, if poorly soluble in water, can be easily separated from the biocatalyst. The use of carefully measured concentrations in hydrophobic reactants becomes facile in such systems, and substrate excess inhibition of the biocatalyst is reduced. 

The direct conversion of propene to its epoxide, in near quantitative yields, with aqueous H202 will be environmentally more benign. One of the unique features of TS-1 as a solid oxidation catalyst is its ability to utilize aqueous H202 as the oxidant for such conversions. This ability of TS-1 derives from the fact that silicalite-1 is hydrophobic, in contrast to the hydrophilic amorphous Ti-Si02. Consequently, hydrophobic reactants, such as alkenes, are preferentially adsorbed by TS-1, thus precluding the strong inhibition by H20 observed with amorphous Ti-Si02. 

Although enzymes function admirably in water (their natural medium), some reactions cannot easily be performed in water owing to insolubility of hydrophobic reactants or equilibrium limitations, e.g., in esterifications and amidations. 

The enzyme may be dissolved in a mixed aqueous-ionic liquid medium, which may be mono- or biphasic or it could be suspended or dissolved in an ionic liquid, with little or no water present. Alternatively, whole cells could be suspended in an ionic liquid, in the presence or absence of a water phase. Mixed aqueous-organic media are often used in biotransformations to increase the solubility of hydrophobic reactants and products. Similarly, mixed aqueous-ionic liquid media have been used for a variety of biotransformations, but in most cases there is no clear advantage over water-miscible organic solvents such as tert-butanol. 

The major problem associated with aqueous catalysis is the limited and often very low solubility of certain organic reactants in water. Much work is needed to find practical solutions for these hydrophobic reactants. Possibilities deserving further attention include the application of fluorous biphasic catalysis or nonaqueous ionic liquid catalysis. The potential of organic reactions compatible with or even promoted by water is not yet fully exploited. 

This paper describes our efforts to apply phase transfer catalysis (PTC) to the synthesis of HMHEC polymers. The potential use of PTC in the manufacture of HMHEC polymers is important because a large portion of the manufacturing cost of HMHEC polymers is the cost of the hydrophobe reactant. Higher alkylation efficiencies would increase the overall reaction efficiency and thus reduce the overall cost of the final HMHEC product. Increased hydrophobe alkylation efficiencies would also reduce the volume of unreacted hydrophobe in the waste stream and reduce disposal costs. 

The main advantage is an enhanced solubility of many, mostly hydrophobic, reactants insoluble or only sparingly soluble in water. Even for reactants with a moderate solubility in water, that solubility can often limit the attainable reaction rate. 

Although water is the natural medium for enzymes, organic solvents are widely used to improve the solubility of hydrophobic reactants or products, and to shift... 

Chaimovich and coworkers have prepared large unilamellar vesicles of DODACl by a vaporization technique which gives vesicles of ca 0.5 pm diameter. These vesicles are much larger than those prepared by sonication, where the mean diameter is 30 nm, and their effects on chemical reactivity are very interesting. The reaction of p-nitrophenyl octanoate by thiolate ions is accelerated by a factor of almost 10 by DODACl vesicles (Table 2), but this unusually large effect is due almost completely to increased concentration of the very hydrophobic reactants in the small region of the vesicular surface and an increased extent of deprotonation of the thiol. There is uncertainty as to the volume element of reaction in these vesicles, but it seems that second-order rate constants at the vesicular surface are similar to those in cationic micelles or in water (Cuccovia et al., 1982b Chaimovich et al., 1984). 

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... 

Reversed micellar systems have certain attributes which can be exploited when considering enzymatically-based synthesis reactions. These systems can solubilize hydrophilic and hydrophobic reactants and, If the reactants Interact with the surfactant layer, higher concentrations can be obtained than Is possible In either an aqueous or an organic environment. Partitioning of reactants between the bulk organic portion of a reversed micellar solution and the micellar core can result In localized high concentrations of polar reactants. This can be used to promote desired reactions, such as the synthesis of dlpeptldes. 

A remarkable feature of water-promoted reactions is that the reactants only need to be sparingly soluble in water, and most of the time, the effects of water occur under biphasic conditions. If the reactants are not soluble enough, co-solvents can be used as well as surfactants. Another possibility for inducing water-solubility lies in grafting a hydrophobic moiety (a sugar residue or carboxylate, for instance) onto the hydrophobic reactant. 

In both cases a negative volume of activation is expected. Both contributions (hydrophobic effects and polarity) could be active in the same reaction (Figure 1), which means a greater destabilization of the hydrophobic reactants in the initial state than in the transition state, and a greater stabilization of a more polar transition state. 

It is also noteworthy that micelle-forming surfactants may solubilize organic compounds sometimes in a very low concentration of the surfactant (still above the CMC). This embedding depends on the charge of surfactant and the charge of reactant. Only hydrophobic reactants may permeate into the hydrophobic core. Important for good solubilization properties is the hydrophile-lipophile balance (HLB) of the surfactant because sufficient water-solubility is required [12] (cf. Sec-... 

Consider the reaction between a hydrophobic reactant A in phase 1, the organic phase, and a hydrophilic reactant B in phase 2, the aqueous phase ... 

The first experiments of an organocatalysed aldol condensation between acetone and 4-nitrobenzaldehyde with aliphatic amino acids in anhydrous conditions (DMSO acetone 4 l) led to the desired adduct in very low yields (<10%). In 2005, Amedjkouh and Cordova independently showed the importance of additional water in the reaction medium to improve the yields. The concept was based on the hypothesis that a molecule of water would participate in a proton relay in the aldolase system, and would allow for a faster hydrolysis of the intermediates. A hydrophobic amino acid might be efficient in aqueous media by strong association with hydrophobic reactants. Furthermore, water would restore the catalyst from its deactivation by condensation with the aldehyde. 

Due to a low solubility of the hydrophobic reactant in the aqueous-phase and/or of the aqueous-phase reagent in the organic phase, the rates of many heterogeneous reactions are generally low. Since the hydrotrope will enhance the solubility of the organic compound in the aqueous phase, the employment of such compounds is very useful for increasing the rates for heterogeneous reactions (11, 13). 

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