Liquid water medium catalysis

Tetrabutylammonium salts frequently form third-liquid phases (or catalyst layers) when used in conjunction with organic solvents with low polarity such as toluene, hexane, and 1-chloro-octane, and with a concentrated aqueous solution of inorganic salts. An excellent example of tri-liquid-phase catalysis was demonstrated by Wang and Weng [97] in the displacement of benzyl chloride and sodium bromide in toluene/water medium catalyzed by Bu4N+Br, in which a third-liquid phase appeared under certain critical conditions with a concomitant sharp increase in the reaction rate. Mason et al. [98] reported that Bu4N+Br uniquely formed a third-liquid phase in a toluene/Bu4N Br /... 

Biocatalysis localization in the biphasic medium depends on physicochemical properties of the reactants. When all the chemical species involved in the reaction are hydro-phobic, catalysis occurs at the liquid-liquid interface. However, when the substrate is hydrophobic (initially dissolved in the apolar phase) and the product is hydrophilic (remains in the aqueous phase), the reaction occurs in the aqueous phase [25]. The majority of biphasic systems use sparingly water-soluble substrates and yield hydrophobic products therefore, the aqueous phase serves as a biocatalyst container [34,35] [Fig. 2(a)]. Nevertheless, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38] (Fig. 2(b), (c)). 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

Carbon dioxide is the most employed substance by far in supercritical fluid processes (SCCO2). Like water, carbon dioxide is an environmentally attractive solvent. The ability to control its solvating power by simple swings in temperature and pressure makes it an ideal medium for homogeneous catalysis. The main problem compared with conventional systems is that such swings, especially in pressure, require costly recompression and care must be taken to control the temperature whilst the pressure swing is occurring so that mixed liquid and gas phases do not form. 

The invention of new methods for catalyst recovery appear likely to further increase the attractiveness of SCCO2 as a reaction medium, potentially in partnership with a second phase such as water, ionic liquid, or PEG. Given the high price of chiral homogeneous catalysts and the particularly clean separations that can be obtained using the biphasic catalysis techniques described in Section 3.3, one can expect industrial interest in this aspect in particular. 

Many enzyme reactions in the pharmaceuticals sector are carried out in an aqueous medium, i.e., the enzyme is dissolved in water, and the reactant is usually a second (water-insoluble) phase. Fundamentally, it would be interesting to learn what would happen if the phases were reversed, i.e., if the reactant were dissolved in some solvent and the enzyme the second phase. Klibanov and coworkers had previously reported on enzymatic catalysis reactions in organic liquids with only scant moisture present to render the enzyme active (Zaks and Klibanov, 1984). 

Lipases are able to work in very different media. They work in biphasic systems and in monophasic (in the presence of hydrophilic or hydrophobic solvents) systems where the water content can vary significantly between aqueous and anhydrous media. They have been tested also in ionic liquid media (Lau et al. 2000 Wasserscheid and Keim 2000 Kamal and Chouhan 2004 Ha et al. 2007), in supercritical fluids (Laudani et al. 2007) and in gaseous media (Cameron et al. 2002). The different media for enzymatic catalysis has been outlined before (see section 1.6) and it will not be further discussed here. However, some examples of modulation of activity and selectivity of lipases by medium engineering will be described in this section. 

This is a biomimetic system and a model system for reaction kinetics, but also an actual medium for phase transfer catalysis. In this system, one of the solvents is usually water, and the other one is a low polar organic liquid (od), such as, for example, nitrobenzene or dichloroethane. 

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