Biphasic systems, solvent applications

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. 

Another environmental issue is the use of organic solvents. The use of chlorinated hydrocarbons, for example, has been severely curtailed. In fact, so many of the solvents favored by organic chemists are now on the black list that the whole question of solvents requires rethinking. The best solvent is no solvent, and if a solvent (diluent) is needed, then water has a lot to recommend it. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water at ambient temperature and pressure can provide substantial environmental and economic benefits. Similarly, there is a marked trend toward the application of organometal-lic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide and ionic liquids. ... 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

Since then, several publications dealing with liquid/liquid biphasic systems or the application of an immobilized enzyme in pure organic solvent have appeared. 

As well as biocatalysis in neat organic solvents and biphasic systems (fundamentals and synthetic applications), the present volume covers new and promising aspects of non-aqueous enzymology that have emerged in recent years, including biocatalysis with undissolved solid substrates or vaporized compounds, the use of ionic liquids as solvents, and the preparative-scale exploitation of oxynitrilases and dynamic kinetic resolutions . For the sake of completeness and comparison,... 

They are good solvents for a broad spectrum of inorganic, organic, and polymeric materials and are immiscible with numerous organic solvents. Thus, applications in process intensification and as non-aqueous polar alternatives in biphasic systems are possible. 

Examples of applying biphasic systems to catalyzed reactions, such as phase-transfer catalysis, overpower the stoichiometric reactions. In a typical catalytic biphasic system, one phase contains the catalyst, while the other phase contains the substrate. In some systems, the catalyst and substrates are in the same phase, while the product produced is transferred to the second phase. In a typical reaction, when the two phases are mixed during the reaction and after completion, the catalyst remains in one phase ready for recycling while the product can be isolated from the second phase. The most common solvent combination consists of an organic solvent combined with another immiscible solvent that, in most applications, is water. However, there are few examples of suitable water-soluble and stable catalysts, and therefore various applications are limited to some extent [192]. Immiscible solvents other than water are recently becoming more applicable in biphasic catalysis because of the better solubility and stability of various catalysts in such solvents. For example, ionic liquids and fluorous solvents have many successful applications in liquid-liquid... 

The limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

In Chapter 7 we have already discussed the use of fluorous biphasic systems to facilitate recovery of catalysts that have been derivatized with fluorous ponytails . The relatively high costs of perfluoroalkane solvents coupled with their persistent properties pose serious limitations for their industrial application. Consequently, second generation methods have been directed towards the elimination of the need for perfluoro solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents [42]. Thus, appropriately designed fluorous catalysts are soluble at elevated temperatures and essentially insoluble at lower temperatures, allowing for catalyst recovery by simple filtration. 

Not surprisingly, the most well developed biphasic system is that using water and organic solvents, despite the first industrial biphasic process involving only organic solvents. Obviously, water is the solvent of choice as it is abundant, cheap, non-flammable, non-toxic and has many other desirable properties such as being polar (and therefore relatively easy to separate from apolar compounds), high thermal conductivity, heat capacity and heat of evaporation. Nevertheless, alternative solvents to water for applications in biphasic catalysis are needed for several reasons ... 

This new experimental technique, using fluorous solvents or fluorous biphasic systems (FBS) with fluorous biphase catalysis (FBC), was developed by Vogt and Kaim [884] and by Horvath and Rabai [885] in 1991 and 1994, respectively. Since then, this method has found many applications in synthetic organic chemistry and has already been reviewed repeatedly [886-893]. Incidentally, temperature-dependent two-phase one-phase transitions are not limited to combinations of fluorous solvents with organic solvents. For example, certain mixtures of water and l-cyclohexylpyrrolidin-2-one form one phase at ambient temperature and a two-phase system at higher temperatures >ca. 50 °C), also allowing interesting separation possibilities. 

It should be emphasized that the conditions found in aqueous polymeric biphasic systems seem to simulate those in biological systems to a good approximation 33>. The properties of both phases of such systems are only slightly different as compared to those of the phases of water-organic solvent systems. Hence, the partition coefficient of a solute in an aqueous polymeric biphasic system is much more responsive not only to the modifications of the molecule structure but to the alterations in the conformation of the molecule as well. The application of the partition... 

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