Alternative biphasic systems

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

A possible alternative for the use of organic solvents (many of which are on the black hst), is the extensive utilization of water as a solvent. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enz5matic procedures in water, at ambient temperature, can provide both environmental and economic benefits. Similarly, there is a marked trend toward organometalhc catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide, and ionic liquids. 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

The surface-active diphosphane 12 was applied in the hydrogenation of methyl a-acetamidocinnamate [Eq. (10)] with [RhCl(COD)]2 as the catalyst precursor in homogeneous methanolic solution and, alternatively, in ethyl acetate-water biphasic systems (96). 

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. 

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. 

Lipophilicity — The affinity of a molecule or a part of a molecule for a lipophilic environment. For molecules and ions it is commonly measured by its distribution behavior in a biphasic system, either liquid-liquid (by determining the logarithm of the partition coefficient log(P) in octan-l-ol/water [i, ii]) or solid/liquid (retention on reversed-phase high-performance liquid chromatography (RP-HPLC), or thin-layer chromatography (TLC) system). (Alternative term is Hydropho-bicity.)... 

An alternative method for preparing 1-alkylisatins consists in the reaction of isatin and alkyl halides in a benzene-chloroform/50% aq. KOH biphasic system, employing tetrabutylammonium hydrogensulfate as the phase transfer catalyst. ... 

The aqueous biphasic hydroformylation concept is ineffective with higher olefins owing to mass transfer limitations posed by their low solubility in water. Several strategies have been employed to circumvent this problem [22], e.g. by conducting the reaction in a monophasic system using a tetraalkylammonium salt of tppts as the ligand, followed by separation of the catalyst by extraction into water. Alternatively, one can employ a different biphasic system such as a fluorous biphasic system or an ionic liquid/scC02 mixture (see later). 

Chapter 7 addresses another key topic in the context of green chemistry the replacement of traditional, environmentally unattractive organic solvents by greener alternative reaction media such as water, supercritical carbon dioxide, ionic liquids and perfluorous solvents. The use of liquid/liquid biphasic systems provides the additional benefit of facile catalyst recovery and recycling. 

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 ... 

Recently, Horvath, and Rabai (89) have reported the use of fluorous biphasic systems as an alternative to aqueous biphasic systems for phase transfer catal5 tic processes in... 

Enzyme-catalyzed reactions based on such biphasic systems have been shown to be promising alternatives for developing green chemical processes because of their physical and chemical characteristics [9]. By combining these media with enzymes, the possibilities of carrying out integral green biocatalytic processes has been already demonstrated [10-12]. Such biphasic systems can be used for both the biotransformation and extraction of products simultaneously, even under extremely harsh conditions, because of the different miscibilities of ILs and SC-CO2. 

PEGs have been extensively used to date in aqueous biphasic systems (ABS). Therefore, it is important to understand their phase behaviour, although this involves many variables (polymer molecular weight, salts, neutral organic molecules, temperature, etc.), it will ultimately lead to a better understanding of chemistry in these alternative solvents. Indeed, Rogers and co-workers have already shown that the distribution of organic solutes in these systems is a function of the difference in polymer concentration between the polymer-rich and polymer-poor (aqueous) phases. Also, a series of near identical ABSs can be prepared even when the salts used are different (they will just possess a... 

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