Organic-aqueous tunable solvents

Another way to omit the fluorous solvent would be to utilize a catalyst immobilization solvent that is not fluorinated, such as water. We demonstrated the application of a phase change after reaction permits facile recycle of hydrophilic catalysts. This method is called OATS (Organic-Aqueous Tunable Solvent) (15). 

Figure 3.12 Biocatalysis using organic-aqueous tunable solvents (OATS).

A recent proposal concerns mixed organic-aqueous tunable solvents (OATS) such as dimethyl ether-water, the solubility of which for substrates can be influenced by a third component such as carbon dioxide. CO2 acts as a antisolvent and as a switch to cause a phase separation and to decant the phases from each other (preferably under pressure). This behavior makes the operation of bi- or multiphase homogeneous catalytic processes easier and more economic the preferential dissolution at modest pressure of carbon dioxide causes phase separation which results in large distribution coefEcients of target molecules in biphasic organic-aqueous systems. This extraordinary behavior lead to a sophisticated flow scheme (Figure 6) [7]. 

A related principle was established by Eckert and Liotta who advocated the use of organic-aqueous tunable solvents (OATSs) such as water-tetrahydrofurane (THE) mixtures [31]. Upon the effect of COj, the water-THE phase splits into relatively immiscible phases and allows thus the separation of the catalyst and aldehyde [32]. 

P. Pollet, R. J. Hart, C. A. Eckert, C. L. Liotta, Organic Aqueous Tunable Solvents (OATS) A Vehicle for Coupling Reactions and Separations. Acc. Chem. Res., 43, 1237-1245 (2010). 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

Given the choice of equipment today and its cost relative to conventional solvent based cleaning, (see Ch. 10 by Huse and Smith), the advantages of SCF cleaning make it very attractive compared to aqueous or organic based systems. Capabilities of solvent tunability... 

SCFs are an environmentally friendly alternative to organic solvents as media for biocatalysis. A key feature of biocatalysis in SCFs is the tunability of the medium [75]. Enzymatic activity in SCFs has been proven and well documented [76]. Limiting factors, which may affect enzymatic activity in supercritical solvent systems, have been identified and are well characterized. A major limitation to the broader use of SCFs is their inability to dissolve a wide range of hydrophilic and ionic compounds, which greatly impedes their ability to carry out biolransformation with polar substrates. The interest in water-in-SCF microemulsion as reaction media stems from the fact that in such systems high concentrations of both polar and apolar molecules can be dissolved within the dispersed aqueous and continuous SCF phases, respectively. 

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