Polarity fluorous solvents

Better retention into the fluorous phase was observed when using a polar fluorous solvent and hexane (>99.82%, insoluble catalyst emulsified in homogeneous reaction mixture) than when using a PFMC and acetone (97.5%), as expected for the ionic... 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

In comparison to traditional biphasic catalysis using water, fluorous phases or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way of combining the specific advantages of homogeneous and heterogeneous catalysis. 

Figure 1.15 t-Butyl- (a) and perfluoroalkyl- (b) substituted betaine dyes for polarity measurements in less polar or highly fluorous solvents... 

Ionic liquids may be used in a similar fashion, but in contrast to the extremely nonpolar fluorous solvents, ionic liquids are polar. They are completely nonvolatile and so cannot be lost to the atmosphere. A range of ionic compounds that are liquid at room temperature and their use in synthetic chemistry are described in Chapter 4. 

Water is particularly suitable for use in biphasic catalysis. It readily separates from organic solvents because of its polarity, density and because of the hydrophobic effect. Water will form biphasic systems with fluorous solvents, some ionic liquids, many volatile organic solvents, and also with scCC>2 [18],... 

Water has several attractive features as a solvent and, as we have said elsewhere, the best solvent is no solvent, but if one has to use a solvent then let it be water. Water is the most abundant molecule on the planet and is, hence, readily available and inexpensive. It is nonflammable and incombustible and odorless and colorless (making contamination easy to spot). It has a high thermal conductivity, heat capacity and heat of evaporation, which means that exothermic reactions can be controlled effectively. It readily separates from organic solvents owing to its polarity, density and because of the hydrophobic effect [12], which makes it eminently suitable for biphasic catalysis. Indeed, water forms biphasic systems with many organic solvents, with fluorous solvents, some ionic liquids and with scC02 [13]. 

Fig. 5-19. Catalyzed chemical reaction between polar educt A and nonpolar educt B and a reagent in a biphasic solvent system with temperature-dependent mutual miscibility of the polar and nonpolar (fluorous) solvents. A more detailed illustration of the experimental possibilities for catalysis in fluorous solvents is given in reference [890].

Fluorous Solvent Polarity Data, Solubility and Miscibility Data... 

Figure 7.2 Perfluoroalkyl-substituted solvatochromic dye used to produce a spectral polarity index (Pg) for fluorous solvents.

Recently a method has been described in the patent literature for the fractionation of essential oils using a fluorinated solvent.Oils studied included clove bud and bergamot, and fractionations could be performed in a semi-continuous mode. Fluorous solvents, as very non-polar media, offer an interesting alternative to the aqueous or alcoholic solvent approaches typically used in natural product extractions. However, it is unlikely that the technique will become widely used in this field. 

The miscibility of perfluoroalkanes and other perfluoro solvents is low with corresponding hydrocarbon solvents and is exploited in fluorous organic biphasic catalysis.27 In some cases, apolar reactants may be dissolved in the fluorous phase and on conversion to higher polarity products a second immiscible phase is formed. Notable examples of catalyzed reactions that are effectively carried out using the fluorous biphase approach are hydroformylations28 and oxidations.29 It should be noted that fluorous solvents are damaging to the environment, however, as with other catalyst immobilization solvents, if they are not lost from the system no damage to the environment takes place. Fluorous biphase systems have not, as yet, been used on an industrial scale. 

This can be exploited for the extractive separation of fluorous-tagged compounds from other substances. The partition coefficient depends on the size of the fluorous tag and on the organic solvent. The preference for the fluorous phase increases with increasing fluorine content and polarity of the organic phase. As the fluorous solvent, FC-72 (a mixture of CeFu isomers) is often used. At room temperature, it forms biphasic systems with solvents such as toluene, dichloromethane or acetonitrile and with aqueous media. Somewhat surprisingly, diethyl ether and tetrahydrofuran are good solvents for fluorous molecules and are miscible with FC-72 at... 

Most of the other biphasic hydrogenations use also water as polar medium [130-145], Interesting variations are the use of aqueous micellar media [146] (see also Section 4.5) and the hydrogenation in aqueous media with Rh complexes which were deposited on aluminophosphate molecular sieves [147]. Other groups employed biphasic systems with fluorous solvents [148-151] or ionic liquids [152-153],... 

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