Fluorous solvent miscibilities

With binary solvent systems, it is common to determine a consolute or upper critical solution temperature above which phase separation cannot occur, whatever the composition. Consolute temperatures are usually found for 50 50 mixtures however, the fiill phase diagrams show that solvents can become miscible in other 

The mixing of two phases is, of course, favorable from an entropic standpoint. From an enthalpic standpoint, intermolecular attractive interactions will always be greater within the pure non-fluorous phase (which has a much greater polarity) than within the pure fluorous phase (which has a much lower polarity). Upon mixing the two phases, the stronger intermolecular interactions in the former will be markedly diluted, and the intermolecular interactions felt by the fluorous molecules will increase only slightly. Hence, no enthalpic gain is to be expected. 

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Fluorous biphasic catalysis was pioneered by Horvath and Rabai [54, 55] who coined the term fluorous , by analogy with aqueous , to describe highly fluori-nated alkanes, ethers and tertiary amines. Such fluorous compounds differ markedly from the corresponding hydrocarbon molecules and are, consequently, immiscible with many common organic solvents at ambient temperature although they can become miscible at elevated temperatures. Hence, this provides a basis for performing biphasic catalysis or, alternatively, monophasic catalysis at elevated temperatures with biphasic product/catalyst separation at lower temperatures. A number of fluorous solvents are commercially available (see Fig. 7.16 for example), albeit rather expensive compared with common organic... 

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].

Table 7.1). Perfluorinated polyethers can also be used as the fluorous phase. However, perfluorinated aromatics are usually miscible with organic solvents and therefore are not used in FBS. It should also be noted that fluorous solvents have a low solubility in water and therefore aqueous-fluorous separations can also be achieved. 

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

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

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... 

The unique properties of highly fluorinated and perfluorinated ( fluorous ) solvents and reagents open several routes to a solution of these problems and to a sustainable green chemistry [1-5]. These properties include their very temperature-dependent miscibility with typical hydrocarbons, their non-toxicity, and their extreme chemical inertness. 

At room temperature, most fluorous solvents are immiscible with other organic solvents, but an increase in temperature typically renders the solvents miscible. The reactants are initially dissolved in a non-fluorinated, organic solvent and the catalyst is present in the fluorous phase. Raising the temperature of the system creates a single phase in which the catalysed reaction occurs. On cooling, the solvents. 

Experiments made with different fluorous solvents proved that the nature of the fluorous phase has practically no effect on the l/b ratio and on the aldehyde selectivity. On the other hand, this factor greatly influences the activity, since the TOP dropped from 3900 h when using IH-perfluorooctane to 3800, 2500, and 2300 h with perfluoromethylcyclohexane (PFMC), perfluoromethyldecalin (PFMD) and per-fluoroperhydrophenanthrene (PFPP) respectively. These observations are explained by the fact that at the reaction temperature (80 °C), PFMD and PFPP are not totally miscible with 1-decene, in contrast with the l-decene-CgFj and 1-decene-PFMC couple [13]. 

Fluorous biphasic catalysis has emerged since the late 1990s as an attractive alternative to traditional catalysis methods [1], Fluorous techniques take advantage of the temperature-dependent miscibility of organic and perfluorocarbon solvents to provide easier isolation of products and recovery of a fluorinated catalyst. The large-scale use of fluorous solvents, however, has drawbacks cost, and concern over environmental persistence. 

In 2001, it has been independently reported [2, 3] that the fluorous solvent can be skipped by designing fluorinated catalysts that themselves have a temperature-dependent phase miscibility - that is, solubility - in ordinary organic solvents. 

Pereluorodecalin is not miscible with a non-fluorous solvent, toluene, or o-xylene, even under reflux conditions. 

Although fluorous reactions can be conducted under heterogeneous liquid/Uquid biphasic conditions, from a rate standpoint it will normally be advantageous to operate under homogeneous monophasic conditions, as in the top sequence in Figure 3.2. For this reason it is important to know at what temperatures various fluorous and non-fluorous solvents become miscible (Table 3.5). 

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