Alternative fluorous solvents

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). 

Thus, the use of catalysts in new green reaction media such as ionic liquids, fluorous solvents, and supercritical carbon dioxide has become a viable alternative to those discussed within the chapters. 

Fluorous Solvents. The fluorous phase is an alternative to the aqueous phase.26 Fluorous (perfluorinated) solvents, such as perfluoroalkanes, perfluoroalkyl ethers, and perfluorinated tertiary amines, have been recognized to be extremely stable and nontoxic and have high density, low solvent strength, and extremely low solubility in water and organic materials. 

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... 

Perfluorinated molecules are prepared from their hydrocarbon analogues by electrochemical fluorination or by fluorination using cobalt trifluoride. Functional perfluorinated molecules are then used to prepare the tagged catalysts and reagents (Figure 7.4). Therefore, in terms of life cycle analysis, fluorous solvents are not as green as a solvent that does not need to be prepared, e.g. water, or a solvent that requires little substrate modification, e.g. a renewable VOC. However, the ability of FBSs to perform efficient separations often reduces the overall amount of solvent that is required in a process and therefore they are considered green alternative 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. 

Diels-Alder reactions have been performed in most alternative reaction media. For certain substrates this reaction is significantly accelerated in fluorous solvents (Figure 1.9) This has been ascribed to a fluorophobic effect, analogous to the better-known hydrophobic effect where there is an inverse relationship between reaction rate and the solubility of reagents. However, it should be noted that in general cycloaddition reactions (including Diels-Alder reactions) are faster in water and this can be attributed to additional hydrogen bond stabilization of the transition state. 

One of the main drawbacks of the Wittig reaction is the formation of unwanted triphenylphosphine oxide. A new route, which makes use of polymer-supported triphenylphosphine and microwave dielectric heating has been developed (Scheme 13), which yields the required alkene without the triphenylphosphine oxide. An alternative strategy for separation of the product alkene from unwanted phosphine oxide by-product is to carry out the Wittig reaction in a fluorous solvent using a perfluorinated ylide such as (45). One drawback of this... 

The replacement of relatively harmful volatile organic solvents with, alternative, less environmentally damaging solvent systems/approaches is generating interest from a green perspective for synthetic chemistry and catalysis. A number of these are reviewed elsewhere in this compilation. Here, we highlight some notable developments in green applications of water, supercritical (sc) fluids, ionic liquids, and fluorous solvents in organometallic chemistry. 

In this respect, fluorous-phase operation is similar to temperature-regulated phase transfer catalysis (see Section 2.3.5) and to special versions of soluble polymer-bound catalysis (see Chapter 7). Alternatively, the temperature-dependent solubilities of solid fluorous catalysts in liquid substrates or in conventional solvents containing the substrates could eliminate the need for fluorous solvents. 

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. 

---