Fluorous biphasic catalysis catalysts

The technique now called fluorous biphasic catalysis was apparently first described in the Ph.D. thesis of M. Vogt in 1991 however, these studies did not become known to the community until sometime later. W. Keim, M. Vogt, P. Wasserscheid, B. Driessen-Holscher, Perfluorinated polyethers for the immobilization of homogeneous nickel catalysts , J. Mol. Catal A Chem. 1999,139,171. 

R. H. Fish, Fluorous Biphasic Catalysis A New Paradigm for the Separation of Homogeneous Catalysts from their Reaction Substrates and products , Chem. Fur. J. 1999, 5,1677. 

These critical aspects of the classical fluorous biphasic catalysis led in recent works to the development of protocols for the conversions with modified catalyst systems in non-fluorinated hydrocarbons as solvents. As part of the BMBE lighthouse project, Gladyzs and coworkers appHed this concept to C - C coupHng reactions (Suzuki reaction) and other metal-catalyzed addition reactions (hydrosilylation, selective alcoholysis of alkynes), which have direct relevance for the synthesis of fine chemicals and specialties [74]. 

Following the publication of the first example of fluorous biphase catalysis by Horvath and Rabai in 1994 [1], the immediate focus was to develop catalysts that would exhibit very biased partition coefficients with respect to fluorous and organic solvents. Such liquids are normally immiscible at room temperature. This was done by attaching ponytails of the formula (CH2)m(CF2) -iCF3 (abbreviated (CH2)mRf )> including arrays emanating from silicon atoms [2]. Catalysis was then effected at elevated temperatures, where fluorous and organic solvents are commonly miscible, with prod-uct/catalysis separation at the low-temperature two-phase limit. 

A chapter written in 1996 covers hydroformylation catalyzed by organometallic complexes in detail,219 whereas a review written 5 years later gives a summary of the advances on hydroformylation with respect to synthetic applications.220 A selection of papers in a special journal issue has been devoted to carbonylation reactions.221 A major area of the research has been the development of fluorous biphasic catalysis and the design of new catalysts for aqueous/organic biphasic catalysis to achieve high activity and regioselectivity of linear or branched aldehyde formation. 

Fluorous biphase catalysis was also applied in Friedel-Crafts acylation with Yb tris(perfluoroalkanesulfonyl)methide catalysts with acid anhydrides.59 Of the aromatics studied, activated compounds and naphthalene (95% conversion) showed satisfactory reactivity. 

The fluorous biphasic catalysis concept was successfully demonstrated first by hydroformylation of 1-decene carried out in perfluoromethylcyclohexane and toluene, which forms a homogeneous liquid phase at 100°C in the presence of catalyst 2 prepared in situ according to Eq. (14.1) 125,133... 

Fig. 1. Fluorous biphasic catalysis part I hydroformylation, hydro-boration, and transesterification with fluorous tagged catalysts.

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... 

In order to perform fluorous biphasic catalysis the (organometallic) catalyst needs to be solubilized in the fluorous phase by deploying fluorophilic ligands, analogous to the hydrophilic ligands used in aqueous biphasic catalysis. This is accomplished by incorporating so-called fluorous ponytails . 

Fluorous biphasic catalysis exploits not only this principle, but also the ability of certain perfluo-rocarbon/hydrocarbon biphasic mixtures to form a homogenous solution at high temperatures. An extremely fluorinated catalyst can thus be applied under homogeneous conditions and recovered from the fluorous phase subsequent to a phase separation step at lower temperatures. 

Fluorous biphasic catalysis without fluorous solvents (filtration of a ther-momorphic fluorous catalyst). This can be used when a fluorous catalyst exhibits significantly different solubility in an organic solvent upon changing the temperature of the system. 

Solubility switching behaviour is one of the main benefits of fluorous biphasic catalysis (discussed in Chapter 7) however, other specially designed catalysts... 

Fish, R. H. (1999) Fluorous biphasic catalysis A new paradigm for the separation of homogeneous catalysts from their reaction substrates and products. Chemistry-A European Journal, 5(6), 1677-1680. 

Fluorous biphasic catalysis is another active area in multiphasic homogeneous catalysis. The term fluorous was introduced [90] as the analogue to the term aqueous, to emphasize the fact that one of the phases of a biphase system is richer in fluorocarbons than the other. Fluorous biphase systems can be used in catalytic chemical transformations by immobilizing catalysts in the fluorous phase. A fluorous catalyst system consists of a fluorous phase containing a preferentially fluorous-soluble catalyst and a second product phase, which may be any organic or inorganic solvent with limited solubility in the fluorous phase (Figure 2.8a). 

For typical fluorous biphase catalysis the most important aspect is the simple recycling and re-use of the catalyst. Fluorous solvents have one special advantage over hydrocarbon solvents, however. Their very high oxygen dissolving capacity, combined with their extreme resistance to oxidative decomposition makes perfluorocarbons in combination with fluorous catalysts the optimum choice for oxidation reactions. Thus, the biomimetic oxidation of olefins with molecular oxygen and 2-methylpropanal as a co-reductand has been achieved with a fluorous cobalt porphyrin catalyst (22) [23], and also even without catalyst [24] (Scheme 3.7). 

This approach has now been applied to a broad range of catalytic reactions, such as hydrogenation, hydroformyla-tion, hydroboration, and many palladium-catalyzed cross-coupling reactions, and excellent catalyst recovery and reuse have been reported. Although there are many comprehensive reviews of fluorous biphase catalysis,the... 

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. 

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