Fluorous biphasic solvent systems

Remarkably, the use of a fluorous biphasic solvent system in combination with a [Rh(NBD)(DPPE)]+-type catalyst (NBD = norbornadiene) copolymerized into a porous nonfluorous ethylene dimethacrylate polymer, resulted in an increased activity of the catalyst relative to a situation when only toluene was used as solvent [30]. The results were explained by assuming that fluorophobicity of the substrate (methyl-trans-cinnamate) leads to a relatively higher local substrate concentration inside the cavities of the polymer when the fluorous solvent is used. That is, the polymer could be viewed as a better solvent than the fluorous solvent system. This interpretation was supported by the observations that (i) the increase in activity correlates linearly with the volume fraction of fluorous solvent (PFMCH) and (ii) the porous ethylene dimethacrylate polymer by itself lowers the concentration of decane in PFMCH from 75 mM to 50 mM, corresponding to a 600 mM local concentration of decane in the polymer. Gas to liquid mass transport limitation of dihydrogen could be mled out as a possible cause. 

Grigg, R. and York, M. (2000) Bimetallic catalytic cascade ring-closing metathesis-intramolecular Heck reactions using a fluorous biphasic solvent system or a polymer-supported palladium catalyst. Tetrahedron Lett., 41, 7255-8. 

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].

The most important biphasic liquid systems are probably those that combine a conventional organic phase with another type of solvent, such as water, a fluorous organic solvent, or an ionic liquid [3]. In those cases the solvent can be considered as the support for the catalyst phase and we have therefore limited the examples in this review to those where the recycled liquid catalyst phase is recovered as a whole. 

Biphasic catalysis in an organic/fluorous solvent system by Pd/dendrimer nanocomposites. 

Fluorous biphasic systems operate on the premise that the catalyst complex is preferentially soluble in the fluorous phase. This is achieved by synthesising fluorinated ligands that have a high weight-percentage of fluorine. It has been reported that for a complex to be preferentially soluble in fluorous solvents it must contain >60... 

Recently, the fluorous biphasic separation technique has been enriched with two new developments, both of which were demonstrated in hydrogenation. The need for a fluorous solvent can be eliminated by using fluorous silica as a fluorous catalyst scavenger. In liquid-liquid biphasic systems, reversible expan-... 

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

In both the Ni(acac)2-[bmim][PF6] and fhtorous biphasic systems, catalyst leaching is very low and several further batch oxidation reactions may be carried out with similar results to those obtained in the first run. In the fluorous biphasic system, the yield of 4-chlorobenzoic acid dropped from 87 % to 70 % by the sixth reaction cycle using [bmim][PF6] as a solvent, the yield was essentially the same after four uses, and no catalyst was found to leach into the organic phase. 

Another environmental issue is the use of organic solvents. The use of chlorinated hydrocarbons, for example, has been severely curtailed. In fact, so many of the solvents favored by organic chemists are now on the black list that the whole question of solvents requires rethinking. The best solvent is no solvent, and if a solvent (diluent) is needed, then water has a lot to recommend it. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water at ambient temperature and pressure can provide substantial environmental and economic benefits. Similarly, there is a marked trend toward the application of organometal-lic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide and ionic liquids. ... 

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]. 

---