Supercritical fluorous catalysis

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. ... 

Recent developments in phosphine synthesis have allowed for catalysis to be carried out in nontraditional media, such as aqueous and fluorous solvents, as well as in supercritical carbon dioxide. Horvath and Gladysz have... 

Supercritical fluids, especially SCCO2 (cf. Section 3.1.13), find increasing interest as environmentally friendly reaction media with unique properties for chemical reactions [285]. The problem of insufficient solubility of the ligand complexes has been solved by an approach similar to fluorous biphasic catalysis (cf. Section 3.1.1.2.1) [286-289]. 

There are various possible approaches for multiphase operation of homogeneous catalysis, to improve their usability and recycle processes with organic/organic, organic/aqueous, or fluorous solvent pairs (solvent combinations), non-aqueous ionic solvents, supercritical fluids, and systems with soluble polymers. Figure 2.2 reports a general scheme of the possibilities for homogeneous catalysis. 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

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. 

The most severe dra wback in homogeneous catalysis is the separation of the catalyst from the reaction mixture. The industrial success of the aqueous two-phase hydroformylation ofpropene to n-butanal [1] in Ruhrchemie AG in 1984 represents the considerable progress in this field. However, aqueous/organic biphasic catalysis has its limitations when the water solubility of the starting materials proves too low, as in hydroformylation of higher olefins (see Chapter 1). To solve this issue, a variety of approaches have been attempted. Additions of co-solvents [2] or surfactants [3, 4] to the system or application of tenside ligands [5, 6] and amphiphilic phosphines [7, 8] are ways to increase the reaction rates. Other approaches such as fluorous biphase system (FBS see Chapter 4) [9], supported aqueous phase catalysis (SAPC see Section 2.6) [10], supercritical CO2 (cf. Chapter 6) [11] and ionic liquids (cf Chapter 5) [12] have also been introduced to deal with this problem. 

In conjunction with the work being done on fluorous biphasic systems, the development of fluorinated ligands has also made an impact in the area of supercritical carbon dioxide (SCCO2) catalysis. Supercritical CO2 has been heavily studied as a reaction medium for organometallic chemistry due to its unique properties. Its... 

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