Fluorous biphasic catalysis ligands

Because of their low solubilities in the aqueous phase, the hydroformylation of higher alkenes (>C2) is still a challenging problem. In addition to fluorous biphasic catalysis, possible solutions, which have been addressed, include the addition of surfactants240,241 or the use of amphiphilic ligands242-244 to enhance mutual solubility or mobility of the components across the phase boundary and thereby increase the rate of reaction. The use of polar solvents such as alcohols,245 p-cyclodextrin,246 cyclodextrin ligands,247 248 thermoregulated phase-transfer... 

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 . 

Phosphorus(III) ligands, in particular phosphines, are the most widely studied ligand system in homogeneous catalysis. It is not surprising, therefore, that Horvath and Rabai chose the derivatisation of a phosphine in their seminal paper [10] and that this class of ligand (Fig. 3) has received significantly more attention than other ligands for fluorous biphase catalysis. 

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]. 

The ligand 4-H F -tpp has been also successfully employed in fluorous biphasic catalysis Kling R, Sinou D, Pozzi G, Choplin A, Quignard F, Busch S, Kainz S, Koch D, Leitner W (1998) Tetrahedron Lett 39 9439 see the relevant chapter in this volume on the FBS approach... 

In the 15 years since its invention, fluorous (biphasic) catalysis has become a well-established area and provides a complementary approach to other variants of biphasic catalysis. Fluorous solvents are nontoxic and environmentally benign. Owing to their low surface tension, they do not form emulsions and due to their high density separate readily from other components in solvent mixtures. All these characteristics could make fluorous biphase catalysis superior to the analogous aqueous systems. Conversely, the same features that make fluorous solvents immiscible with many organic liquids make them bad solvents of organometallic catalysts, too, and chemical modifications of the catalysts (ligands) are required to attain sufficient solubility. Moreover, to retain the catalyst in the fluorous phase... 

Ferrocene diphosphine ligands fluorous biphase catalysis fluxional behavior... 

The application of perfluorous polyethers in biphasic catalysis was first described by Vogt (133), who also synthesized ligands based on hexafluor-opropene oxide oligomers to create metal complexes that are soluble in the perfluorous polyethers. The solvophobic properties of the fluorous solvent were successfully incorporated in the metal complexes catalytic oligomerization and polymerization reactions with nickel and cobalt complexes were demonstrated. 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

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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