Fluorous metallic catalysis complexes

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

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 value of perfluoroalkyl-derivatised ligands in FBS catalysis depends on the preferential solubility of the catalyst in a fluorous solvent. Initially, this was established, qualitatively, by 31P NMR spectroscopic studies [37,49], but very recently [52] the fluorous partition coefficients of metal complexes of the trialkyl phosphine (la) have been measured analytically, from which both a fluorine content of > 60% and the number of perfluoroalkyl groups, which shield the hydrocarbon domain of the complex, are crucial for a high fluorous partition. 

Adsorption on silica gel surfaces or silica gels coated with water or thin layers of ionic liquids has been used to immobilize transition metal complexes % ionic interactions and hydrogen bonding. Reversed-phase silica gels were used to retain catalysts by hydrophobic interactions. Support of catalysts on fluorous reversed-phase silica gel by the solvophobic nature of perfluoroalkyl chains is a new and promising approach with potential in catalysis and combinatorial chemistry. 

Another attractive line of application of macromolecular complexes as catalysts is the so-called fluorous catalysis, whose idea was proposed in the early 1990s [163-168]. The main idea of such catalysis is that a catalyst is soluble (immobilized) in the perfluorocarbon phase, whereas the product is soluble in an organic solvent. If a suitable solvent for substrates, e.g. toluene or benzene, is used, then the system at sufficiently high temperature is homogeneous, and on cooling, there is phase separation. As a result, the solution of the metal complex in fluorocarbon is readily separable from the reaction products and can be reused. 

Several monographs and many reviews have appeared on various aspects of organometallic catalysis by transition metal complexes in aqueous (1-28), fluorous (29-33), and ionic liquid (33-48). These are listed noncomprehensively among the references up till the end of May 2009. 

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