Fluorous metallic catalysis

Metallic catalysis plays many important roles in modern organic chemistry. Metal-catalyzed carbon-carbon bond and carbon-hetero bond formation reactions have been widely used in the synthesis of pharmaceuticals, natural products, agrochemicals, and materials. However, these metallic catalysts are usually expensive and toxic, and tend to contaminate both the product and the environment. So many industrial appHcation of metallic catalysis, especially in pharmaceutical industry, are restricted. For example, only less than 2 ppm of the metallic residue is allowed 

---

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. 

In comparison to traditional biphasic catalysis using water, fluorous phases or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way of combining the specific advantages of homogeneous and heterogeneous catalysis. 

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

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]. 

Fig. 22. Schematic illustration of the approach used to carry out fluorous biphasic catalysis using dendrimer-encapsulated metal nanoparticles modified on their exterior with perfluoroether ponytails. Note that the ponytails can be attached by either electrostatic or covalent means. Reprinted with permission from Ref. 103 Copyright 2000 American Chemical Society...

JJJ Juliette, IT Horvath, JA Gladysz. Transition metal catalysis in fluorous media practical application of a new immobilization principle to rhodium-catalyzed hydroboration. Angew Chem hit Ed Engl 36 1610-1612, 1997. 

Homogeneous Catalysis in Fluorous Solvents Using Dendrimer-Encapsuiated Metal Particles... 

In this section we describe two approaches for using dendrimer-encapsulated metal particles to perform biphasic catalysis. The first is hydrogenation catalysis using dendrimers rendered soluble in the fluorous phase by electrostatic attachment of perfluoroether groups [103]. The second demonstrates the use of perfluoroether groups covalently hnked to the dendrimer exterior to carry out a Heck reaction [100]. 

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. 

---