Fluorous immobilization

As expected, 1 is more active than 4, and is recovered in quantitative yield by extraction with perfluoromethylcyclohexane. Although 2 and 3 are more active than 4, they cannot be recovered by extraction with any fluorous solvents. The amide condensation proceeds cleanly in the presence of 5 mol% of 1 the desirable amide has been obtained in 95% yield by azeotropic reflux for 15 h. In addition, the corresponding N-benzylamide has been obtained in quantitative yield by heating 4-phenylbutyric acid with benzylamine in the presence of 2 mol% of 1 under azeotropic reflux conditions for 4 h. Based on these results, the re-use of 1 has been examined for the direct amide condensation reaction of cyclohexanecarboxylic add and benzylamine in a 1 1 1 mixture of o-xylene, toluene, and perfluorodecalin under azeotropic reflux conditions with removal of water for 12 h [ Eq. (2) and Table 2] [5]. After the reaction has been completed, the homogeneous solution is cooled to ambient temperature to be separated in the biphase mode of o-xylene-toluene/ perfluorodecalin. The corresponding amide is obtained in quantitative yield from the organic phase. Catalyst 1 can be completely recovered from the fluorous phase and re-used in the recyclable fluorous immobilized phase. 

Table 2 Recovery and re-use of in the recyclable fluorous immobilized phase...

Figure 10.1 Recycle system of 3,5-(C,oF2i)2C5HjB(OH)2 in the fluorous immobilized phase.

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. 

Finally - and perhaps most importantly - the fluorous tagging of the catalyst that introduces affinity for the fluorous phase can be a very mild immobilization technique, as there is no direct covalent link with a support and the sepa-... 

The technique now called fluorous biphasic catalysis was apparently first described in the Ph.D. thesis of M. Vogt in 1991 however, these studies did not become known to the community until sometime later. W. Keim, M. Vogt, P. Wasserscheid, B. Driessen-Holscher, Perfluorinated polyethers for the immobilization of homogeneous nickel catalysts , J. Mol. Catal A Chem. 1999,139,171. 

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. 

The range of ligands developed for ionic liquid catalysis is much smaller than that for other immobilization solvents such as water and fluorous phases as off the shelf ligands and catalysts can often be used in ionic liquids. For example, a number of catalysts that were developed to operate in organic solvents under homogeneous conditions are salts themselves and do not need to be modified for use in ionic liquids [25],... 

Aldehydes may be converted to carboxylic acids using Ni(acac)2 immobilized in [bmim][PF,5], with oxygen as the oxidant, as shown in Scheme 9.14 [27], A similar reaction has also been performed using perfluorinated solvents, and it was found that there was little difference between the two systems [28], However, the Ni(acac)2 catalyst could not be used directly in the fluorous solvent and therefore the 1,3-diketonate was modified with long perfluorinated chains prior to use to ensure solubility. 

Another way to omit the fluorous solvent would be to utilize a catalyst immobilization solvent that is not fluorinated, such as water. We demonstrated the application of a phase change after reaction permits facile recycle of hydrophilic catalysts. This method is called OATS (Organic-Aqueous Tunable Solvent) (15). 

Cornils, B. (1997) Fluorous biphase systems - the new phase-separation and immobilization technique. Angew. Chem. Int. Ed. Engl, 36, 2057. 

Hyperbranched polymers can also be used for supramolecular immobilization (Scheme 15). Yet another approach for the noncovalent immobilization has been presented by Tzschucke and coworkers who used interactions between fluorous phase silica (FPS) and perfluoro-tagged palladium... 

Tin(rv) bis(perfluorooctanesulfonyl)amide is a practical catalyst for transesterification and direct esterification using an equimolar ratio of the reactants in a fluorous biphase system (Equation (103)). The tin amide is completely recovered and reused in the immobilized fluorous phase without loss of its catalytic activity.269... 

An elegant alternative approach for noncovalent interaction relies on fluorous-lluorous interactions. A glycan array of monosaccharides and disaccharides bearing anomeric fluorous tags was noncovalently immobilized on fluorous-derivatized glass slides (19, 20). The attachment method is compatible with a wide range of functional groups and has been successfully used to probe carbohydrate-protein interactions. 

This review is divided into four main sections, covering the Heck, Stille, and Suzuki reactions, with miscellaneous reactions being included at the end. Processes featuring alkynes in copper co-catalyzed Sonogashira-type couplings have been included in the section on Heck reactions. This review does not cover carbon-carbon bond formation processes using immobilized catalysts. Similarly, fluorous-phase syntheses " and those on polyethylene glycol " are excluded. 

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