Ligand fluorous

Scheme 7.86 Vinylation of enamides employing fluorous ligands.

Fluorous ligands introduce an ease of purification in that the tagged phosphine ligand, the palladium catalyst complexed ligand, and the oxidized ligand can be completely removed by direct fluorous solid-phase separation (F-SPE) prior to product isolation. Similarly, an example of a fluorous palladium-catalyzed microwave-induced synthesis of aryl sulfides has been reported, whereby the product purification was aided by fluorous solid-phase extraction [91]. 

Fig. 7.6 Recycling ofthe fluorous ligand in aminocarbonylations (reproduced with permission from [100]).

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

The hydrogenation of a number of 1-alkenes using fluorous derivatives of Wilkinson s catalyst has been investigated. Initial experiments established the effect of the fluorous ligands on catalytic activity [15], The ligands evaluated in this study are illustrated in Figure 8.7. 

The turnover frequency was found to be greatest for the two fluorous ligands and the control ligand with the trimethylsilyl function was found to be less effective than triphenylphosphine. These fluorous derivatives of Wilkinson s catalyst were used to hydrogenate 1-octene in perfluoromethylcyclohexane (PP2). The reaction was carried out a number of times in order to evaluate the efficiency of the system in terms of recycling and reuse [16], It was found that as the number... 

Table 4 Pd-leaching in perfluorohexane with fluorous ligands...

Acknowledgements We would like to thank Prof. Dr. Leitner (RWTH Aachen) for the supply of PEG-modified ligands and Prof. Dr. Gladysz (University of Erlangen-Nurnberg) and Prof. Dr. Dinjus (FZ Karlsruhe) for the donation of fluorous ligands. 

Potential applications of fluorous phases are also limited by the low solubilities of reactants in them 8-10), and modifications of catalysts with fluorous ligands necessary for some applications are expected to be expensive (6). 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

Larhed and coworkers employed fluorous triphenylphosphine as a ligand and Mo(CO)6 as a CO source for palladium-catalyzed hydrazidocarbonyla-tions of aryl iodides and bromides (Scheme 9) [36]. The fluorous ligand was recovered by extraction with fluorous solvent FC-84. 

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