Fluorous reagents and ligands

Figure 7.4 Commercially available fluorous building blocks for the preparation of fluorous tagged substrates, reagents and ligands.

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. 

More examples are found for varied oxidation processes mainly for various epoxidations carried out by metal catalysts bearing F-modified ligands, such as porphyrins,139 Ru perfluoroacetylacetonate salt,140 or salen complexes,141 142 or using the 3 selenium compound as catalyst.143 The potential for enantioselective transformations offering an easy way to recover precious chiral reagents and catalysts was demonstrated in enantioselective epoxidation using fluorous chiral salen... 

Another approach to facilitate the recovery of catalytic systems relies on the use of fluorinated analogues of classic chiral ligands. The recycling options offered by the fluorous catalysts have been explored in the field of asymmetric addition of dialkylzinc reagents to aldehydes in presence of titanium tetraisopropoxide. In 2000, the groups of Chan ° and Curran reported independently the synthesis of perfluoroallqrl-substituted BINOL ligands and their evaluation in the titanium-mediated enantioselective addition of diethylzinc to aromatic aldehydes in fluorous biphasic system (Scheme 7.27). 

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