Hydrophilic ligands, aqueous catalysis

In order to perform fluorous biphasic catalysis the (organometallic) catalyst needs to be solubilized in the fluorous phase by deploying fluorophilic ligands, analogous to the hydrophilic ligands used in aqueous biphasic catalysis. This is accomplished by incorporating so-called fluorous ponytails . 

Water solubility of all the aforementioned catalysts was due to the hydrophilic nature of ligand. In a second class, the aqueous nature originates from direct interaction of water molecules with the metal center [61]. Although this area of aqueous catalysis is not as extensive, there are several representative examples illustrating its importance and potential, as well as the variation of the metal center. One of the earlier examples involved the hydrogenation of maleic and fumaric acids with [RhCl (H20)6 ]3-" and [RuCl (H20)6 n]3" [62]. These simple catalysts revealed key mechanistic information that is applicable to many other systems, such as the requirement of alkene complexation prior to H2 activation. After this important discovery, many advances have been made in the areas of hydrogenation and polymerization reactions using these types of catalysts. 

The transfer of the phosphine-assisted catalytic processes to aqueous media prompts the development of specific hydrophilic ligands. The most important rationale for the application of such ligands is the development of phase-separation techniques. In the biphasic liquid-liquid technique, the hydrophilic phosphine works as an effective extractor of palladium to the aqueous phase. However, numerous recent works coming primarily from Genet s group (vide infra) show that many important Pd-catalyzed reactions can be made to run under very mild conditions in homogeneous aqueous media if carried out in the presence of hydrophilic phosphines—essentially aqueous phosphine-assisted catalysis. 

The area where aqueous-organic biphasic catalysis has had the greatest impact is in oxidation reactions arguably the most important industrial catalysed reaction. Many oxidation catalysts lend themselves well to the biphasic technique, as they do not require any modification to induce water solubility.15 Other complexes that are active oxidation catalysts are insoluble in water and in order to induce water solubility hydrophilic groups are attached to the periphery of the ligand. Some examples of such modified ligands are shown in Figure 2. 

In this section catalysts containing monophosphines for aqueous-phase catalysis will be presented, the influence of ligand variation on catalyst activity being emphasized. Syntheses of hydrophilic monodentate phosphine ligands will be discussed very briefly. Reference to catalysts containing other ligands (see Sections 3.2.2 to 3.2.6) is made only where appropriate. 

Another possible way to separate they catalyst from the fatty products was found by Davis [52-54] and further investigated by Fell [55]. This new method is supported aqueous-phase catalysis (SAPC cf. Section 4.7). On a hydrophilic support, e.g., silicon oxide with a high surface area, a thin aqueous film is applied which contains the water-soluble rhodium catalyst, for instance HRh(CO)L3 with sodium TPPTS ligands. Oleyl alcohol and syngas react at the organic/aqueous interface and form the formylstearyl alcohol in a yield of 97%. The catalyst can be separated from the product by simple filtration without loss of activity. 

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