Hydrosoluble ligands

Of particular significance to achieve environmentally benign systems would be the use of water as a solvent, but this is also challenging in view of the lack of solubility of the alkanes and, commonly, also of the metal catalysts. The approach followed in the author s group often involves the use of hydrosoluble ligands at appropriate metal centers, which can lead to the formation of water-soluble catalyst precursors. Examples are indicated in the following sections. 

Scheme 5.3 The hydrosoluble ligand m-TPPTC and types of cyclodextrins used In aqueous hydroformylation of lone chain olefins [12].

The role of the surfactants has been evaluated with other hydrosoluble catalysts. It was found that the activity of the system depends on the surfactant/ligand parr, the highest conversion being achieved with CTAB/TPPMS couple (Conv. = 80% 3% corresponding to a DS = 0.105 0.005) [55]. 

Of particular importance for industry is the hydroformylation of alk-l-enes and one of the best processes developed so far for C2-C5 olefins employs a biphasic system in combination with a hydrosoluble rhodium complex bearing TPPTS as ligand [69-71], From this statement, Wasserscheid et al. investigated in the early 2000s the possibility of using ILs supported catalysts for this reaction [71-75], owing to the discovery by Chauvin that good linear/branched selectivities could be achieved... 

The effect of the catalyst and the hgands has not been discussed yet however, it plays an important role. For example, by replacing the classical phosphine ligands by a hydrosoluble sulfonated triphenylphosphine (TPPTS), Safi and Sinou carried out a reaction with butadiene monoxide and ethyl acetoacetate in a two-phase aqueous-organic medium.f The recycled water layer containing the catalyst could be reused with very high efficiency for a second reaction. As a matter of fact, the corresponding ethyl ester of the 2-acetyl-6-hydroxy-4-hexenoic acid was isolated in 80% and 87% yield (Z/E = 15 85) after the first and the second reaction, respectively. 

An attractive approach for the telomerization of butadiene with methanol using hydrosoluble NHC-Pd complexes was reported by Pinel and coworkers [83], In their work, they described the preparation and the use of diiFerent hydrosoluble (NHC)-Pd-based complexes a hydrosoluble imidazolinium-che-lated palladium complex and (NHC)-Pd complexes bearing one hydrosoluble phosphine ligand and one classical carbene ligand (Figure 4.27). In the presence of water, hydrosoluble (NHC)-Pd complexes were not efficient for telomerization reaction, while the latter exhibited very high activities. 

Steps have been taken toward eco-friendly catalytic systems active in alkane functionalization under mild conditions, preferably in aqueous media, by using hydrosoluble catalysts obtained from ligands that are water soluble. Systems can operate usually with a green oxidant (aqueous hydrogen peroxide) and in partially aqueous media. 

Similar yields, under the same conditions, were obtained for the water-soluble Fe(II) complexes 19-21 bearing the C-functionalized tris(pyrazolyl)methane HOCH2C(pz)3 (Table 22.1, [5e]). The hydroxo group of the scorpionate ligand imparts hydrosolubility that allows them to operate also in pure aqueous media (without any organic solvent, although less effectively). 

Water-soluble iridium hydrides containing PTA, TPPMS, and TPPTS ligands have been described and a few notes on these compounds have been presented in Section 1.1 of this review." A remarkable hydrosoluble hydride is [Cp Ir(bpy)H] 821 prepared by Ogo and co-workers by controlled protonation of the aquo complex [Cp Ir (bpy)(H20)] 505 with carboxylic acids. ° Compound 821 has been used as a robust and highly active catalyst... 

To date, only some hydrosoluble vitamins are known to interact as ligands with metal ions and, with some exceptions, very few solid phase stmctures of these complexes have been obtained (Scheme 2). Indeed, only a few cadmium complex structures have been described. 

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