Ligands water-solubilizing

Hydrogenation reactions in water have been extensively studied and many of the water-solubilizing ligands described in Chapter 5 have been tested in aqueous-organic biphasic hydrogenation reactions. One of the earliest catalysts used was the water-soluble analogue of Wilkinson s catalyst, RhCl(tppms)3 (tppms = monosulfonated triphenylphosphine), but many other catalysts have since been used including [Rh(cod)(tppts)2]+, [Rh(cod)2]+ and [Rh(acac)(CO)2]+ (cod = cyclooctadiene). 

According to Eq. (32), the factors that determine the degree of solubilization of the solute are the complexation constant and the solubilities of the solute, the ligand, and the complex. As a result, the most useful ligands for solubilization in aqueous media are highly water soluble, and produce soluble complexes. 

The di-(TPPDS)239 and tri-sulfonated phosphines (116) (Scheme 8) (TPPTS)239 are also known, the latter being the most widely used water-solubilizing phosphine ligand, as of 2002. The synthesis of (116) is ostensibly similar to that of (115), although more recently it has been shown that addition of orthoboric acid to the sulfonation mixture reduces the amount of tertiary phosphine oxide side products.240 This general sulfonation procedure has been used for the preparation of various ditertiary (e.g., (117))241 242 and chiral sulfonated phosphines (e.g., (118)).241,243-245 Carboxylic acid- and carboxylate-modified phosphines (e.g., (119) and (120)) are also known.246,247... 

Diphosphine diversity was also exploited by atropisomeric biphenyl ligands. An interesting approach toward water solubilization was disclosed for one of these BINAP-type diphosphines (Scheme 1, MeOBIPHEP-S) [12] (cf. Section 3.2.1). The indolylsulfonyl group withstands the Grignard reaction and phosphine oxide re-... 

Carboxylate groups were used in many cases to achieve water-solubilization of simple achiral phosphines. With the synthesis of the water-soluble polymer shown in Eq. (2) this methodology was extended into the field of chiral ligands [28a]. Acylation of the diphosphine (2S,4S)-4-diphenylphosphino-2-diphenylphosphinome-thylpyrrolidone (PPM) with poly(acrylic acid) (PPA) yields the hydrophilic macroligand PAA-PMM [28a,b] and PAA-pyrphos [28c]. 

The general flow scheme for this alternative is shown in Figure 15. After the homogeneous reaction, catalyzed for instance by a rhodium catalyst containing triphenylphosphine monosulfonic acid as complex ligand, the solubilizer methanol is distilled off. The catalyst system now becomes insoluble and is separated by extraction with water in the third unit. The products C and D, in this case the aldehydes, can be separated as the second liquid phase. After evaporation of the aqueous catalyst solution to dryness (unit 4) the catalyst is dissolved in the solvent methanol for a new reaction step (unit 5). 

In the first stage, 1-butene is preferentially converted to valeraldehyde. This takes place in a low-pressure process, in which the catalyst used is a rhodium complex compound containing water-soluble phosphines as ligands. A solubilizer is also added. In the second stage, the 2-butene not converted in the first stage is hydroformylated with a cobalt catalyst. Part of the valeraldehyde obtained is then hydrogenated to amyl alcohol [109]-[111]. 

---