Aqueous conditions hydrophilic ligands

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. 

As noted above, the addition of alcohols improves the reaction activity in the two-phase hydroformylation of higher olefins with Rh/TPPTS catalysts. Reaction selectivity, however, is diminished if the operating conditions are otherwise similar. This is illustrated in Figure 1, which compares the conversion and selectivity of 1-octene hydroformylation over the (acac)Rh(CO)2/TPPTS catalyst in water alone and 50% aqueous methanol as the solvent [10, 11], Selectivity can be improved in the aqueous methanol system when ligands other than TPPTS are used, as for example with ionic phosphites of the type 1 cited above. These ligands differ significantly from TPPTS in that they are expected to be amphiphilic in character the ionic portion is hydrophilic while the rest of the ligand is relatively hydrophobic. 

Whereas the polyketones were usually reported to precipitate from the aqueous phase, under emulsion conditions latexes of ethylene-undecenoic acid-CO ter-polymers and of 1-alkene-CO copolymers can be obtained, which are film-forming at room temperature [12]. Using a miniemulsion technique (cf. Section 6.12.3), water-insoluble diphosphine ligands can be employed without the necessity for hydrophilic modification. 

Contrary to PS, polyacrylamide is a hydrophilic polymer. This may increase the ligand exchange rate, i.e., the column efficiency. Thus on the polyacrylamide-CH2-L-pro-line-Cu(II) macroporous microspheres, the enantiomers of valine, threonine, isoleucine, serine, phenylalanine, tyrosine, tryptophan, and asparagine were completely resolved in less than 1 hr with water as the eluent. The efficiency was markedly improved. However, the methylene bridge between the ligand and the matrix was not stable under acidic and basic conditions. The same sorbent, but with a long spacer, -CH2CH20CH2CH(0H)CH2-, was prepared with L-proline content 1.76 mmol/g of dry polymer." When the polymer was soaked in 0.1 M HCl, 0.1 M NaOH or 1 M NH3, aqueous solutions, respectively, at... 

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