Ligand hydrophilicity

Hydrogenation of iV-acyl a-amino acids in 87-91% ee has been accomplished with Rh bound to copolymers (43) with chiral phosphine ligands, as shown in Scheme 16. The same % ee was obtained with Rh catalysts prepared from analogous soluble ligands. Hydrophilic poly(2-hydroxy-ethyl methacrylate) and poly(iV,iV-dimethylacrylamide) provided polar environments for the hydrogenations. The chiral polymeric catalyst can be recycled. Earlier studies with polymers based on chiral biphosphine monomer (44) showed that polar comonomers were required for high % ee catalysis. ... 

A neutral ligand whose exterior is hydrophobic and whose interior is hydrophilic. 

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

When water-miscible ionic liquids are used as solvents, and when the products are partly or totally soluble in these ionic liquids, the addition of polar solvents, such as water, in a separation step after the reaction can make the ionic liquid more hydrophilic and facilitate the separation of the products from the ionic liquid/water mixture (Table 5.3-2, case e). This concept has been developed by Union Carbide for the hydroformylation of higher alkenes catalyzed by Rh-sulfonated phosphine ligand in the N-methylpyrrolidone (NMP)/water system. Thanks to the presence of NMP, the reaction is performed in one homogeneous phase. After the reaction. 

Porous glass (PG) modified with covalently adsorbed poly(p-nitrophenyl acrylate), as described in Sect. 4.1, turned out to be a highly suitable carrier for immobilization of various biospecific ligands and enzymes. When the residual active ester groups of the carrier were blocked by ethanolamine, the immobilized ligands when bound to the solid support via hydrophilic and flexible poly(2-hydroxyethyl acrylamide). The effective biospecific binding provided by the ligands... 

At the same time, Schmidtchen et al. compared cationic phosphine ligands containing the hydrophilic guanidinium (4.3, 4.4) and the anionic phosphine ligand TPPTS for this palladium-catalyzed coupling reaction. They found that the cationic ligands were effective for the coupling reaction but less efficient than TPPTS 43... 

---