Immobilization systems ligand selection

The functionalized ligands were tested for various hydrogenation reactions (see Scheme 12.17). Ir-Josiphos bound to silica gel as well as to a water-soluble complex produced TONs in excess of 100000 and TOFs up to 20000h for the Ir-catalyzed hydrogenation of 2-methyl-6-ethyl aniline (MEA) imine to give an intermediate for (SJ-metolachlor [45a]. The polymer-bound Ir complex was much less active, and in all cases the ee-values were comparable to those for the homogeneous catalyst Selected results are summarized in Table 12.3. However, no immobilized system could compete with the homogeneous catalyst which is used to produce >10000 tonsy of enantioenriched (. S J-metolachlor and which, under optimized condi-... 

The following three separation methods of product from the Ir-catalyst were evaluated distillation, extraction and filtration. For the last two options the preparation of new modified extractable or immobilized xyliphos ligands was necessary. However, lower activity and selectivity of these xyliphos derivatives and the additional development work that would have been required led to the decision to stay with the already well optimized soluble xyliphos system. After the hydrogenation step, a continuous aqueous extraction is performed to neutralize and eliminate the acid from the crude product. After flash distillation to remove residual water the catalyst is separated from (S)-NAA in a subsequent distillation on a thin film evaporator (see Fig. 13). From the organic distillation residue, Ir could in principle be recovered whereas the chiral ligand decomposes. Owing to the very low catalyst concentrations, Ir recovery is not economical. 

An innovative procedure for the reproducible detection of metal ions using complexation schemes at PMEs was devised by Abruna and coworkers. " The systems developed rely on the presence of a redox couple in the polymer film in addition to immobilized complexing ligand sites. The redox couple, selected to have a formal potential removed from the metal-ligand complex to be detected, acts as an internal standard to allow facile evaluation of saturation effects, normally problematic in complexation studies with PMEs. The redox couple also increases the between- and within-film reproducibility by using ratiometric methods to... 

Salen ligands have also been used in the titanium-catalyzed trimethylsilyl-cyanation of benzaldehyde. The complexes were immobilized by substitution of a chloride with a surface silanol from the support. In the first study on this reaction [38], the most efficient ligand was the non-symmetrical salen Im (Fig. 11) (94% ee), whereas the selectivity obtained with the symmetrical ligand la was significantly lower (72% ee). In a recent paper, the immobilization of different titanium species, including monomeric and dimeric systems with... 

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

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