Anchored Catalyst Systems

Anchored Catalyst Systems. - Catalyst Preparation. Several review articles have dealt with the preparation of supported catalysts and the details of all these preparations will not be dealt with in this Chapter. 

Sasson and Rempel [97] showed that the system [(PPh3)3RuCl2]/secondary alcohol is suitable for the selective transformation of 1,1,1,3-tetrachloro into 1,1,3-trichloro compounds. Similarly, Blum and coworkers [98, 99] employed [(PPh3)3RuCl2] as well as polystyrene-anchored Rh, Ru and Ir complexes for the hydrogen transfer from alcohols to trihalomethyl compounds, leading to dihalo-methyl derivatives. For example, one of the Cl atoms of 2,2,2-trichloro-l-phenyl-ethanol was displaced by H at 140-160 °C in 2-propanol. The polymer-anchored catalysts proved to be resistant to leaching [99]. 

These noncovalently anchored catalysts in general exhibit a behavior similar to their covalently bound analogues, but can now be separated from the support after the reactions by a simple filtration step. So far, these immobilized systems have not been used in continuous flow reactors. 

Some of the polymers slowly change their helicity in solution. A chiral crown ether-potassium ferf-butoxide combined system was reported to cause polymerization of methyl, tert-butyl, and benzyl methacrylate to form isotactic polymers that had high rotation values (164). Detailed scrutiny, however, raised questions about the result (135, 165). At first, in the presence of the initiator, the oligomers exhibit considerable activity, but after removal of the catalyst, the optical activity decreases. This decrease may be attributed to unwinding of the helixes in the chain the helicity could be caused by the anchored catalyst. 

The resulting materials are particularly effective catalysts. Not only are the initial turnover frequencies high (up to 500 catalytic cycles per hour) but the stability of the anchored catalysts in such systems was also shown beyond doubt by two research teams (359,377). 

The significant improvement in polymerization activity probably represents the single most important advantage of a chemically anchored catalyst. These catalyst systems show high efficiency, presumably because the active transition metal compound resides only on the support surface and thus permits availability of a larger concentration of active sites for polymerization. Reaction of a transition metal compound with a support surface provides an anchoring device preventing destruction of potential sites by mutual interaction. 

Conversion over PdICe-modified H-ZSM-5 - A way to promote the noble metal function is the addition of rare earth metals to the catalyst system. It has been reported that rare earth metals can act as anchors for noble metals and thus enhance dispersion and accessibility of noble metal sites in a zeolite. " The product distribution for a-limonene conversion over first Ce-exchanged, afterwards Pd-exchanged, H-ZSM-5-(55) is given in Table 7. 

The field of (alkane) isomerization has been thoroughly exploited by the petrochemical industry, resulting in many publications. The use of immobilized chiral catalysts, however, is still not common nowadays and was only described by Firmenich [85, 86]. They presented the first example of the use of a chiral diphosphino ligand, neither C2-symmetric nor atropic, for the rhodium(I)-cat-alyzed preparation of citronellal 40 via isomerization of allylic amines or alcohols (Scheme 16). The catalyst systems used were polymer-anchored BINAP... 

Even though most of the supported ionic liquid catalysts prepared thus far have been based on silica or other oxide supports, a few catalysts have been reported where other support materials have been employed. One example involves a polymer-supported ionic liquid catalyst system prepared by covalent anchoring of an imidazolium compound via a linker chain to a polystyrene support [79]. Using a multi-step synthetic strategy the polymeric support (e.g. Merrifield resin among others) was modified with l-hexyl-3-methylimidazolium cations (Scheme 5.6-4) and investigated for nucleophilic substitution reactions including fluorina-tions with alkali-metal fluorides of haloalkanes and sulfonylalkanes (e.g. mesylates, tosylates and triflates). 

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