Immobilization of phosphines

Several approaches toward immobilization of phosphine-free ruthenium-based metathesis catalysts bearing a coordinating ether group have been made over the past 3 years [61]. This aspect has been covered in a recently published review by Blechert and Connon [8d] and will therefore not be discussed here. 

Silica was used as support material for the immobilization of phosphine ligands. As the surface of the silica is able to react with the phosphine ligands, an improved spacer/linker system had to be applied in order to shield the substituents and to prolong the lifetime of the catalyst. The spacer/linker system is therefore designed to prevent entirely the contact of the catalysts with the aggressive surface. Two new linker systems were tested to achieve this goal. 

Figure 2 (a-c) Immobilization of phosphine ligands to a silica support. 

Even under the most inert atmosphere conditions, the 31CP/MAS spectrum of the immobilized ligand showed a major signal at 6 = 42 ppm (wrt 85% H3POO characteristic of phosphine oxide rather than phosphine. This could be quantitatively reduced by HSiCl3 and this surface reaction monitored by NMR but the subsequent exchange reaction (equation [5]) generated substantial quantities of phosphine oxide and a number of different isomeric complexes were f ormed. 

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

Many other modifications, particularly of the Rh and Mel catalysed carbonylation of MeOH, have been proposed and some of these have been operated commercially or may have been tested at significant pilot plant scale. These include, for example, the use of phosphine oxide species such as PPh30 [20] as promoters and systems involving immobilizing the Rh on ion exchange resins [21]. Numerous examples of ligand modified catalysts have been described, particularly for Rh, though relatively few complexes have been shown to have any extended lifetime at typical process conditions and none are reported in commercial use [22, 23]. The carbonyl iodides of Ru and Os mentioned above in the context of the Cativa process are also promoters for Rh catalysed carbonylation of MeOH to AcOH [24]. 

Fig. 7. Schematic representation of the non-covalent immobilization of ligands to a dendrimer support and the actual supramolecular dendritic complex containing 32 phosphine ligands 21).

Other means of immobilizing ferrocenyl phosphine ligands inclnde covalent see Covalent Bonds) attachment to either sihca gel see Silica) or solid snpport poly(ethylene glycol) (TantaGel), or confinement within mesoporons MCM-41. " ... 

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