Phosphine anchored catalyst

Figure 40. Electron lines from phosphine anchored catalyst as a function of oxygen plasma etching exposure.

Based on the data listed in Table 20.1, a value of 0.42% P was calculated for an anchored catalyst having three triphenylphosphine ligands, 0.28% P with two phosphine groups and 0.14% with one triphenylphosphene. An analytical value of 0.37% P was found which indicates that all three triphenyl-phosphines (TPP) are present in the catalyst as depicted by 4 in Scheme 20.2. However, only 0.11% P was found in the catalyst sample taken after catalyst pre-hydrogenation indicating that only one TPP is present on the active entity. Because of steric constraints between the bulky TPP and the HP A, it would appear that the TPP should be in the axial position as in 5. A proposed reaction mechanism for the anchored Wilkinson based on that shown in Scheme 20.1 is shown in Scheme 20.2. 

An example of such an analysis of rhodium phosphine ligand anchored catalysts is presented (58). A survey scan of a copolymer containing a diphosphine ligand prior to introduction of the metal is shown in Figure 1, spectrum h. The presence of phosphorus in the polymer is evident from this spectrum. 

Figure 38. Accumulated surface scans from phosphine anchored rhodium catalyst.

Figure 39. Electron spectrum from phosphine anchored rhodium containing catalyst after 30-min exposure to oxygen plasma etching.

Here triphenylphosphine, the most important ligand in organometallic catalysis, is coupled to the benzene rings of cross-linked polystyrene. An anchored catalyst is then formed by coordination of the phosphine group to the metal center of a rhodium complex (Eq. 6-2). 

The principal objectives of this review are to illustrate the range of reactions known to be catalyzed by phosphine-supported catalysts, to discuss critically performances of anchored catalysts in comparison to their soluble analogs, to consider the effects of the support itself, and to indicate a few areas where research activity might be fruitful. References are intended to be illustrative. 

Schweb and Mecking have reported one of the first examples of noncovalent anchoring of catalysts to soluble polymeric supports in 2001. This noncovalently anchored catalyst featured phosphine ligands that were bound by multiple sulfonate groups to soluble polyelectrolytes using electrostatic interactions. The catalyst system was employed in the hydroformylation of 1-hexene and exhibited typical selectivity for a bis-triphenylphosphine-bound rhodium catalyst. The complex was readily recovered and recycled by ultrafiltration. Independently, Reek et al. described similar systems, but these systems made use of a soluble... 

This is an ion-exchanger like the sulfonated polymer. The siUca surface can also be functionalized with phosphine complexes when combined with rhodium, these give anchored complexes that behave like their soluble and polymer-supported analogues as catalysts for olefin hydrogenation and other reactions ... 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Hydroformylation of vinyl acetate to give mainly the branched product in >90% ee has been achieved using a rhodium catalyst containing binaphthol and phosphine ligands anchored to polystyrene. 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers.117 They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles118 and tuned catalysts (TunaCat).119 A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity.120... 

Free-radical-catalysed additions have also been reported, and provide a genuine alternative to the more familiar base-catalysed addition routes. Thus the secondary diphosphine (28) readily adds to diphenylvinylphosphine in the presence of AIBN to give (29).25 Similarly, addition of di(pentafluorophenyl)phosphine to diphenylvinylphosphine affords26 the diphosphine (30). Sequential addition of silanes and secondary phosphines to terminal aco-dienes under the influence of u.v. light affords the silylalkylphosphines (31), which may be anchored via silicon to the surface of inorganic oxides and used as polymeric catalysts.27... 

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