Palladium catalysts dendrimer polymers

Supported catalysts involving palladium on carbon and dendrimer-encapsulated palladium and a polymer-supported phosphine palladium catalyst have facilitated C-C coupling reactions in SCCO2. Polymer-tethered substrates or amine bases have also been successfully used for the Mizoroki-Heck and Suzuki-Miyaura reactions in SCCO2. For example, REM resin underwent a Mizoroki-Heck reaction with iodobenzene to yield, after cleavage, ( )-methyl cinnamate 48 (74%) (Scheme 88). It is assumed that SCCO2 acts as a good solvent that swells the polymers and exposes reactive sites. 

Dendrimers are branch-like polymers (see Chapter 5, Section 5.8) and have been used as nano-reactorsThese branched molecules have large voids in their structure that can incorporate a catalytic species into the inner nanoscale environment of the dendrimer. The dendrimer poly(propylene imine) (PPl) has been used as a nanoreactor for Heck reactions and alkyl aminations. 4-Dipheny Iphosphinobenzoic acid is fixed into the voids of the dendrimer via ionic contacts between the positive charges on protonated amine functional groups and the deprotonated benzoic acid and used to anchor the palladium catalyst (Figure 2.36). 

Another advantage of dendrimer-based catalysts concerns their easy recovery by stabilization at the surface of a polymer. The principal activities in dendritic catalysis lie in homogeneous catalysis, including Kharash addition of CC14 to methacrylate, palladium-catalyzed allylic alkylation, hydrogenation of olefins, hydroformylation, cyclopropanation, and oxidation.258 Dendrimers with redox-active cores have been proposed as promising materials for miniaturized information-storage circuits.259... 

Polymer-stabilized palladium nanoparticles (or nanoclusters) [125-127] have recently received increasing attention in the field of synthetic organic chemistry [128, 129]. Thus, for example, the poly(iV-vinyl-2-pyrrolidone) (PVP)-supported Pd particle catalyzed the Suzuki-Miyaura coupling in water [130]. Poly(amidoamine) (PAMAM) dendrimer-encapsulated palladium nanoparticles were designed and prepared to provide highly selective catalysts for hydrogenation of olefins [131-133]. Hyperbranched aromatic amides (aramids) and PS-DVB-methacryloylethylenesulfonic acid resin have also been... 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

The same catalyst precursor PPI(G2)-[(diphosphine)PdMe2)i5 was employed by Reetz and co-workers for Heck reactions [9]. By addition of diethyl ether the polymer-bound catalyst could be precipitated and isolated by filtration. Upon its repeated use for catalysis, a slight decrease in activity was observed. By contrast to analogous low molecular-weight catalysts that were not polymer-bound, no formation of palladium black was observed with the dendrimer-bound catalyst. 

Dendrimer-encapstdated catalysts are another area of active research for polymer-supported catalysts. The nanoparticles are stabilized by the dendrimers preventing precipitation and a omeration. Bimetallic nanoparticles with encapsulated metals (dendrimer-encapsulated catalyst DEC) from commercially available fourth-generation PAMAM dendrimers and palladium and platinum metal salts were prepared via reduction by Crooks and co-workers [34], following previous work in this area [35], The simultaneous incorporation of Pt and Pd reflects the concentrations in solution. The bimetallic DECs are more active than the physical mixture of single-metal DEC [35, 36] in the case of the hydrogenahon of allyl alcohol in water, with a maximum TOP of 230 h compared to TOP = 190 h obtained for monometallic palladium nanoparticles (platinum TOP = 50 h ). 

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