Supported Palladium Catalysts in Other Coupling Reactions

Supported Palladium Catalysts in Other Coupling Reactions 

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]. 

Several authors successfully demonstrated the potential of micro- and meso-porous silica materials as well as zeolites as carriers. Corma et al. anchored car-bapalladacycles on high surface area inorganic supports and converted aryl 

The rather classic catalyst palladium on activated carbon has been applied by Sun and Sowa et al. [183] and Heidenreich et al. [158] without additional ligands. This simple system was able to convert (mainly activated) aryl chlorides in mixtures of water and an organic solvent (DMA and NMP, respectively). Lysen et al. were able to convert aryl chlorides in pure water and without addition of any ligand [184,185]. Suzuki reactions using Pd/C in an aqueous medium were also reported by Arcadi et al. (in the presence of surfactants) [186] and the group of Leadbeater, who applied microwave techniques [187]. Microwave (as weU as ultrasound) conditions were also employed by Cravotto and Palmisano et al. [188] The substrate scope of Suzuki-type reactions in the presence of Pd/C was extended to halopyri-dines and haloquinoUnes by Tagata and Nishida [189]. 

The majority of the papers on Suzuki and Sonogashira reactions catalysed by supported palladium do not focus on mechanistic aspects and the corresponding contributions are thus limited in this respect Nevertheless, it is reasonable to assume from the limited number of such investigations that the principal conclusions from detailed mechanistic studies of Heck reactions by supported catalysts are valid also for this group of coupling reactions the active species is Pd leached into solution during the reaction and re-deposited at the end (see next section). 

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