## 6 Palladium
### Catalysts Applicable Towards Several Cross-Coupling Reaction Types  
Several catalysts have been developed for more than one cross-coupling reaction; a brief overview of these will be given here. [PERSON] and co-workers have developed plugs where functionalized resin is embedded within high-density polyethylene, to afford polymer-bound reagents in a format more easy to handle than the classical polymer beads [63]. In a more recent report, aminomethylstyrene resin plugs were treated with palladium(II) acetate; the loaded Pd(II) was subsequently reduced to Pd(0) using hydrazine and finally entrapped into the resin via cross-linking with succinyl chloride, to form a catalyst with the structure shown in Figure 2 (structure **30**) [64]. Analysis of the plugs with a microscope revealed that smaller palladium nanoparticles were found at the edge of the beads with the bead size increasing towards the middle of the bead. The plugs were subsequently evaluated in three different cross-coupling reactions, i.e. the Suzuki, Heck and Sonogashira reactions, mainly involving aryl iodides, with good results. Attempted coupling of an aryl bromide in the Heck reaction was less successful. The plugs could be recycled up to three times without any major loss of activity. [PERSON] _et al_. applied polystyrene-supported diarylphosphine palladium complexes **31** in the same three cross-coupling reactions, also with success [65]. Best results were obtained when the aromatic substituent was _ortho_-tolyl (Sonogashira, Suzuki) or _meta_-tolyl (Heck). The recyclability of the catalysts was found to be excellent and the activity was similar to that of palladium(0) tetrakistriphenylphosphine in solution. Figure 2: Catalysts developed for several types of cross-coupling reactions. [PERSON] _et al._ have performed Suzuki couplings as well as copper-free Sonogashira reactions in water using a palladium(II) azo complex tethered to polystyrene (Figure 2, structure **32**) [66]. The use of potassium carbonate as the base was found to be important for the yield, and after optimization both aryl bromides and aryl iodides could be applied in the two reactions with good results. Aryl chlorides only gave coupling product in the Suzuki reaction, however. [PERSON] and [PERSON] have prepared a rather unusual class of supported palladium catalysts, where a ligand system incorporating a pyridine ring as well as one or two _N_-heterocyclic carbene moieties are attached to a PEGA-resin (a polyethylene glycol dimethylacrylamide copolymer) using a dipeptide tether and a Rink amide linker (monocarbene ligand shown in Figure 2, see structure **33**) [67]. The catalytic system **33** was stable enough to allow Suzuki-coupling to be carried out in water at 50-65 \({}^{\text{o}}\)C for 6 h. Only aryl iodides and bromides were used as substrates. The corresponding dicarbene catalyst was cleaved off the resin before application, and employed in microwave-mediated Suzuki coupling, allowing the reaction time to be reduced to 10 min, albeit at a higher reaction temperature (90-100 \({}^{\text{o}}\)C). Catalyst **33** was also used in the coupling of aryl iodides with terminal alkynes, i.e. the Sonogashira reaction, with good results. The chirality of the tether was not exploited in this work, but opens up the possibility of investigating this class of ligands in asymmetric metal-catalyzed transformations. [PERSON], [PERSON] and co-workers have exploited palladium complex **34**, with an \(N\),_S_-donor ligand, in Suzuki and Heck reactions, comparing different types of support materials [68]. Aryl bromides and aryl iodides functioned well in both reactions, irrespective of the support used, _i.e._ Merrifield resin, Wang resin or a ground macroporous polymer monolith material, with the Wang resin providing the highest turnover numbers (up to 19950). The use of aryl chlorides afforded very little product, however.