## 6 Palladium
### Suzuki Coupling  
There is a plethora of methods for performing the Suzuki reaction on solid phase, either using polymer bound catalysts or with the substrate tethered to a support, thus only a few more recent methods will be mentioned here. Several groups have targeted the problem of coupling aryl chlorides with boronic esters, the chlorides in general being more problematic than the corresponding aryl iodides or bromides, but of interest as coupling partners due to their lower price and easier availability. Scheme 17 shows some of the more recent polymer supported catalysts developed for this purpose. Air-stable palladium complex **20** (Scheme 17) was prepared by [PERSON] and co-workers and applied in the formation of biparlic compounds using as little as 0.4 mol% palladium in the reaction [40,41]. Reaction conditions involved the use of cesium fluoride as the base and reflux in toluene containing a small amount of water for 20 hours. The catalyst could be recycled up to seven times without any loss of efficiency. [PERSON] and colleagues instead applied a carbene bound to macroporous polystyrene as the ligand to palladium in similar coupling reactions, albeit with somewhat different reaction conditions [42]. When complex **21a** and phenylboronic acid were heated together with either _ortho_- or _para_-substituted aryl chlorides in a 2:1 mixture of dimethylformamide and water, using potassium carbonate as the base, the desired coupling products were produced in reasonable yields. The methodology was also applicable to aryl iodides and aryl bromides. A related polymer bound palladium species was reported by [PERSON] _et al._, who prepared a soluble silver carbene polymer that could be used as a carbene transfer reagent for the preparation of catalyst **21b**, also for application in the Suzuki coupling of aryl chlorides [43]. By using isopropanol as the solvent, the polymeric support was rendered non-soluble. Performing the reaction at 80 \({}^{\circ}\)C with sodium _tert_-butoxide as the base afforded biparl products in high yields after 8-18 h. A slightly different approach was taken by [PERSON] and [PERSON], who prepared a main-chain NHC-palladium polymer (**22**) where the palladium catalyst is incorporated into the polymer backbone itself [44]. The Suzuki reaction of both activated and deactivated aryl chlorides could be performed in water at 80-90 \({}^{\circ}\)C with as little as 0.05 mol% of catalyst **22**, albeit with rather long reaction times. An aryl fluoride was also coupled successfully using this method. **Scheme 17. ** Polymer-supported palladium catalysts developed for the Suzuki coupling of aryl chlorides with boronic acids. Continuing on the topic of Suzuki reactions in water using supported catalysts, [PERSON] and [PERSON] used a salen-palladium complex attached to Merrifield resin (**23**, Figure 1) for the Suzuki reaction of aryl bromides with phenylboronic acid under aqueous conditions [45]. The reaction conditions were optimized carefully and a 30:1 mixture of water and toluene was found to be the optimal solvent, with a reaction time of 24 h and heating to 90 \({}^{\circ}\)C affording coupling products in high yields. Notable is that no phosphine ligand is required for this reaction and very little leaching of palladium takes place (less than 1 ppb). [PERSON] and [PERSON] applied palladium chloride attached to polymer-supported diphenylphosphine **24** in the coupling of sodium tetraphenylborate with various aryl bromides using water as the solvent in conjunction with focused microwave irradiation [46]. Potassium carbonate was found to be the optimum base, and surprisingly enough, the use of only water as the solvent gave better results than various mixtures of water and organic solvents (benzene, toluene, ethanol). The catalyst loading in the reaction corresponded to 1 mol% palladium, and the catalyst could be recycled up to ten times without any deterioration of its performance.