Solid support catalysts aryl iodide coupling

Another successful approach to catalyst immobilisation involves attachment of the carbene precursor to a peptide on solid support. Treatment with base generates the corresponding carbenes that undergo in situ complexation to Pd(ll) centres (Scheme 6.33). Again, the main drawback of this approach was the low reactivity of the catalytic system that only allowed the coupling of aryl iodides and bromides [116], The reasons for this outcome are in need of further studies. 

Three-dimensional aminomethyl-polystyrene can act as an insoluble solid support for an aminomethylphosphine palladium complex, and this enabled the Sonogashira coupling of aryl iodides with a catalyst loading of 1.6 mol%. Moreover, the catalyst could be recycled four times without any loss of activity [48]. 

In a completely different approach to reactions on a solid support, Villemin and Caillot [70] ground the catalyst Pd(OAc)2 with KF on alumina and used the support thus formed to carry out coupling in the absence of solvent using microwave heating these included reactions involving aryl iodides and either tetramethyltin or tributylvinyltin, and the organotin by-product remained on the support. 

Another solid-support Suzuki coupling is the hydroboration of vinyl polystyrene with 9-BBN in THF to provide [29] polymer supported borane. This borane is coupled to give different aryl, vinyl, or alkyl iodides. Pd(OAc)2 is employed as the catalyst for aryl iodides and PdCl2(dppf) in other cases to yield the coupled adduct in 55-85% yield (Scheme 31.7) on the basis of the mass of the product cleaved or by comparison with IR integration to standards. 

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