Polymer-supported Stille chemistry

Brody and Finn [58] used a polystyrene-bound aryltin in a biaryl synthesis under catalysis by a new palladacyde complex. 

The polymer support does not necessarily have to be insoluble in the reaction medium. Thus, Sieber et al. [67] used poly(ethyleneglycol) as a soluble polymer matrix an iodobenzoate moiety linked to the support was allowed to react with tributylphenyltin to determine optimum conditions ((Ph3P)2PdCl2 (10%)/LiCl (10equiv.)/DMF/80°C). Precipitation into ether allows the removal of side products, excess reagents, and organotin by-products, leaving the polymer, which is recovered in 99% yield. Similar work has been carried out in an aqueous medium [68]. 

Coupling on a polymeric support can of course readily be applied to library building by combinatorial techniques. Several groups have reported work of this type thus Havranek and Dvofak [69] have carried out repeated coupling of 3-substituted 3-(tributylstannyl)allyl alcohols with substrates linked to a TentaGel S OH resin to obtain a 21 x 21 library of skipped dienes and a 21 x 21 x 21 library of skipped trienes. 

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. 

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Under certain condition, however, reactions are still preferably conducted in solution. This is the case e.g., for heterogeneous reactions and for conversions, which deliver complex product mixtures. In the latter case, further conversion of this mixture on the solid support is not desirable. In these instances, the combination of solution chemistry with polymer-assisted conversions can be an advantageous solution. Polymer-assisted synthesis in solution employs the polymer matrix either as a scavenger or for polymeric reagents. In both cases the virtues of solution phase and solid supported chemistry are ideally combined allowing for the preparation of pure products by filtration of the reactive resin. If several reactive polymers are used sequentially, multi-step syntheses can be conducted in a polymer-supported manner in solution as well. As a further advantage, many reactive polymers can be recycled for multiple use. 

By replacing insoluble cross-linked resins with soluble polymer supports, the well-estabhshed reaction conditions of classical organic chemistry can be more readily apphed, while still fadhtating product purification. However, soluble supports suffer from the hmitation of low loading capacity. The recently introduced fluorous synthesis methodology overcomes many of the drawbacks of both the insoluble beads and the soluble polymers, but the high cost of perfluoroalkane solvents, hmitation in solvent selection, and the need for specialized reagents may hmit its apphcations. 

Current developments in combinatorial chemistry and rapid screening techniques promote new systems in the field of polymer-supported catalysts (cf. Section 3.1.3) [279]. A drawback for this technology is still the leaching tendency of the catalyst which is clearly shown in recycling experiments (cf. Section 3.1.1.3) [280]. Polycondensation of organo-functionalized silanes and polysiloxanes leads to covalent support of catalyst on the surface, also known as the sol-gel process [281-283]. Van Leeuwen and co-workers reported a phosphine ligand with a xanthene backbone which was functionalized with a propyltrialkoxysilane... 

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