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Organic Polymers, Oligomers, and Catalysis
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Cross-linked polystyrene is probably the most popular polymeric support for heterogenized catalysts. For instance, the chloromethylated cross-linked polystyrene, also named Merrifield resin, offers a convenient functionalization point. Thus, several ligands for designing homogeneous catalysts, including carbenes, xyliphos, and phosphines ligands were covalently immobilized on cross-linked polystyrene matrices. The corresponding Ru, Ir, and Pd catalysts were synthesized and their activity was tested for hydrogenation, ring closing metathesis and coupling reactions. Heterogenized Ru-based Grubbs and Hoveyda catalysts could reach comparable turnover numbers as their homogeneous counterparts for catalyzing various ring closing metathesis reactions [79]. However, no recycling could be performed, due to the degradation of the catalyst. Immobilization of iridium-based xyliphos complexes on polystyrene allowed catalyzing the asymmetric hydrogenation of imines [80]. In that case, the heterogenization process resulted in much less active catalysts than homogenous competitors, but a similar enantioselectivity of around 75% was obtained. Although several attempts for immobilizing palladium complexes on polystyrene have been performed, the formation of palladium nanoparticles after one catalytic run is suspected [81–86]. This type of polymer-supported nanoparticles is discussed later in this chapter.
Solid-phase synthesis of sulfur containing heterocycles
Published in Journal of Sulfur Chemistry, 2018
Merrifield resin (Figure 3) is a cross-linked polystyrene resin which contains a chloromethyl functional group. It is typically available as white beads. These beads are swelled in suitable solvents (dimethylformamide, ethyl acetate, dimethylsulfoxide) to displace the reagents with chloride substituents. Merrifield resin was synthesized by copolymerization of styrene and 4-vinylbenzyl chloride or by chloromethylation of polystyrene [26].