Easily Recycled Coupling Catalysts

A new family of phosphane ligands could help bring down the high cost of palladium-catalyzed coupling reactions. The electron-rich phosphane ligands used in such transformations are often made via costly multistep syntheses, so finding a way to easily reuse them would be a boon to coupling chemists. Matthias Beller and colleagues at the Leibniz Institute for Catalysis at the University of Rostock, in Germany, have developed recyclable imidazolium phosphanes that work well in palladium-catalyzed C–O, C–N, and C–C bond-forming reactions (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201001787). The sterically hindered ligands are insoluble in 1,4-dioxane at room temperature, but their cationic nature drives the formation of a soluble complex at 100 °C. In the case of palladium-catalyzed hydroxylation, Beller’s group found that this solubility helps the complex remain in solution, ready to be reused, once the phenolate reaction products are removed via precipitation. In the case of Sonogashira coupling, where the product was soluble in 1,4-dioxane, the ligand precipitated out in the reaction flask by simply cooling it down. Beller notes that the ligands are stable to air and moisture and can be recycled several times without significant loss of reactivity.