Palladium Catalysts [Cross-coupling Reaction using Transition Metal Catalysts]

(2) Higher activity palladium catalysts
　Generally, cross coupling reactions are promoted by using electron-rich and sterically hindered ligands. For example, palladium catalysts when coordinated by tertiary alkyl phosphines such as the tert-butyl group, cyclohexyl group and so on are used, show high catalytic activities in cross-coupling reactions. They are effective to use when low reactive aryl chlorides or sterically hindered aromatic halides are employed as reagents. The monomer state of sterically hindered alkyl phosphines is chemically unstable, but they are stabilized by coordinating them with palladium complexes.

　Recently, N-heterocarbene (NHC) ligands have been used for cross-coupling reactions because they activate palladium catalysts more effectively than tertiary alkyl phosphines. Palladium-NHC complexes show not only high activity but also high structual stability. In addition, they keep their complex formation without decomposition even after work-up, which is an advantage because it makes removing these complexes easy.

(3) Palladacycle catalysts
　In some cases the active palladium species can’t be efficiently prepared by the combination of palladium complexes and ligands. To solve this problem, palladacycle catalysts have been developed and used for more efficient preparation of active palladium species. As a property, they easily release the coordinated ligand with treatment of some bases, and afford the related zero-valent active palladium species. On the other hand, they form structurally stable complexes caused by their cyclic structures constructed of the ligand with intramolecular coordinating ability and the palladium species. In addition, their catalytic ability is improved by the addition of bases and they show high turnover frequency.