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The study of coupling reactions using organopalladium complexes has been advanced as well as that using organonickel complexes. Generally, the reactivity of organopalladium complexes is lower compared with organonickel complexes. However, they have higher chemical stability for oxidations and this makes them easy to use. Therefore, palladium complexes are most commonly used for cross-coupling reactions.
(1) Palladium complexes generally used in cross coupling reactions Palladium-phosphine complexes are well used in cross-coupling reactions and commercially available. Pd(PPh3)4 and PdCl2(PPh3)2 are used without any treatment. Also, palladium catalysts are used prepared in situ from the precursors such as Pd(OAc)2 and Pd2(dba)3·CHCl3, with appropriate amounts of phosphines. When using divalent palladium complexes such as Pd(OAc)2, once they are reduced to zero-valent palladium species by some organometallic reagents, phosphines and amines, then their catalytic reactions can start.
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(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.
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