Aryl fluorides, Kumada reaction

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

Recently, nickel-based catalysts have featured more prominently in coupUng reactions of aryl fluorides. In 2001, Herrmann was able to demonstrate Kumada coupUngs of aryl fluorides, including electron-rich aryl fluorides, using the tetra-fluoroborate salt of the N-heterocyclic carbene IPr as the supporting ligand for nickel (Equation 2.69) [113]. 

Herrmann and co-workers reported the activation of aryl C-F bonds by [(NHC)Ni] systems under Kumada-Corriu-type coupling conditions. Screening of catalytic conditions revealed that yields with [(IPr)2Ni] were poorer than when the catalyst was generated in situ from [Ni(acac)2]/IPr-HBF4. Interestingly, the electronic influence of the aryl fluorides on the reaction rate as demonstrated by Hammett ct -values indicated different pathways for the NHC-modifled catalytic system versus the analogous Ni-phosphine systems. 

Ni(0) carbenes are also effective for the Kumada-Corriu coupling reaction between aryhnagnesium reagents and aryl chlorides or fluorides in ca. 35 catalytic turnovers (equation 5). The yields are generally high even at room temperature (ca. 30 turnovers over 18h) only when the aryl chloride is congested (substituents ortho to Cl) does the yield suffer. It is noteworthy, however, that very similar yields were obtained when the N-heterocarbene ligands were replaced by t-BusP. 

---