Mixed NHC/Phosphine Ni(II) Complexes: Synthesis and Their Applications as Versatile Catalysts for Selective Cross-Couplings of ArMgX with Aryl Chlorides, Fluorides, and Methyl Ethers

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• 作者：Jie Zhang ; Jin Xu ; Yanchao Xu ; Hongmei Sun ; Qi Shen ; Yong Zhang
• 刊名：Organometallics
• 出版年：2015
• 出版时间：December 28, 2015
• 年：2015
• 卷：34
• 期：24
• 页码：5792-5800
• 全文大小：580K
• ISSN：1520-6041

文摘

New methods for the preparation of mixed NHC/phosphine Ni(II) complexes have been developed. It was shown that the quaternary ammonium cation in the easily available Ni(II) complexes [NEt₄][Ni(PPh₃)X₃] (X = Cl and Br) can act as a good leaving group in reactions of [NEt₄][Ni(PPh₃)X₃] with the bulky ItBu (ItBu = 1,3-ditertbutylimidazol-2-ylidene) or IPr [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] ligand, resulting in the corresponding mixed NHC/PPh₃ Ni(II) complexes Ni(PPh₃)(ItBu)X₂ (X = Cl, 1; X = Br, 2) or Ni(PPh₃)(IPr)Br₂ (3) in high yields. The PPh₃ ligand in these obtained mixed NHC/PPh₃ Ni(II) complexes can be easily substituted by a more electron-donating phosphine ligand, i.e., PCy₃, resulting in the corresponding mixed NHC/PCy₃ Ni(II) complexes Ni(PCy₃)(ItBu)Br₂ (4) and Ni(PCy₃)(IPr)Br₂ (5) in high yields. The crystal structures of these Ni(II) complexes have been characterized, which revealed a trans disposition of the NHC ligand to the phosphine ligand. The catalytic behaviors of them on varying the carbene ligand (ItBu vs IPr) as well as the phosphine ligand (PPh₃ vs PCy₃) were investigated in the cross-coupling of aryl Grignard reagents with a wide range of electrophiles. In addition to a significant synergic effect on their catalytic activities, high selectivity for the activation and transformation of C–Cl, C–F and C–O bonds was achieved based on the rational structural design. Complex 2 showed the highest catalytic activity for the cross-coupling of aryl chlorides and fluorides with aryl Grignard reagents, but exhibit little activity for the cross-coupling of aryl methyl ethers with aryl Grignard reagents. On contrast, complex 4 showed great potential for the aryl methyl ethers involved cross-coupling reactions, although its reactivity for the activation of the C–X bond is very poor. The difference in catalytic activity between 2 and 4 has been successfully employed to construct oligoarenes by selective cross-coupling reactions.