Ni(II)-Catalyzed Intramolecular C–H/C–H Oxidative Coupling: An Efficient Route to Functionalized Cycloindolones and Indenoindolones

Nickel­(II)-catalyzed intramolecular
C­(sp2)–H/C­(sp3)–H and C­(sp2)–H/C­(sp2)–H oxidative couplings
in indoles are achieved via chelation
assistance. These reactions provide access to biologically relevant
five- and six-membered substituted cyclopentaindolones, carbazolones,
and indenoindolones in high yields and good chemoselectivity employing
an air-stable and defined nickel catalyst, (bpy)­Ni­(OAc)2. The oxidative cyclizations proceeded either through a six-membered
or an unconventional seven-membered nickelacycle. An extensive mechanistic
investigation by experiments and theoretical calculations revealed
a facile indole’s C(2)–H nickelation and a rate-limiting
reductive elimination process. This intramolecular oxidative cyclization
operates via a probable Ni­(II)/Ni­(III) pathway involving single-electron
oxidation of nickel without the participation of a carbon-based radical.

二价镍催化的吲哚分子内C(sp²)-H/C(sp³)-H与C(sp²)-H/C(sp²)-H氧化偶联反应通过螯合辅助策略实现。该类反应以空气稳定且结构明确的(bpy)Ni(OAc)₂为催化剂，能够以高收率与良好的化学选择性得到具有生物相关性的五元、六元取代环戊并吲哚酮、咔唑酮及茚并吲哚酮类化合物。氧化环化过程可通过六元环或非常规七元环镍杂环中间体进行。通过实验与理论计算相结合的深入机理研究表明，吲哚C(2)-位的C-H镍化过程易于发生，而还原消除步骤为整个反应的决速步。该分子内氧化环化反应可能遵循Ni(II)/Ni(III)催化循环路径，涉及镍中心的单电子氧化，且全程无碳基自由基参与。