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