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.