Pyrroloindolone Synthesis via a Cp*Co^III-Catalyzed Redox-Neutral Directed C–H Alkenylation/Annulation Sequence

A unique synthetic utility of a Cp*CoIII catalyst in comparison with related Cp*RhIII catalysts is described. A C2-selective indole alkenylation/annulation sequence proceeded smoothly with catalytic amount of a [Cp*CoIII(C6H6)]­(PF6)2 complex and KOAc. Intramolecular addition of an alkenyl-Cp*Co species to a carbamoyl moiety gave pyrroloindolones in 58–89% yield in one pot. Clear difference was observed between the catalytic activity of the Cp*CoIII complex and those of Cp*RhIII complexes, highlighting the unique nucleophilic activity of the organocobalt species. The Cp*CoIII catalysis was also suitable for simple alkenylation process of N-carbamoyl indoles, and broad range of alkynes, including terminal alkynes, were applicable to give C2-alkenylated indoles in 50–99% yield. Mechanistic studies on C–H activation step under Cp*CoIII catalysis with the aid of an acetate unit as well as evaluation of the difference between organo-CoIII species and organo-RhIII species are also described.