Pyrroloindolone Synthesis via a Cp*CoIII-Catalyzed Redox-Neutral Directed C-H Alkenylation/Annulation Sequence

A unique synthetic utility of a Cp*Co-III catalyst in comparison with related Cp*Rh-III catalysts is described. A C2-selective indole alkenylation/annulation sequence proceeded smoothly with catalytic amount of a [Cp*Co-III(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*Co-III complex and those of Cp*Rh-III complexes, highlighting the unique nucleophilic activity of the organocobalt species. The Cp*Co-III 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*Co-III catalysis with the aid of an acetate unit as well as evaluation of the difference between organo-Co-III species and organo-Rh-III species are also described.