Heteroannulation Route to β- and γ-Carbolinones from Indolecarboxamides

Significance

Reported is the palladium-catalyzed dehydrogenative annulation of indole-2- and 3-carboxamides with symmetrical alkynes leading respectively to β- and γ-carbolinones. The reaction is carried out using Pd(OAc)₂ as a catalyst and TBAB as a base at 50 ˚C under air atmosphere for β-carbolinones, whereas it requires a 100 ˚C heating under O₂ atmosphere for the
γ-ones. Free (NH)-indoles do not undergo this ­reaction. Both primary and a variety of alkyl-­substituted amides were successfully coupled with symmetrical alkyl and aryl acetylenes. When unsymmetrical alkynes were used, better regio­selectivities (79-89%) were obtained for the β-carbolinones than for the γ-carbolinones (57-67%). A mechanism was suggested without evidence.