Synthesis of 5-Azaindolizine Derivatives by the Palladium-Catalyzed Intermolecular Formal [3+2] Cycloaddition of Alkylidenecyclopropanes with 1,2-Diazines

The palladium-catalyzed formal [3+2] cycloaddition reaction of alkylidenecyclopropanes with 1,2-diazines proceeded smoothly to give the corresponding 5-azaindolizine derivatives in good to allowable yields. For example, in the presence of 5 mol % of Pd(PPh₃)₄, the reaction of 1-propyl-hexylidenecyclopropane with phthalazine or with pyridazine proceeded at 120 °C without solvent, and the corresponding 2-(1-butylpentyl)pyrrolo[2,1-a]phthalazine or 6-(1-butylpentyl)pyrrolo[1,2-b]pyridazine was obtained in 61% or 49% yield, respectively.