Synthesis of Isoindolo[2,1- a ]indoles by the Palladium-Catalyzed Annulation of Internal Alkynes

A wide variety of substituted isoindolo[2,1- a ]indoles have been prepared in good to excellent yields by the palladium-catalyzed annulation of internal alkynes by imines derived from o -iodoanilines and benzaldehydes. The mechanism of this interesting annulation process appears to involve (1) oxidative addition of the aryl iodide to Pd(0), (2) alkyne insertion, (3) addition of the resulting vinylic palladium intermediate to the C−N double bond of the imine, (4) either electrophilic palladation of the resulting σ-palladium intermediate onto the adjacent aromatic ring originating in the internal alkyne or oxidative addition of the neighboring aryl carbon−hydrogen bond, and (5) reductive elimination of the tetracyclic product and Pd(0).