Palladium(0)-catalyzed tandem cyclization of allenenes: direct construction of tricyclic heterocycles through aromatic C--H activation.

Palladium(0)-catalyzed tandem cyclization of allenenes is described. Treatment of allenenes with an aryl halide, potassium carbonate, and catalytic [Pd(PPh(3))(4)] in dioxane afforded tri- or tetracyclic heterocycles in moderate to good yields through insertion of arylpalladium(II) halide into the allenic moiety, intramolecular carbopalladation, and aromatic C--H bond activation. The substituent on the olefin terminus has proven to be essential for the success of the tandem cyclization. The reaction with heterocyclic aryl halides such as iodopyrazine or 4-bromo-1-methylindole afforded tri- or tetracyclic heteroaromatic products in good yields.