Microwave-assisted Palladium-catalyzed cyclization of allenyl indoles

As a part of our study directed to the development of intramolecular palladium-catalyzed procedure in obtaining polyheterocyclic systems, and to study the reactivity of indole derivatives, the present work is regarding the reactivity of N-allenyl 2-indolecarboxamides. Heteroannulation processes involving an allenyl functional group have been extensively studied in the past years, in particular allenamides represent a versatile and effective class of building blocks for generating nitrogen-containing heterocycles. The interest of our substrates is due to the presence of the allenyl group tethered to the indole nucleus which nitrogen atom should be suitable nucleophile to give intramolecular cyclization. The pathway proceeds with the involvement of a -allylpalladium intermediate, generated by carbopalladation of the allene moiety. The complex plays a fundamental role in terms of regio- and stereoselectivity. The subsequent step occurs following divergent outcome but in both cases the carbon in the  position of the 1,2-diene is involved. The reaction is totally regioselective giving only the exo-cyclized product. The key step, in the presence of an aryl iodide, consists of a carboamination process, to give imidazo[1,5-a]indole derivatives. The second strategy, without the intervention of the aryl iodide, consists of a new hydroamination reaction that proceeds exclusively under microwave irradiation. The alternative hypothetical product arising from the involvement of indole C-3 nucleophile to the -allyl complex is never observed.