Nickel-Catalyzed Cycloisomerization of 1,5-Allenynes

We report the Ni-catalyzed cycloisomerization of 1,5-allenynes. Substrates containing terminal alkynes afford cyclopentene derivatives, whereas internal alkynes lead to the formation of two consecutive C-C bonds to give fused 5-5 bicyclic compounds. The reaction shows a wide scope. Experimental and computational mechanistic studies suggest a Ni(0)-Ni(II) catalytic cycle. The reaction starts with an oxidative cyclometallation followed by a beta-hydrogen elimination. The trans arrangement of alkenyl and hydride ligands is key to allow formation of the second carbocycle. image