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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 β‐hydrogen elimination. The trans arrangement of alkenyl and hydride ligands is key to allow formation of the second carbocycle. [ABSTRACT FROM AUTHOR]

A diastereoselective synthesis of borylated bicyclic 5‐5 fused ring systems by a domino Ni‐catalyzed hydroborylative polycyclization of allenynes has been developed. The reaction provides two new C−C and one C−B bonds and constitutes an atom‐economical method. It occurs rapidly, in absence of inert atmosphere, with an inexpensive Ni‐based catalyst and HBpin as boron source. The procedure showed excellent tolerance to functional groups and a wide scope. Derivatization of the borylated bicycles has been performed to explore the synthetic utility of the products. Experimental and DFT‐calculation results suggest that the process starts by an oxidative cyclometallation involving Ni(0) and both the allene and the alkyne moieties, followed by a stereoselective σ‐metathesis with HBpin. Formation of the second ring takes place through a non‐fully coplanar 1,2‐insertion into the Ni−H bond to give a nickelacyclohexene which finally evolves by reductive elimination. [ABSTRACT FROM AUTHOR]