Nickel‐Catalyzed Hydroborylative Polycyclization of Allenynes: an Atom‐economical and Diastereoselective Synthesis of Bicyclic 5‐5 Fused Rings

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 sigma-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.