Regioselective [2+2+2] Cycloaddition Reaction Using Allene-ynes with Simple Allenes under Nickel Catalysis

A Ni-catalyzed [2+2+2] cycloaddition reaction between allene-ynes and various mono-, di- and tri-substituted allenes is described. This protocol effectively differentiates allenyl pi components and shows broad substrate generality to give highly functionalized carbocycles. A DFT study played a key role in revealing a detailed reaction mechanism that controls site-, regio- and stereoselectivity, which are thought to originate in the substituent effect on pi-bonds in the early transition state.