Efficient nickel-catalyzed [2 + 2 + 2] cycloaddition of CO2 and diynes.

A mild and general route for preparing 2-pyrones from CO2 and diynes is described. Under only 1 atm of CO2, excellent yields of pyrone are obtained using catalytic amounts of Ni(COD)2 and imidazolylidene ligand, IPr. In addition, stoichiometric reactions between the isolated complex Ni(IPr)2, diynes, and CO2 suggest that the pathway involves initial reaction with CO2 as the key step.