Nickel-catalyzed [2 + 2] Cycloaddition Reaction of Bulky Enones with Simple Alkynes. The Effect of Bulkiness of Substituent Attached at β-Carbon

Abstract In the presence of a catalytic amount of [Ni(cod)2] and PCy3, the [2 + 2] cycloaddition of enones bearing a bulky substituent group at the β-position with simple alkynes occurs to give cyclobutenes. The bulky group at the β-position can suppress the insertion of either an enone to give a cyclohexene or an alkyne to give a cyclohexadiene.