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

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 beta-position with simple alkynes occurs to give cyclobutenes. The bulky group at the beta-position can suppress the insertion of either an enone to give a cyclohexene or an allcyne to give a cyclohexadiene.