Enediyne dimerization vs Bergman cyclization.

High-level quantum chemical calculations reveal that the dimerization of enediynes to 1,3-butadiene-1,4-diyl diradicals is energetically more favored than the corresponding Bergman cyclization of enediynes. Moreover, the activation barrier of both reactions can be drastically reduced by the introduction of electron-withdrawing substituents like fluoro groups at the reacting carbon centers of the triple bonds.