Merging and bifurcation of 4+2 and 2+4 cycloaddition modes in the archetypal dimerization of butadiene. A case of competing bispericyclic and diradical paths

The dimerization of butadiene has been explored by using DFT methods at the B3LYP level with the 6-311+G** basis set. A concerted bispericyclic TS for the endo pathway and a concerted pericyclic TS for the exo pathway are the lowest passes for the dimerization and occur at almost the same energy thus accounting for the lack of stereochemical preferences in the dimerization. Diradical paths involving two unswitched transoid butadiene moieties are competing and account for the formation of minor amounts of trans-1,2-divinyl cyclobutane and 1,5-cycloctadiene.