DIELS-ALDER DIMERIZATION OF 1,3-BUTADIENE - AN AB-INITIO CASSCF STUDY OF THE CONCERTED AND STEPWISE MECHANISMS AND BUTADIENE ETHYLENE REVISITED

Ab initio CASSCF calculations with the 3-21G basis set have been used to locate transition structures for concerted and stepwise mechanisms of the Diels-Alder and [2 + 2] dimerizations of 1,3-butadiene. The butadiene-ethylene reaction was also studied with six-orbital/six-electron CASSCF calculations and the 3-21G and 6-31G* basis sets. The CASSCF calculations appear to slightly overestimate the stabilities of biradicals relative to concerted mechanisms, based upon comparisons with experimental data and expectations about correlation energies of different species. Energies were also evaluated with QCISD(T)/6-31G* calculations. All the theoretical results taken together lead to the conclusion that the transition structure for the butadiene-ethylene reaction leading to the biradical intermediate is 5 kcal/mol above the concerted synchronous transition structure. The transition states leading to various isomeric diradicals are slightly lower in energy than the concerted transition structures for the butadiene dimerization. The barriers to cleavage and closure of biradicals increase as the radical stability increases. Thus, the cleavage and closure barriers calculated at the CASSCF/3-21G level are 4-5 and 1-2 kcal/mol, respectively, for the biradical formed from two butadienes, as compared to 2 and <1 for the biradical intermediate formed in the butadiene-ethylene reaction. This contrasts to the ethylene dimerization, for which Doubleday et al. found no potential energy barrier for the 1,4-butanediyl intermediate to undergo either cleavage or ring closure.