Ring expansion in methylcyclopentadiene radicals. Quantum chemical and kinetics calculations

Ring expansion processes in 1-, 2-, and 5-methylenecyclopentadiene radicals and isomerizations among the three isomers were studied by the Becke three-parameter hybrid method with Lee-Yang-Parr correlation functional approximation (B3LYP). Structure, energy, and frequency calculations were carried out with the Dunning correlation consistent polarized double ζ (cc-pVDZ) and augmented aug-cc-pVDZ basis sets. The potential energy surfaces for ring expansion in methylenecyclopentadiene radicals consist of several intermediates and transition states. The process that takes place by insertion of the methylene group into the cyclopentadiene ring in the three isomers occurs via two principal mechanisms. One mechanism is associated with cleavage of the five-membered ring of the cyclopentadiene ring. In the second mechanism, the transition states of the first stage consist of a newly formed three-membered ring fused to the original cyclopentadiene ring. In all the three isomers of methylenecyclopentadiene, the reaction pathways leading to ring expansion include intermediates that via additional transition states lead to the production of the cyclohexadienyl radical. The latter, by a very fast H-atom ejection produces benzene. The structure and energetics of the various pathways are shown. The isomerization processes 1-methylenecyclopentadiene ⇔ 5-methylenecyclopentadiene and 2-methylenecyclopentadiene ⇔ 1-methylenecyclopentadiene involve a single 1,2-H atom shift and proceeds in one step without intermediates. The ring expansion in 1-methylenecyclopentadiene proceeds much faster via isomerization to 5-methylenecyclopentadiene than via direct formation of cyclohexadienyl.