The mechanism of the retro-Diels-Alder reaction in 4-vinylcyclohexene cation radical

Butadiene cation radicals are produced symmetrically from the ring and side-chain of the vinylcyclohexene cation radical near the onset of the fragmentation. The appearance energies of C₄H₆^+? and C₄H₂D₄^+? from (3,3,6,6-D₄)vinylcyclohex ene were measured as 11.07 ± 0.05 and 11.06 ± 0.06 eV, respectively. This sets the barrier to retro-Diels-Alder decomposition at 1140 kJ mol^?1 above the energy of 1 and 44 kJ mol^?1 above the thermochemical threshold corresponding to C₄H₆^+? + C₄H₆. Topological molecular orbital calculations indicate that this lowest-energy path involves a sequential rupture of the C₃C₄ and C₅C₆ bonds, with a calculated barrier of 211 kJ mol^?1. The second, two-step reaction channel proceeds by subsequent fission of the C₅C₆ and C₃C₄ bonds with a barrier of 299 kJ mol^?1. This channel is found experimentally as a break on the ionization efficiency curve at 12.1 eV. Both the supra-supra and the supra-antara pericyclic reactions go through energy maxima and are therefore forbidden. The supra-supra process is the most favorable route for decomposition from the first excited state, the activation energy being 333 kJ mol^?1. The preference for the two-step mechanism is due to hyperconjugative stabilization of intermediate molecular configurations.