Energy barriers to the Diels–Alder cycloadditions and cycloreversions of cation-radicals in the gas phase

Abstract

The mechanism of the retro-Diels–Alder reaction of substituted bicyclo[2.2.1]hept-2-ene cation-radicals can be related to the energetics of the fragmentation. Concerted synchronous or asynchronous mechanisms are operative in reactions which occur at their thermochemical thresholds. The reverse process, the Diels–Alder reaction of a cation-radical with a molecule, has a negligible activation energy for symmetrical and slightly unsymmetrical systems, [(Z)-1,2-dihydroxyethene]⁺˙+ cyclopentadiene, [cyclopentadiene]⁺˙+ vinyl alcohol, [(E)-1-hydroxybuta-1,3-diene]⁺˙+ cyclopentadiene, and [(Z)-1-hydroxybuta-1,3-diene]⁺˙+ cyclopentadiene. The retro-Diels–Alder decomposition of the highly unsymmetrical 2-vinylbicyclo[2.2.1]hept-5-en-2-ol cation-radical proceeds in two steps and overcomes a barrier of 58–67 kJ mol^–1 above the thermochemical threshold. The effect of bond-dissociation energies on the reaction mechanism is discussed.