Synthesis of the covalent benzene-carbon monoxide cycloadduct, norborna-2,5-dien-7-one. Correlation of kinetic and thermodynamic stabilities in cycloreversion reactions

endo-Tricyclo[4.2.1.0^2,5]non-7-ene-3,4,9-trione (14) is prepared from the 5,5-diethoxy-cyclopcntadiene-diethyl maleate Diels-Alder adduct. Photolysis of the trione in CD₂Cl₂ solution at 192 K forms norborna-2,5-dien-7-one (2). The latter substance is thermally unstable and decomposes to benzene and CO with ΔG‡ =15 kcal mol^-1. This appears to be the lowest activation energy for a cycloreversion yet measured. A correlation is found for the kinetic and thermodynamic stabilities in a series of 18 orbital symmetry allowed cycloreversions by plotting ΔG‡ vs ΔH_r.