Thermal Decomposition Kinetics of Dicyclopentadiene‐1,8‐dione: The Reaction Path through Quantum Chemical Calculation

Thermal decomposition kinetics of dicyclopentadiene‐1,8‐dione 7 implied an intramolecular competition between α,β‐ and β,γ‐double bond to assist the CO elimination. Experimental thermolysis of 7 in dioxane gave 3a,7a‐dihydro‐1H‐inden‐1‐one (cis‐bicyclo[4.3.0]nona‐2,4,7‐triene‐9‐one), CO gas, and a very small amount of indanone. This result suggested β,γ‐double bond favored the extrusion of CO gas. Calculations of several density functional theory (DFT) levels of theory and CBS‐QB3 method were employed. Two mechanisms were considered: a one‐step concerted pathway and a stepwise mechanism involving [1,3] and [1,5] hydrogen sigmatropic migrations. The CAM‐B3LYP/6‐31G(d,p) calculation reasonably agrees with the experimental kinetic parameters. The mechanism appears to be unimolecular in one step concerted through a five‐membered cyclic transition state. Isomerization of product cis‐bicyclo[4.3.0]nona‐2,4,7‐triene‐9‐one yielding 1‐indanone is presented and described. Calculation from substrate 7 may explain in a similar way the mechanism of decomposition of compounds 1‐6 . The present work may well promote to the possibility of carrying out experimental research works on the thermal decarbonylation kinetics in a liquid solution and in the gas phase of β,γ‐unsaturated aliphatic ketones.