Theoretical investigation of cheletroptic decarbonylation reactions

In this study, B3LYP is used to calculate the decarbonylation reactions of the bicyclo-[2.2.1]hepta-2,5-dien-7-one (7-norbornadienone, 1) and its related extended fused aromatic analogues 2-5. On the basis of our results, all of the reactions tend to proceed synchronously to expel CO, forming the corresponding aromatic hydrocarbons. It is found that the more exothermic the reaction is, the less of a reaction barrier it needs to overcome. Moreover, upon a decrease of the reaction exothermicity, the structure of the transition state is farther away from the reactant, and the reaction barrier increases. The results agree well with the Hammond postulate as well as the Bell-Evans-Polanyi principle. Studies predict an activation energy of 27.83 kcal/mol for 5, so that the production of pentacene from compound 5 might proceed at elevated temperatures such as 400 K.