Theoretical study of the furfuryl benzoate and furfuryl acetate pyrolysis

In the present work, the pyrolysis reaction mechanism of both furfuryl benzoate and furfuryl acetate was evaluated at the M06/6-311++g(d,p) level. The uncommon methylenecyclobutenone compound and either the benzoic or the acetic acid were determined as the products of a multistep process consisting in two [3 + 3] rearrangements and a subsequent hydrogen α-elimination step, through a cyclic 5-membered transition state (TS), being the latter the rate-limiting step for both reactants. Furthermore, a deeper analysis on the basis of the reaction force formalism showed that the TS is formed in two stages: The first one is characterized by the weakening of the C─O bond, and the second one is where the H atom is transferred from the C atom to its nearest O atom. The H─O bond formation was determined to contribute the most to the electronic activity occurring during the TS formation as suggested by a reaction electronic flux analysis. Accordingly, natural bond orbital calculations showed that the most significant changes occur in the charge distribution of the O and H atoms. Finally, a negligible effect of the substituting group on the reaction was determined since similar activation energies were obtained for the pyrolysis of furfuryl benzoate and furfuryl acetate; however, a minor difference was evidenced in the reaction force results. In this sense, the structural contribution to the activation energy is larger than the electronic one for the furfuryl benzoate reaction, w^SR ₁ > w^SR ₂, whereas the opposite is observed for the furfuryl acetate reaction, w^SR ₁ > w^SR ₂.