Azulene–Naphthalene, Naphthalene–Naphthalene, and Azulene–Azulene Rearrangements

The mechanism(s) of thermal rearrangement of azulenes have been enigmatic for several decades. Herein, we have employed density functional theory (DFT) calculations at the M06–2X/6–311+G(d,p) level together with single-point calculations at the CCSD(T) level to assess possible mechanisms of the experimentally observed azulene and naphthalene automerizations. Of the two mechanisms proposed for naphthalene automerization, it is found that the benzofulvene (BF) route is favored over the naphthvalene mechanism by ∼6 kcal/mol and is energetically lower than the norcaradiene–vinylidene mechanism (NVM) for the azulene–naphthalene rearrangement (Ea ∼ 76.5 (74.6) kcal/mol). Moreover, contrary to older reports, we observe that a pathway involving indenylcarbene intermediates is a viable, alternate mechanism. Therefore, the naphthalene automerization is expected to take place during azulene pyrolysis, especially under conditions of low-pressure FVP, where it will be aided by chemical activation. Furthermore, thermal azulene–azulene isomerization is feasible through vinylidene–acetylene–vinylidene (VAV), dehydrotriquinacene (DTQ), and azulvalene (AV) pathways with activation energies lying below that required for the azulene–naphthalene conversion, i.e., the NVM. These results, together with the previously published NVM, provide reasonable explanations for most of the products of the thermal azulene–naphthalene rearrangement.