Thermal Rearrangement of Azulenes to Naphthalenes: A Deeper Insight into the Mechanisms.

The thermal rearrangement of azulene to naphthalene has been the subject of several experimental and computational studies. Here, we reexamine the proposed mechanisms at the DFT level. The use of different functionals showed that the HF-exchange contribution significantly affects reaction energies and barrier heights. Accordingly, all proposed pathways were investigated with the optimal method, M06-2X/6-311+G(d,p), which confirms the norcaradiene-vinylidene mechanism (A) as the dominant unimolecular route ( ≈ 76 kcal/mol) able to account for the major products of pyrolyses using C- or substituent-labeled azulenes. Moreover, a facile vinylidene-acetylene interconversion will scramble the terminal carbon atoms in the vinylidene. Several other potential intramolecular reaction mechanisms (B-E) are ruled out because of higher activation energies (>84 kcal/mol) and failure to reproduce the results obtained with substituted and C-labeled azulenes and benzazulenes. These experimental results also demonstrate that the proposed free radical or H atom-induced intermolecular methylene walk and spiran mechanisms cannot be major contributors, especially under flash vacuum pyrolysis conditions.