Stone–Wales Rearrangements in Polycyclic Aromatic Hydrocarbons: A Computational Study

Mechanisms for Stone–Wales rearrangements (SWRs) in polycyclic unsaturated hydrocarbons containing a pentafulvalene core have been studied using density functional, coupled cluster, and multiconfigurational methods. At the BD­(T)/cc-pVDZ//(U)­M06-2X/cc-pVDZ level of theory, free energies of activation (at 1000 °C) range from ca. 70 kcal/mol for the model system pentafulvalene → naphthalene (<b>1</b> → <b>2</b>) to >110 kcal/mol for the degenerate SWR of pyracyclene (<b>3</b>). Systems studied that do not contain a pyracyclene subunit are predicted to have Δ<i>G</i>^⧧ less than about 90 kcal/mol and to proceed by a carbene-type mechanism. Substrates containing a pyracyclene subunit should proceed via a cyclobutyl mechanism, and appropriate benzannelation of <b>3</b> lowers the activation free energy considerably. Computed Δ<i>G</i>^⧧ values are consistent with experimental observations reported for known systems. SWRs of two untested substrates, cyclopent­[<i>fg</i>]­aceanthrylene (<b>18</b>) and dicyclopenta­[<i>fg</i>,<i>op</i>]­tetracene (<b>21</b>), are predicted to have Δ<i>G</i>^⧧ < 95 kcal/mol and thus to be accessible via flash vacuum pyrolysis.