Nonacethrene Unchained: A Cascade to Chiral Contorted Conjugated Hydrocarbon with Two sp3-Defects

A dihydro precursor of helical diradicaloid nonacethrene undergoes a reaction cascade triggered by an oxidant to a chiral contorted polycyclic aromatic hydrocarbon named hypercethrene. In this ten-electron-oxidation process, four σ bonds, one π bond, and three six-membered rings are formed in a reaction sequence of up to nine steps to yield a 72-carbon-atom warped framework, comprising two configurationally locked [7]helicene units, fluorescent peropyrene unit, and two precisely installed sp3-defects.The key intermediate in this cascade is a closed nonacethrene derivative with one quaternary center, presumably formed via an electrocyclic ring closure of nonacethrene, which— when activated by oxidation—undergoes an oxidative dimerization of phenalenyl to peropyrene. By controlling the amount oxidant used, two intermediates and one side product could be isolated and fully characterized, including single-crystal X-ray diffraction analysis, and one intermediate was detected by electron paramagnetic resonance spectroscopy. In concert with density functional theory calculations, these intermediates support the proposed reaction mechanism. Compared to peropyrene, the absorption and emission of hypercethrene are slightly red-shifted on account of extended conjugation and the fluorescence quantum yield of 0.45 is decreased by a factor of ~2. Enantiomerically enriched hypercethrene displays circularly polarized luminescence with a CPL brightness value of 8.3 M–1cm–1. This unexpected reaction cascade demonstrates that the reactivity of “unchained”diradicaloid compounds, which is typically considered an undesired feature, can be well-defined and employed as a useful, step-economic synthetic tool toward novel carbon nanostructures.