Coronene (pictured) is a polycyclic aromatic hydrocarbon (PAH) that consists of seven fused benzene rings. Coronene subunits can be found, e.g., in carbon nanoribbons, nanowires, or nanotubes. It is also of interest in astrochemistry, where PAHs can be found around carbon-containing stars, and in combustion chemistry, where coronene has been found in soot. Understanding the formation of coronene at high temperatures is, thus, an interesting research target.

Patrick Hemberger, Paul Scherrer Institute, Villigen, Switzerland, Alexander M. Mebel, Florida International University, Miami, USA, Ralf I. Kaiser, University of Hawaii at Ma̅noa, Honolulu, USA, and colleagues have combined molecular beam experiments and electronic structure calculations to investigate how coronene can be obtained via gas-phase synthesis. The team used a chemical microreactor that mimics the environment in combustion flames and carbon-rich envelopes around stars. They performed reactions of 1-triphenylenyl with acetylene, 4-pyrenyl with vinylacetylene, and phenanthrene with phenyl to study the possible gas-phase preparation of benzo[e]pyrene (C₂₀H₁₂). The 1-benzo[e]pyrenyl radical was reacted with acetylene to obtain a benzo[ghi]perylene (C₂₂H₁₂), and finally, a reaction of the 7-benzo[ghi]perylenyl radical ([C₂₂H₁₁]^•) with acetylene gave coronene (C₂₄H₁₂).

The team combined these experimental results with calculations of potential energy surfaces (PES) leading to the observed products. They found that the complex chain of reactions involves three different aryl-radical-mediated ring annulation mechanisms, i.e., hydrogen abstraction–acetylene addition (HACA), hydrogen abstraction–vinylacetylene addition (HAVA), and phenyl addition–dehydrocyclization (PAC). According to the researchers, the work could provide a strategy for exploring the radical chemistry of complex PAHs in deep space.

• Gas-Phase Synthesis of Coronene through Stepwise Directed Ring Annulation,
  Shane J. Goettl, Lotefa B. Tuli, Andrew M. Turner, Yahaira Reyes, A. Hasan Howlader, Stanislaw F. Wnuk, Patrick Hemberger, Alexander M. Mebel, Ralf I. Kaiser,
  J. Am. Chem. Soc. 2023.
  https://doi.org/10.1021/jacs.3c03816