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. 2019 Apr 3;10(1):1510.
doi: 10.1038/s41467-019-09224-8.

Gas phase synthesis of [4]-helicene

Long Zhao  1 , Ralf I Kaiser  2 , Bo Xu  3 , Utuq Ablikim  3 , Wenchao Lu  3 , Musahid Ahmed  3 , Mikhail M Evseev  4 , Eugene K Bashkirov  4 , Valeriy N Azyazov  4 , Marsel V Zagidullin  4 , Alexander N Morozov  5 , A Hasan Howlader  5 , Stanislaw F Wnuk  5 , Alexander M Mebel  4   5 , Dharati Joshi  6 , Gregory Veber  6 , Felix R Fischer  6   7   8
Affiliations

Gas phase synthesis of [4]-helicene

Long Zhao et al. Nat Commun. .

Abstract

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields [4]-helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C18H13). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Molecular structures of helicenes. a (M)-[4]-helicene; b (P)-[4]-helicene; c (M)-[5]-helicene; d (P)-[5]-helicene; e (M)-[6]-helicene and f (P)-[6]-helicene
Fig. 2
Fig. 2
Comparison of photoionization mass spectra recorded at a photoionization energy of 9.50 eV. a 4-phenanthrenyl ([C14H9]•) - vinylacetylene (C4H4) system; b 4-phenanthrenyl ([C14H9]•) - helium (He) system. The mass peaks of the newly formed C18H12 (m/z = 228) species along with the 13C-substituted counterparts (m/z = 229) are highlighted in blue
Fig. 3
Fig. 3
Photoionization efficiency (PIE) curves relevant to the formation of [4]-helicene. a m/z = 228 and b m/z = 229. Black: experimentally derived PIE curves; blue: [4]-helicene (benzo[c]phenanthrene) reference PIE curve; cyan: 4-vinylpyrene PIE curve; yellow: overall fit. The overall error bars (gray area) consist of two parts: ±10% based on the accuracy of the photodiode and a 1 σ error of the PIE curve averaged over the individual scans
Fig. 4
Fig. 4
Potential energy surface (PES) for the 4-phenanthrenyl [C14H9]• reaction with vinylacetylene (C4H4). This PES was calculated at the G3(MP2,CC)//B3LYP/6-311G(d,p) level of theory for the channels leading to [4]-helicene p1, 4-vinylpyrene p2, 4-((E)-but-1-en-3-yn-1-yl)phenanthrene p3, and 4-(but-3-en-1-yn-1-yl)phenanthrene p4. The relative energies are given in kJ mol–1
Fig. 5
Fig. 5
Mechanism. Schematic presentation of a vinylacetylene mediated helicene propagation to [5]-helicene and [6]-helicene starting from [4]-helicene (benzo[c]phenanthrene)
See this image and copyright information in PMC

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