Synthesis of 2-phospha[7]helicene, a helicene containing a phosphinine core

Helical polyaromatic substances, helicenes, attract attention due to their extraordinary optoelectronic properties, which predestinate them for applications in the field of materials engineering. However, ideal properties are rarely found in unsubstituted carbohelicenes, and it is therefore necessary to further modify these substances. In this work, a new method was developed by which an unusual phosphinine (phosphabenzene) ring can be introduced into the helicene structure.

Phosphahelicene (4-Phenyl-6-methyl-2-phospha[7]helicene) was prepared from starting 2‑bromobenzo[c]phenanthrene in 12 synthetic steps in a total yield of 12%. In the described synthesis, the appropriate helicene precursor bearing an alkyne function is first prepared. The key step of the developed synthetic strategy is the acid-initiated cyclization of this alkyne yielding an ortho-condensed phosphorus heterocycle, which is subsequently aromatized to form the target molecule. The structure of the first phosphahelicene with a terminal phosphinine ring was confirmed by X-ray analysis and the basic optical properties of this compound were subsequently investigated.

Synthesis of 4-phenyl-6-methyl-2-phospha[7]helicene

• Beránek, T.; Jakubec, M.; Sýkora, J.; Císařová, I.; Žádný, J.; Storch, J.: Synthesis of 2-Phospha[7]helicene, a Helicene with a Terminal Phosphinine Ring. Org. Lett. 2022, 24(26), 4756–4761. https://doi.org/10.1021/acs.orglett.2c01723