Dirhodium complexes with a “paddlewheel” structure, such as Rh₂(OAc)₄, can be used to promote a variety of reactions, e.g., carbene or nitrene transfers, C–H activations, or cycloisomerizations. They can serve as catalysts in redox reactions, both for radical processes and two-electron oxidations. However, two-electron oxidations are less favorable for a dirhodium(II) complex, and fewer examples of this type of reaction are known.

Rui Wu, Zhejiang Sci-Tech University, Hangzhou, China, Shifa Zhu, South China University of Technology, Guangzhou, and Zhejiang Sci-Tech University, and colleagues have developed a method for the ligand-free dirhodium-catalyzed [2+2+2] cycloaddition of 1,6-diynes and alkynes to give fused arenes (general product structure pictured). The team reacted a variety of 1,6-diynes with a group of the type O, NR, or CR₂ in the backbone with different alkynes to obtain dihydrobenzofuran, isoindoline, or indane derivatives, respectively. Rh₂(OAc)₄ was used as the catalyst (without an additional ligand) and 1,2-dichloroethane (DCE) as the solvent, and the reactions were performed at 80 °C.

Under these conditions, the desired fused arenes were obtained in mostly moderate to good yields. Based on mechanistic studies, the team proposes a reaction mechanism that involves an oxidative cyclization between one rhodium of the dirhodium complex and the diyne, followed by coordination and insertion or [4 + 2] cycloaddition and a reductive elimination to give the product. According to the researchers, the work represents a new application of dirhodium complexes.

• Dirhodium-Catalyzed [2 + 2 + 2] Cycloaddition of 1,6-Diynes and Alkynes,
  Xiang Gao, Wendeng Li, Yang Chen, Rui Wu, Shifa Zhu,
  J. Org. Chem. 2024.
  https://doi.org/10.1021/acs.joc.4c01780