Catalytic transfer of carbon atoms enables spirofusion of C-C σ-bonds.

Rhodium-catalyzed spiro-carbon-atom insertion converts biphenylene into complex spirocyclic structures by activating C-C σ-bonds with a single carbon atom.

• Why it matters: Constructing spirocyclic frameworks is important for materials, ligands, and natural products, but controlled insertion of a carbon atom into C-C σ-bonds has been challenging, limiting structural diversity.
• What they did: The authors used a rhodium catalyst and N-isocyanoiminophosphorane as an atomic carbon equivalent to activate C-C bonds in biphenylene, forming four new C-C bonds at a spiro center, enabling homo- and cross-spirofusion.
• The result: This catalytic method establishes a new approach for building complex spirocyclic frameworks through C-C σ-bond activation, with a single carbon atom acting as a central linchpin.