Catalytic Carbon-Atom Transfer Enables Spiro-Fusion of C–C σ-Bonds

The catalytic transfer of a single carbon atom offers a powerful means to reconfigure molecular frameworks, yet controlled insertion of a carbon atom into C–C σ-bonds remains a longstanding challenge. Here we report a rhodium-catalyzed spiro-carbon-atom transfer that converts planar biphenylene feedstocks into rigid three-dimensional architectures. Using N -isocyanoiminophosphorane (PINC) as an atomic carbon equivalent, the reaction forms four new covalent bonds at a single spiro center through sequential C–C bond activation. The transformation enables both homo- and cross-spiro-fusion of biphenylenes that are otherwise difficult to access. The strategy can be further extended to reactions involving combined σ- and π-bond activation, providing complementary molecular frameworks. These results establish catalytic carbon-atom transfer as a general approach for transforming simple monocyclic feedstocks into structurally complex spirocyclic molecules, with a single carbon atom serving as a central linchpin.