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

Spirocyclic frameworks, polycyclic structures in which two rings share a single carbon atom, constitute key structural motifs across diverse areas, including optoelectronic and hole-transporting materials1-3, ligand design for coordination complexes4-6, and numerous natural products7. In a spiro skeleton, the two molecular units adopt an orthogonal arrangement, generating a rigid three-dimensional (3D) architecture. Introducing a single carbon atom to join two components to form the spiro centre therefore represents a direct strategy for constructing such frameworks. Previous carbon-atom-transfer approaches have largely relied on π-bond reactivity, whereas controlled insertion of a carbon atom into C-C σ-bonds remains challenging8-11. Here we report a rhodium-catalysed spiro-carbon-atom insertion reaction that converts biphenylene substrates into spirocyclic structures through sequential C-C bond activation. Using N-isocyanoiminophosphorane (PINC)12-14 as an atomic carbon equivalent, the reaction forms four new C-C bonds at a spiro centre. The transformation enables both homo- and cross-spirofusion of biphenylenes and can be extended to reactions combining σ- and π-bond activation. These results establish a catalytic strategy for constructing structurally complex spirocyclic frameworks through C-C σ-bond activation, with a single carbon atom serving as a central linchpin.