On‐Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross‐conjugated π‐electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non‐contact atomic force microscopy, and time‐of‐flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction. The first [1+1+1] cycloaddition reaction has been achieved on the Ag(111) surface to afford the aza[3]radialenes. The chlorine substituents in the isocyanides ensured the selectivity and orientational order of the products via the steric hindrance‐directed molecular assembly.