[2 + 2 + 2] Cycloaddition Reactions Using 3d Metal Catalysts: Photo‐ and Electrocatalytic Systems

The [2 + 2 + 2] cycloaddition of unsaturated substrates represents one of the most atom‐economic strategies for the construction of carbo‐ and heteroaromatic frameworks. While traditionally dominated by thermally driven processes relying on cobalt, nickel, and other transition metals, recent advances demonstrate that alternative modes of energy input fundamentally expand the reactivity landscape. This perspective highlights the emergence of photo‐ and electrochemically enabled cyclotrimerization reactions with a particular focus on 3d metal catalysts. Visible‐light irradiation and electrochemical activation provide distinct pathways for catalyst generation, redox modulation, and mechanistic control, enabling milder reaction conditions, altered kinetics, and unconventional selectivity patterns. Beyond simple replacement of thermal activation, these approaches unlock light‐gated, redox‐neutral, or mediator‐assisted catalytic cycles and facilitate access to previously inaccessible reactivity, including directly activated transition metal catalysts and a small number of transition metal‐free protocols. By comparing mechanistic details across photochemical and electrochemical platforms, we outline how energy‐controlled catalysis reshapes classic cobalt‐, iron‐, manganese‐, and nickel‐mediated cycloadditions. The integration of reaction engineering, sustainability considerations, and mechanistic insight suggests that energy‐programmable catalysis will play a defining role in the future evolution of [2 + 2 + 2] cycloaddition chemistry.