Halogen Atom Transfer (XAT)-Enabled Transition-Metal-Free Radical Annulation for the Construction of Phenanthrene-Derived Polycyclic Aromatic Hydrocarbons (PAHs)

A transition-metal-free radical annulation strategy enabled by halogen atom transfer (XAT) has been developed for the direct construction of phenanthrene-derived polycyclic aromatic hydrocarbons (PAHs) from 2-iodobiaryls and terminal alkynes. This visible-light-mediated protocol proceeds under mild conditions and exhibits broad substrate scope, high regioselectivity, and excellent functional group tolerance. Importantly, the method can be applied iteratively to achieve homologative aromatic extension, allowing efficient access to structurally diverse and complex PAHs in just a few steps. Mechanistic studies support a radical pathway involving α-aminoalkyl radical-mediated XAT, vinyl radical cyclization, and a quantum tunneling-enabled aromatization process. This work provides a practical and scalable approach to PAHs that circumvents the limitations of conventional transition metal catalysis.