Photoexcited Intramolecular Annulative Cross-Coupling via an EDA Complex Mechanism

Transition-metal-free cross-coupling reactions have emerged as an important green and sustainable methodology in modern organic synthesis. Herein, we report a metal- and additive-free strategy for the intramolecular C-halogen/C-H cross-coupling to construct phenanthrene cores. This method employs readily accessible E/Z-mixed diarylethenes, which involve a rapid E → Z isomerization followed by radical cyclization under mild conditions. Mechanistic studies support a chain pathway involving Z/E photoisomerization followed by electron donor-acceptor (EDA) complex-mediated single-electron transfer (SET) and radical cyclization. This work provides a practical route to phenanthrene cores while offering detailed mechanistic insights into metal-free, photoinduced C(Ar)-halogen bond activation.