Sustainable Electron Donor‐Acceptor Complex‐Mediated Decarboxylative C─C Coupling of Cinnamic Acids

Developing catalyst‐free photochemical transformations that efficiently harness visible light remains an important challenge in sustainable organic synthesis. Herein, we report a visible‐light‐induced decarboxylative C─C coupling of readily available cinnamic acids with aryl diazonium salts for the stereoselective synthesis of stilbene derivatives. The reaction proceeds under transition‐metal‐ and photosensitizer‐free conditions using ethanol as a green solvent and K 2 CO 3 as an inexpensive base, providing a broad range of stilbenes in good to excellent yields with excellent E ‐selectivity. Remarkably, simply changing the irradiation wavelength enables access to the corresponding Z ‐stilbenes as the major stereoisomer, providing a convenient wavelength‐controlled stereodivergent protocol. The synthetic utility is further enhanced by gram‐scale synthesis and chromatographic purification‐free product isolation through simple recrystallization. Experimental studies combined with DFT calculations support an electron donor–acceptor (EDA)‐complex‐mediated photoinduced single‐electron‐transfer process that generates alkenyl and aryl radicals for subsequent radical–radical coupling. This work demonstrates the potential of EDA‐complex photochemistry as a practical and sustainable platform for catalyst‐free C─C bond formation.