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 K₂CO₃ 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.