Mechanistic Study of a Photocatalyzed C-S Bond Formation Involving Alkyl/Aryl Thiosulfate

This study presents thioether construction involving alkyl/aryl thiosulfates and diazonium salt catalyzed by visible-light-excited [Ru(bpy)₃Cl₂] at room temperature in 44-86 % yield. Electron paramagnetic resonance studies found that thiosulfate radical formation was promoted by K₂CO₃. Conversely, radicals generated from BnSH or BnSSBn (Bn=benzyl) were clearly suppressed, demonstrating the special property of thiosulfate in this system. Transient absorption spectra confirmed the electron-transfer process between [Ru(bpy)₃Cl₂] and 4-MeO-phenyl diazonium salt, which occurred with a rate constant of 1.69×10⁹ M^-1 s^-1. The corresponding radical trapping product was confirmed by X-ray diffraction. The full reaction mechanism was determined together with emission quenching data. Furthermore, this system efficiently avoided the over-oxidation of sulfide caused by H₂O in the photoexcited system containing Ru²⁺. Both aryl and heteroaryl diazonium salts with various electronic properties were investigated for synthetic compatibility. Both alkyl- and aryl-substituted thiosulfates could be used as substrates. Notably, pharmaceutical derivatives afforded late-stage sulfuration smoothly under mild conditions.