Visible-light photocatalytic preparation of alkenyl thioethers from 1,2,3-thiadiazoles and Hantzsch esters: synthetic and mechanistic investigations

Herein, we disclose a protocol to synthesize trisubstituted alkenyl thioethers through a direct S-alkylation of 1,2,3-thiadiazoles with C-radical precursors, 4-alkyl-1,4-dihydropyridines (DHPs), driven by visible-light photocatalysis. A broad range of primary, secondary, and tertiary C-radical precursors, DHPs, are suitable for this reaction and the desired products can be obtained in good to excellent yields under mild conditions. Remarkably, high stereoselectivity with Z-alkenyl thioethers was achieved in the presence of a Cu(OAc)(2) catalyst. Synergistic experimental and computational studies were carried out to shed light on the mechanisms of this reaction, in which the quenching pathway of an excited photocatalyst (*Ru-II) could be altered in the presence of the Cu(OAc)(2) catalyst. A reductive quenching pathway (Ru-II/*Ru-II/Ru-I/Ru-II) is proposed in the absence of the Cu(OAc)(2) catalyst while an oxidative quenching pathway (Ru-II/*Ru-II/Ru-III/Ru-II) is suggested with the assistance of the Cu(OAc)(2) catalyst. In addition, the origin of the Z-selectivity of the product is discussed.