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

Abstract

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.