Visible-Light Photoredox Catalyzed, Facile Synthesis of 2-Amino-1,3,4-Thiadiazoles from Aldehydes and Thiosemicarbazides

A metal-free, visible light-mediated protocol has been developed towards the synthesis of 2-amino-1,3,4-thiadiazoles (ATZ) from commercially available aldehydes and thiosemicarbazides. This one-step conversion has been achieved by the condensa-tion of aldehydes with substituted thiosemicarbazides in the presence of 2,4,6-tri(p-tolyl) pyrylium tetrafluoroborate (TPP-TFB) as a photoredox catalyst. Different aldehydes/hetero aldehydes, as well as thiosemicarbazides, have reacted smoothly under the reaction conditions to afford the 2-amino-1,3,4-thiadiazoles in good to excellent yields. The practicality of the de-veloped method has been successfully extended for the convenient one-pot synthesis of commercial herbicide Tebuthiuron on a gram scale. A series of control experiments, cyclic voltammetry, and fluorescence studies have been carried out to investi-gate the reaction mechanism.