First (3+2)-Cycloadditions of Thiochalcones as C=S Dipolarophiles: Efficient Synthesis of 1,3,4-Thiadiazoles via Reactions with Fluorinated Nitrile Imines

Fluorinated nitrile imines generated in situ from the corresponding fluoral-derived hydrazonoyl bromides smoothly react with monomeric aryl/hetaryl-substituted thiochalcones yielding 2,3-dihydro-1,3,4-thiadiazoles in a chemo- and regioselective manner. The elaborated protocol can be efficiently applied starting with precursors functionalized with both electron-donating and electron-withdrawing groups located at the para position of the aryl ring. In contrast, the non-fluorinated nitrile imines do not enter the (3+2)-cycloaddition reactions with title thiochalcones.