First Exclusive Stereo? and Regioselective Preparation of 5?Arylimino?1,3,4?Selenadiazole Derivatives: Synthesis, NMR analysis, and Computational Studies

Isoselenocyanates are valuable coupling partners required for preparing key chemical intermediates and biologically active molecules in an accelerated and effective way. Likewise, (Z)-2- oxo-N-phenylpropanehydrazonoyl chlorides have been em- ployed in numerous one-step heteroannulation reactions to assemble the structural core of several various kinds of heterocyclic compounds. Here, we describe the inverse electron demand 1,3-dipolar cycloaddition reaction of isoselenocyanates with a variety of substituted (Z)-2-oxo-N-phenylpropanehydra- zonoyl chlorides to generate, regioselectively and stereoselec- tively, a series of 5-arylimino-1,3,4-selenadiazole derivatives comprising a multitude of functional groups on both aryl rings. The synthetic method features gentle room-temperature con- ditions, wide substrate scope, and good to high reaction yields. The selenadiazoles were separated by gravity filtration in all instances and chemical structures were validated by multi- nuclear NMR spectroscopy and high accuracy mass spectral measurements. First conclusive molecular structure elucidation of the observed 5-arylimino-selenadiazole regioisomer was verified by single-crystal X-ray diffraction analysis. Crystal- structure measurement was successfully carried out on (Z)-1-(4- (4-iodophenyl)-5-(p-tolylimino)-4,5-dihydro-1,3,4-selenadiazol-2- yl)ethan-1-one and (Z)-1-(5-((4-methoxyphenyl)imino)-4-(4- (methylthio)phenyl)-4,5-dihydro-1,3,4-selenadiazol-2-yl)ethan-1- one. Likewise, the (Z)-geometry of the hydrazonoyl chloride reactant was proven by X-ray diffraction studies. As representa- tive examples, crystal-structure determination was carried out on (Z)-2-oxo-N-phenylpropanehydrazonoyl chloride and (Z)-N- (3,5-bis(trifluoromethyl)phenyl)-2-oxopropanehydrazonoyl chloride. Density functional theory calculations at the B3LYP- D4/def2-TZVP level were conducted to support the noted experimental findings and suggested mechanism.