Combined Experimental and Theoretical Studies of the 1,3-Dipolar Cycloaddition Reaction Between Phenyldiazomethane and 4-Methylthioaurone

This research paper describes the synthesis of new spiropyrazoline bearing a 1-pyrazoline framework. In literature, only a few research works are devoted to synthesizing 1-pyrazoline heterocycle since this heterocycle undergoes spontaneous isomerization to its 2-pyrazoline isomeric form. Herein, the spiro 1-pyrazoline has been synthesized from thioaurone derivative through a regiospecific 1,3-dipolar cycloaddition (1,3-DC) with phenyldiazomethane. The structure of the obtained stable 1-pyrazoline has been established using IR and (1H and 13C) NMR spectroscopy and corroborated by mass spectrometry. Furthermore, the reaction's mechanism and regioselectivity have been investigated using DFT calculations at the B3LYP/6-31G(d) level. The outcomes have shown that the formation of the regioisomer 5 is more favorable than 5’, which is in good agreement with the observed regioselectivity.