Formation of 1-(thiazol-2-yl)-4,5-dihydropyrazoles from simple precursors: synthesis, spectroscopic characterization and the structures of an intermediate and two products

Two new 1-(thiazol-2-yl)-4,5-dihydropyrazoles have been synthesized from simple precursors, and characterized both spectroscopically and structurally. In addition, two intermediates in the reaction pathway have been isolated and characterized, one of them structurally. The molecules of the intermediate (E)-1-(4-methoxyphenyl)-3-[4-(prop-2-ynyloxy)phenyl]prop-2-en-1-one, C₁₉H₁₆O₃ (I), are linked by a combination of C-H⋯O and C-H⋯π(arene) hydrogen bonds to form ribbons. The products (RS)-5-(4-methoxyphenyl)-1-(4-phenythiazol-2-yl)-3-[4-(prop-2-ynyloxy)phenyl]-4,5-dihydro-1H-pyrazole, C₂₈H₂₃N₃O₂S (II), and (RS)-5-(4-methoxyphenyl)-1-[4-(4-methylphenyl)thiazol-2-yl]-3-[4-(prop-2-ynyloxy)phenyl]-4,5-dihydro-1H-pyrazole, C₂₉H₂₅N₃O₂S (III), are closely related-differing only by presence or absence of a methyl group at the arylthiazolyl substituent-and crystallize in an isomorphous setting. Both molecules contain an effectively planar dihydro-pyrazole ring, and possess an overall T-shaped structure, which is a characteristic of triaryl-substituted 4,5-dihydro-1-(thiazol-2-yl)pyrazole compounds. The crystal packing is characterized by intermolecular C-H⋯S and C-H⋯π (aryl/alkyne) interactions. A combination of two C-H⋯π(arene) hydrogen bonds links the product molecules into sheets.