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.