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 center dot center dot center dot O and C-H center dot center dot center dot pi(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-2yl]-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 center dot center dot center dot S and C-H center dot center dot center dot pi(aryl/alkyne) interactions. A combination of two C-H center dot center dot center dot pi(arene) hydrogen bonds links the product molecules into sheets.