Tautomerism of pyrazoline-5-thione and triazoline-5-thione studied by variable temperature photoelectron spectroscopy

Abstract

CLAUDE GUIMON, GENEVIEVE PFISTER-GUILLOUZO, and MIKAEL BEGTRUP. Can. J. Chem. 61, 1197 (1983.Photoelectron spectra at variable temperatures demonstrate that the rate of thermal isomerization of pyrazoline-5-thione ( l a ) into methylthiopyrazolc(s) in the gaseous phase is mctal catalysed. Similar isomerizations of triazolethione (20 and 30) are at best only slightly so.Furthermore, photoelectron spectra and MNDO calculations show that gaseous methylthiopyrazoles (16 and l c ) and mcthylthiotriazolcs (26 and 2c) exist predominantly in planar form for the 6 isomer and in nonplanar form for the c isomer. The AH" for the rotameric equilibria is 2.7-4.2 kJ.mol-'. CLAUDE GUIMON, GENEVIEVE PFISTER-GUILLOUZO et MIKAEL BEGTRUP. Can. J. Chem. 61, 1197Chem. 61, (1983. L'analysc des spectres photoelectroniques enregistrts a I'aide d'une "sonde i ternptrature variable" montre que la transformation en phase vapeur de la pyrazoline-thione-5 ( l a ) en mtthylthio-pyrazole(s) est catalysie par les parois metalliques de la sonde. Cette rtaction d'isomerisation est, semblc-t-il, nettement plus difficile dans le cas des triazolethiones (20 et 30).D'autre part les spectres exptrimentaux et les calculs thtoriques rtalists I'aide de la mtthode MNDO montrent que les composts S-mtthylks ( l b , l c , 26 et 2c) existent prtfkrentiellement sous une forme plane dans le cas des isomkres 6 et une forme non plane dans celui des isornkres c et qu'ils prtsentent un tquilibre entre rotameres (AH" cornpris entre 2.7 et 4.2 kJ. mol-I).

Synthesis and tautomerism of spiro-pyrazolines

“…5 In addition to the synthesis of pyrazoles and pyrazoline derivatives, tautomerism is another area of equal interest due to their high application potential in biological systems, chemical reactivity, and molecular recognition. 8-10 Although, the importance of tautomerism (imine-enamine) has been described very well for pyrazole related ring systems, 9-11 it's rarely observed in analogous spiro-pyrazolines. As of today, only three reports exist on the structural changes observed in spiro-pyrazolines.…”