2-Methylthio-imidazolins: a rare case of different tautomeric forms in solid state and in solution

Abstract

The synthesis and structure elucidation of two new compounds, 2-(methylthio)-1,3-diazaspiro[4.4]non-2-ene-4-one (1) and 2-(methylthio)-1,3-diazaspiro[4.4]non-2-ene-4-thione (2), are presented. Both compounds crystalized in monoclinic crystal system. Compound 1 formed plate-like colorless crystals, while compound 2 gave yellow needles. The structural and spectral characteristics of both compounds are studied by IR and NMR spectroscopy and quantum chemistry. The optimized geometry, harmonic vibrational frequencies, and NMR shielding are calculated by DFT employing B3LYP functional using 6-311++G(d,p) basis set. Our results support the hydrogen bonding pattern observed in reported crystalline structure. Evidences are given indicating that the tautomeric form in solution is different from that one in solid state. The substantial difference between positions of the characteristic C=N band in nonpolar solvent and crystalline phase for both compounds suggests that in solid state a “conjugated tautomeric form” exists, while in solution phase there is “non-conjugated tautomeric form”. In polar solvents, both tautomeric forms exist. It is suggested to call this phenomenon desmokatatropy—from Greeks δeσμός (bond) κατάσταση (state) and τρόπος (change).