Synthesis and Crystal Structures of New 1,4-Disubstituted 1,2,4-Triazoline-5-thiones

Introduction of sulfur into the 5-position of 1,4-disubstituted quaternary 1,2,4-triazolium salts (1–9; Cl, Br, I, BF₄, PF₆, CH₃OSO₃ were used as anions) by two methods was investigated. The syntheses of nine 1,4-disubstituted 1,2,4-triazoline-5-thiones 10–18 are reported (1, 10: R¹ = CH₃, R² = CH₃; 2, 11: R¹ = NH₂, R² = CH₃; 3, 12: R¹ = NH₂, R² = CH(CH₃)₂; 4, 13: R¹ = N(CH₃)₂, R² = CH₃; 5, 14: R¹ = N(CH₃)₂, R² = CH(CH₃)₂; 6, 15: R¹ = CH₃, R² = NH₂; 7, 16: R¹ = OCH₂Ph, R² = CH₃; 8, 17: R¹ = OCH₂Ph, R² = CH₂CH₃; 9, 18: R¹ = CH₃, R² = CH₂Ph). Compounds 11–17 represent 1-amino, 4-amino, 4-dimethylamino, and 4-benzyloxy-1,2,4-triazoline-5-thiones, whereas 10 served as a reference compound. Thione 18 was identified as an unexpected by-product in the synthesis of 16 and was also prepared independently. Thermolysis of 10 in air gave 1,4-dimethyl-1,2,4-triazolium hydrogensulfate. Crystal structures of eight 1,4-disubstituted 1,2,4-triazoline-5-thiones were determined by single-crystal X-ray diffraction. Intermolecular hydrogen bonds (C–H···S, C–H···N, N–H···N, N–H···S) were observed in the solid state. The solvent-dependent ¹H NMR chemical shifts of signals of 10 and 13 were satisfactorily correlated with the Kamlet-Abboud-Taft π* and β parameters in ten solvents. From the lack of correlation with the α parameter and from the C=S bond length (average 1.67 Å) a significant contribution of a mesoionic imidazolium-2-thiolate resonance structure seems unlikely.