Synthesis and crystal structures of new 1,3-disubstituted imidazoline-2-thiones

Article abstract of DOI:10.5560/ZNB.2013-3184

Two methods (MeOH=K²CO³, pyridine=Et³N) were assessed for the introduction of sulfur into the 2-position of 1,3-disubstituted quaternary imidazolium salts 1-9 (Cl, I, BF⁴, PF⁶, CH³OSO³ were used as anions) to yield nine 1,3-disubstituted imidazoline-2-thiones 10-18 (1, 10: R¹ = CH ³, R² = CH³; 2, 11: R¹ = OCH ²Ph, R² = CH³; 3, 12: R¹ = OCH ³, R² = CH³; 4, 13: R¹ = OCH ³, R² = OCH³; 5, 14: R¹ = NH ², R² = CH²Ph; 6, 15: R¹ = NCHPh, R² = CH³; 7, 16: R¹ = NH², R ² = CH³; 8, 17: R¹ = NCHPh, R² = NCHPh; 9, 18: R¹ = NH², R² = OCH³). Compounds 11-18 represent N-alkyloxy and N-amino imidazoline-2-thiones, whereas 10 served as reference compound. The first method was advantageous for the conversion 1 → 10 due to faster reaction, whereas in the reaction 2 → 11 considerable amounts of by-products were formed. Pure thiones 11, 14, 16, 17, and 18 were obtained only by the second method. Both methods worked for the synthesis of the methoxy derivatives 12 and 13 from 3 and 4, and the benzylideneamino derivative 15 from 6. 1-Amino-3- methylimidazoline-2-thione (16) was also prepared by hydrolysis of the benzylideneamino derivative 15. Crystal structures of seven 1,3-disubstituted imidazoline-2-thiones were determined by singlecrystal X-ray diffraction. Intermolecular C-H...S contacts were identified and, additionally, N-H...S interactions in aminothiones 14 and 16. The ¹H NMR shifts 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 a parameter and from the C=S bond length (average 1.68 A ) a significant contribution of a mesoionic imidazolium-2-thiolate resonance structure seems unlikely.