The Fluoride Anion-Catalyzed Sulfurization of Thioketones with Elemental Sulfur Leading to Sulfur-Rich Heterocycles: First Sulfurization of Thiochalcones

Fluoride anion was demonstrated as a superior activator of elemental sulfur (S8) to perform sulfurization thioketones leading diverse sulfur-rich heterocycles. Due solubility problems, reactions must be carried out either in THF using tetrabutylammonium fluoride (TBAF) or DMF cesium (CsF), respectively. The reactive sulfurizing reagents are situ generated, nucleophilic fluoropolysulfide anions FS(8?x)?, which react with the C=S bond according carbophilic addition mode. Dithiiranes formed thereby, existing an equilibrium ring-opened form (diradicals/zwitterions) key-intermediates, undergo step-wise dimerization afford 1,2,4,5-tetrathianes intramolecular insertion, case thioxo derivatives 2,2,4,4-tetramethylcyclobutane-1,3-dione ring enlarged products. In catalyzed by TBAF, water bounded via H-bridges and forming thereby its stable hydrates is involved secondary leading, e.g., 2,2,4,4-tetramethyl-3-thioxocyclobutanone formation some unexpected products such dithiolactone dithiocarboxylate. contrast thioketones, their ?,?-unsaturated analogues, i.e., thiochalcones slow inefficient. However, alternative protocol triphenylphosphine (PPh3) applied catalyst, offers attractive approach synthesis 3H-1,2-dithioles 1,5-dipolar electrocyclization situ-generated thiocabonyl S-sulfides. All occur under mild conditions can considered methods for preparation rich heterocycles ring-size.