Expedient sulfurization with elemental sulfur and an unexpected conversion of 2,3-diarylcyclopropenethiones using tetrabutylammonium fluoride (TBAF) as a source of the fluoride anion

Elemental sulfur, upon treatment with tetrabutylammonium fluoride (TBAF, 20 mol%) in THF solution at room temperature, undergoes activation via ring opening, leading to in situ-generation of the red colored, highly reactive fluoropolysulfide anion FSx-. The latter easily reacts with sterically congested 2,3-diarylcyclopropenethiones and five membered 3H-1,2-dithiole-3-thiones are formed selectively as single products in good to excellent yields. The analogous course of sulfurization via ring expansion was observed in the case of 2,3-bis(4chlorophenyl)cyclopropenone and in this case the expected 3H-1,2-dithiol-3-one derivative was isolated in 83% yield. The elaborated protocol can replace the already reported, less convenient procedure, based on application of DMF as a solvent and potassium fluoride as an activator. Treatment of 2,3-diphenylcyclopropenethione with TBAF in the absence of sulfur resulted in the unexpected formation of the (E,Z)- and (Z,Z)-isomers of the dimeric a-benzoylthioketene (desaurines), and a tentative mechanism involving participation of the initially formed, reactive (a-thioketene)phenylmethylene (a carbene species) as a key intermediate, is postulated.