Studies on Reactions of Thioketones with Trimethyl(trifluoromethyl)silane Catalyzed by Fluoride Ions

Abstract

Treatment of 2,2,4,4-tetramethylcyclobutane-1,3-dione (6) in THF with CF3SiMe3 in the presence of tetrabutylammonium fluoride (TBAF) yielded the corresponding 3-(trifluoromethyl)-3-[(trimethylsilyl)oxy]cyclobutanone 7 (Scheme 1) via nucleophilic addition of a CF anion at the CO group and subsequent silylation of the alcoholate. Under similar conditions, the ‘monothione' 1 reacted to give thietane derivative 8 (Scheme 2), whereas in the case of ‘dithione' 2 only the dispirodithietane 9, the dimer of 2, was formed (Scheme 3). A conceivable mechanism for the formation of 8 is the ring opening of the primarily formed CF3 adduct A followed by ring closure via the S-atom (Scheme 2). In the case of thiobenzophenones 4, complex mixtures of products were obtained including diarylmethyl trifluoromethyl sulfide 10 and 1,1-diaryl-2,2-difluoroethene 11 (Scheme 4). Obviously, competing thiophilic and carbophilic addition of the CF anion took place. The reaction with 9H-fluorene-9-thione (5) yielded only 9,9′-bifluorenylidene (14; Scheme 6); this product was also formed when 5 was treated with TBAF alone. Treatment of 4a with TBAF in THF gave dibenzhydryl disulfide (15; Scheme 7), whereas, under similar conditions, 1 yielded the 3-oxopentanedithioate 17 (Scheme 9). The reaction of dithione 2 with TBAF led to the isomeric dithiolactone 16 (Scheme 8), and 3 was transformed into 1,2,4-trithiolane 18 (Scheme 10).