Reactions of polyfluorinated thietanes: Selective synthesis of 4-R-2,2-bis(trifluoromethyl)thietane-1-S-oxides and 2-substituted 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes

Oxidation of 4-substituted 2,2-bis(trifluoromethyl)thietanes by m-chloroperoxybenzoic acid results in selective formation of the corresponding S-oxides in 65-86% yield. Oxidation of 4-C₂H₅S-2,2-bis(trifluoromethyl)thietane under mild conditions led to selective formation of 4-C₂H₅SO₂-2,2-bis(trifluoromethyl)thietane, which under more rigorous conditions was selectively converted into trans-4-C₂H₅SO₂-2,2-bis(trifluoromethyl)thietane-1-S-oxide. Reaction of 4-substituted 2,2-bis(trifluoromethyl)thietanes with activated aluminum powder results in a highly selective ring expansion process, producing the corresponding 5-fluoro-4-(trifluoromethyl)-2,3-dihydro-2-alkoxythiophenes in 58-93% yield. These compounds were also prepared in 61-85% yield using a “one-pot” procedure, starting from sulfur, hexafluoropropene and the corresponding vinyl ether without isolation of any intermediates. Both 2-i-C₃H₇O- and 2-t-C₄H₉O- 5-fluoro-4-(trifluoromethyl)-2,3-dihydrothiophenes were converted into 2-fluoro-3-trifluormethylthiophene by reaction with P₂O₅.