The dithiole series. Part 6. Intermediacy of spirothiopyrans in reactions of 3,5-disubstituted 1,2-dithiolylium salts with cationic heterocycles containing reactive methyl groups

Abstract

The self-condensation of 3-methyl-5-phenyl-1,2-dithiolylium perchlorate in the presence of base gives a 2-thiophenacylidene-2H-thiopyran. A mechanism involving the intermediacy of a spiro-[1,2-dithiole-3,2′-2H-thiopyran] is proposed, and evidence is presented that the related condensations of 4,6-diaryl-2-methylthiopyrylium salts with 3,5-diaryl-1,2-dithiolylium salts proceed via spirobi-2H-thiopyrans. A stable spirothiopyran is formed in the reaction of 2,3-dimethylbenzothiazolium perchlorate with 3,5-diphenyl-1,2-dithiolylium perchlorate. Spiro-1,2-dithioles, generated by the reactions of certain bidentate nucleophiles with 2-chloro- or 2-methylthio-5-phenyl-1,2-dithiolylium salts, lose one atom of sulphur to give thiophenacylidene heterocycles, thus providing further support for the mechanism proposed for the original self-condensation.