Ring Cleavage and Ring Contraction of Nitro-Substituted l-Methyl-2,5-diphenyl-l,4-dithiinium Tetrafluoroborates

1 -Methyl-2,5-diphenyl-l, 4-dithiinium tetrafluoroborate (2) underwent selective abstraction of H-3 in aqueous phosphate buffer at pH 6.8 to yield the ring cleavage product (Z)-1-(phenylethynylthio)-2-(methylthio)-1 - phenylethylene (4). Methylation of 3-nitro-2,5-diphenyl-l,4-dithiin (5) with MeI-AgBF4 yielded as the major product l-methyl-6-nitro-2,5-diphenyl-l,4-dithiinium tetrafluoroborate (8), which also underwent ring cleavage in acetonitrile on an alumina column, in pH 6.8 phosphate buffer or with triethylamine, to form (Z)-l-(phenylethynylthio)- 2-(methylthio)-2-nitro-l-phenylethylene (14). The methylation of 5 also yielded a minor product, 3-(dimethylsulfonio)-5-nitro-2,4-diphenylthiophene tetrafluoroborate (13), apparently resulting from ring contraction of 1 -methyl-3-nitro-2,5-diphenyl-1,4-dithiinium tetrafluoroborate (9), which appeared to be unstable. Demethylation of 13 with triethylamine yielded 3-(methylthio)-5-nitro-2,4-diphenylthiophene (12). More fully substituted dithiins, including 3,6-dinitro- and 3-bromo-6-nitro-2,5-diphenyl-l,4-dithiins, could not be methylated, whereas the 3,6- dibromo analogue (16) afforded only a 3% yield of l-methyl-3,6-dibromo-2,5-diphenyl-l,4-dithiinium tetrafluoroborate (17).