1-ALKYL-2,5-DIPHENYL-1,4-DITHIINIUM SALTS AND THEIR AMBIDENT BEHAVIOR TOWARD BASES

Alkylation of 2,5-diphenyl-1,4-dithiin (5) by methyl, methyl-d3, and ethyl iodides in the presence of silver 2,4,6-trinitrobenzenesulfonate, mercuric iodide, silver perchlorate, or silver tetrafluoroborate gave high yields of the 1-alkyl-2,5-diphenyl-1,4-dithiinium salts (6a-g), while 3-bromo-2,5-diphenyl-1,4-dithiin (14) was similarly methylated at the S-1 site. 1-Methyl-2,5-diphenyl-1,4-dithiinium tetrafluoroborate (6d) was readily demethylated by common nucleophiles, including dimethyl sulfide and piperidine, and was further attacked at S-4 by n-butyllithium with ring scission to form 1-butylthio-2-methylthio-1-phenylethylene (8), which was also synthesized by an unambiguous route. Sodium hydride selectively abstracted H-3 of 6d to yield quantitatively the ring-cleavage product, 1-phenylethynylthio-2-methylthio-1-phenylethylene (13a), and the methyl-d3 analog (6e) behaved similarly. The nmr spectra of the salts (e.g., 6a) revealed that H-3 of the dithiinium ring is strongly deshielded (δ 8.75) via d-orbital conjugation of the sulfonium sulfur, while H-6 remains more normally olefinic (δ 6.90) suggesting a lack of cyclic conjugation. © 1968, American Chemical Society. All rights reserved.