1,2,3-Thiadiazol-3-ium-3-methanide (ylide) 1,3-dipoles: cycloaddition–rearrangement sequences leading to substituted 1-(2-vinylthioethenyl)pyrazole systems: azolium 1,3-dipoles

Abstract

Alkylation of 4,5-diaryl-1,2,3-thiadiazoles and 1,2,3-benzothiadiazoles with trimethylsilylmethyl trifluoromethanesulfonate occurred at N-3. Treatment of the salts with CsF generated new 1,2,3-thiadiazol-3-ium-3-methanide 1,3-dipoles. These gave in situ cycloaddition–rearrangement reactions with some alkyne and alkene dipolarophiles leading to new substituted 1-(2-vinylthioethenyl)pyrazole systems, where a second molecule of dipolarophile was added at the thiol SH which was generated by opening of the thiadiazole ring. ¹H, ¹³C and ¹⁵N NMR spectra are reported, as well as X-ray crystal structures on dimethyl 1-{(Z)-2-[(Z)-1,2-bis(methoxycarbonyl)vinylthio]-1,2-diphenylethenyl}-1H-pyrazole-3,4-dicarboxylate 6a and 1-[(Z)-2-cyanoethylthio-1,2-diphenylethenyl]-4,5-dihydro-1H-pyrazole-3-carbonitrile 10. The X-ray data on 6a suggested a weak chalcogen effect from the pyrazole 2-N to the nearby S-atom.