Thiadiazolium ylides: Substituted 2H-1,3,5-thiadiazines and 1,4,5- trisubstituted-imidazoles from 1,2,4- and 1,2,5-thiadiazolium-2- unsubstituted methanide (ylide) systems: ring expansions and ring interconversions via sulfur-nitrogen heterotriene intermediates. Mechanistic ab initio calculations. Azolium 1,3-dipoles

Quaternisation of 3,5-diaryl-1,2,4-thiadiazoles with trimethylsilylmethyl triflate at 40 8C occurred at N-2. Separate desilylation of the salts resulted in a ring expansion to substituted 2H-1,3,5-thiadiazines 5. Heating of these with ethanolic sodium ethoxide caused sulfur extrusion and ring contraction to 2,4-disubstituted imidazoles 6. 3,4-Diaryl1,2,5-thiadiazoles were less reactive to alkylation and trimethylsilylmethylation required heating at 80 8C. Treatment of the salts with CsF unexpectedly gave 1-trimethylsilylmethyl-4,5-diarylimidazoles 21. H, C, N NMR spectra are described and the mechanisms were studied by ab inito calculations with the GAUSSIAN94 series of programmes using the HF/6-31G* theoretical level.