A new thiatriazine isomer: Synthesis, tautomerism, and molecular structureof 3,6-diphenyl-4H-1,2,4,5-thiatriazine as a precursor to the 1,2,4,5-thiatriazinyl radical

The diphenyl derivative of 4H-1,2,4,5-thiatriazine (5) was prepared by oxid ative cyclization of 9. The molecular structure of 5, obtained by X-ray dif fraction [orthorhombic, Pna2(1), a = 9.7746(13) Angstrom, b = 21.692(2) Ang strom, c = 5.6580(8) Angstrom], compares favorably with that predicted with ab initio calculations. The thiatriazine 5 was used as a precursor to the 3,6-diphenyl-1,2,4,5-thiatriazinyl radical (4) through either oxidation wit h PbO2, or conversion to and reduction of sulfiminyl chloride 6 with Ph3Sb. The weak ESR quintet (alpha(N) = 1.03 mT, g = 2.0103) observed in the latt er case correlates well with the molecular structure of 4, but the results of DFT calculations are ambiguous. Ab initio calculations show that 4H-1,2, 4,5-thiatriazine (I-4H) is the most stable tautomer and is the second most stable isomer among the six possible thiatriazines. All isomeric thiatriazi nyl radicals exhibit similar spin distribution patterns. 1,2,4,5-Thiatriazi ny1 radical (I-R) is calculated to be 23.1 kcal/mol less stable than the mo st stable 1,2,4,6 isomer II-R.