A New Thiatriazine Isomer:  Synthesis, Tautomerism, and Molecular Structure of 3,6-Diphenyl-4H-1,2,4,5-thiatriazine as a Precursor to the 1,2,4,5-Thiatriazinyl Radical

Abstract

The diphenyl derivative of 4H-1,2,4,5-thiatriazine (5) was prepared by oxidative cyclization of 9. The molecular structure of 5, obtained by X-ray diffraction [orthorhombic, Pna2(1), a = 9.7746(13) A, b = 21.692(2) A, c = 5.6580(8) A], 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 with PbO(2), or conversion to and reduction of sulfiminyl chloride 6 with Ph(3)Sb. The weak ESR quintet (a(N) = 1.03 mT, g = 2.0103) observed in the latter 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 thiatriazinyl radicals exhibit similar spin distribution patterns. 1,2,4,5-Thiatriazinyl radical (I-R) is calculated to be 23. 1 kcal/mol less stable than the most stable 1,2,4,6 isomer II-R.

1,2,4,6-Thiatriazinyls: Stable Free Radicals With Well-Defined Epr Spectra

“…Kaszinsky has reported recently on a second isomer, but it does not form a stable free radical. 6 Hence the 1,2,4,6-thiatriazinyls, planar ring compounds with substituents in the 3 and 5 position, remain the only isomers for which stable radicals can be generated. Recent detailed computational studies have placed thiazyl radicals, including thiatriazinyls, on a firm theoretical footing.…”

Stable heterocyclic radicals

“…1,2,4,5-Thia(seleno)triazinyl radicals are not described in literature. According to the quantum chemical calculations, this type of a radical should be one of the most stable thiazinyl radicals, 142 however, all attempts to synthesize the diphenyl derivative 92 by the oxidation of 3,6-diphenyl-1,2,4,5-thiatriazine (93) by lead dioxide, thiantrenium perchlorate or iodine have failed.…”

New π-Delocalized Persistent Radicals

“…Considering the structural complexity and chemical sensitivity of the potential liquid crystalline molecules, we focused on oxidation of 1,2-thiazines as precursors for radical generation, rather than reduction of the appropriate sulfiminyl chlorides ( Figure 2). The 1,2-thiazines are generally stable, they can be prepared in a regiospecific manner and their purification is straightforward [6,8,10]. In contrast, the sulfiminyl chlorides and salts are known to be reactive, highly moisture sensitive and many are strong oxidizing reagents, which may be incompatible with some functional groups [11,12] Thus far, we have synthesized nine 1,2-thiazines as precursors to five classes heterocyclic radicals (A, B, E, F, and H) shown in Chart I [6,8,10,13].…”

“…The pyrazino derivative 2c and halogenated radicals 2d and 2e were formed only in low yields, impractical for preparative work. Attempts to generate 1 or 5b by this method were frustratingly unsuccessful [6,13]. The efficiency of radical generation under these conditions would appear to depend upon the N-H homolytic bond dissociation energy (HBDE) of the precursor (Table 1 and Figure 3).…”

“…Most the heterocycles shown in Figure 1 are derived from previously unknown ring systems. Thus, in the first phase of the project, we focused on the development of synthetic methodologies for the construction of selected heterocyclic skeletons [5][6][7][8]. In parallel, we have developed a computational protocol to evaluate spin distribution, hyperfine coupling constants (hfcc), and redox properties of the heteroaromatic radicals [9].…”