Benzoxathiete and related structures: experimental and quantum chemical studies

Abstract

The products, plausible intermediates, and their mechanisms of formation in the aprotic diazotization of 2-[(2-acetoxyethyl)-sulfinyl (and sulfonyl)]anilines with isoamyl nitrate have been investigated by experimental and quantum chemical methods. Oxidation of 1,2,3-benzthiadiazole with hydrogen peroxide in acetic acid affords 1,2,3-benzthiadiazole-1-oxide, 13. The thermal stability of 13 up to 135 °C, together with EPR evidence which disfavors an aryldiazenyl radical precursor, discount 13 as an intermediate in the formation of benzoxathiete 14 or its valence tautomer 17 during aprotic diazotization of the sulfinylanilines. Electron paramagnetic resonance evidence in conjunction with spin trapping indicates an intermediate arylaminyl radical. This evidence, taken together with abinitio calculations of optimized geometries energies and energy differences for biradical intermediate 15, favors a mechanism of formation of benzoxathiete 14 via aryldiazoate anhydride 5 thence to rapid ring closure of carbon-centered radical 8. Further oxidation of 1,2,3-benzthiadiazole-1-oxide, 13, with hydrogen peroxide in methanol – acetic acid affords biphenylene and dibenzo-1,4-oxathiane-S-oxide, 38. Rose bengal sensitized photooxidation of 1,2,3-benzthiadiazole affords 13, biphenylene, and 38. Formation of the latter, in which one of the original S—O bonds has been broken, requires the formation of benzoxathiete-S-oxide, 34, and its rapid valence tautomerism to ketosulfine 36 and (2 + 4) cycloaddition of 36 to the simultaneously generated dehydrobenzene to give 38. Both ring closure of singlet biradical 35 to 34 and valence tautomerism of the latter to 36 are predicted by abinitio calculations to be facile and exothermic. In contrast to the aprotic diazotization of 2-[(2-acetoxyethyl)sulfinyl]anilines, the reaction of isoamyl nitrite with the corresponding sulfonyl anilines may plausibly follow a mechanism via 1,2,3-benzthiadiazole-1,1-dioxide 33 owing to the thermal instability of the latter and supported by abinitio treatments of the energetics of the processes involved. In addition EPR evidence, in conjunction with spin trapping of carbon centered radicals, support the viability of the pathway via 23, 25, 28, and 16 to biphenylene 20. The abinitio calculations of the energy differences between the reaction intermediates and estimates of the activation energies elucidated several aspects of these novel reactions. Keywords: benzoxathiete, abinitio calculations, valence tautomerism.