Formation of Benzoxathiete under Mild Conditions and Its Valence Tautomerism in Solution to Monothio-o-benzoquinone: An Experimental and Quantum Chemical Study

Article abstract of DOI:10.1021/ja00183a039

Aprotic diazotization of 2-<(2-acetoxyethyl)sulfinyl>aniline in dimethoxyethane affords products that include biphenylene and dibenzo-1,4-oxathiane.Detection of these products is consistent with the formation of benzoxathiete and the valence tautomerism to monothio-o-benzoquinone with concomitant formation of dehydrobenzene by a competing pathway.The latter was independently trapped with 1,3-diphenylisobenzofuran and with 9,10-dimethylanthracene.The requirement for SO group participation in the formation of benzoxathiete is established by comparison with the behavior of the analogous thioether and sulfone compounds.EPR and spin-trapping experiments confirm the intermediacy of both oxygen- and nitrogen-centered free radicals which is consistent with the homolytic pathways proposed for the diazotization process.Parallel aqueous diazotization of 2-<(2-acetoxyethyl)sulfinyl>aniline affords vinyl acetate, phenol, and halobenzene consistent with the generation of dehydrobenzene but not benzoxathiete under these conditions.Spin-trapping/EPR studies gave no evidence for free-radical components in the protic diazotization reaction.Ab initio calculations using the 3-21G^* and 6-31G^* basis sets within the Hartree-Fock approximation, as well as the MP2/3-21G^* method, predict an energetically feasible tautomerism of benzoxathiete to monothio-o-benzoquinone.The 3-21G^* calculations reveal the presence of a biradical intermediate for this reaction which, as a singlet, features an energy higher than the benzoxathiete by 33 kcal/mol, while as the corresponding triplet it proves to be lower in energy than the benzoxathiete by 2.5 kcal/mol.This process, however, might be symmetry forbidden.By contrast, the symmetry-allowed <8s+2s> cycloreversion pathway of benzoxathiete to dehydrobenzene and SO is energetically much less favorable.