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A Novel Condensed Heterocyclic Quinone with a Dibenzofuranobisthiadiazole Skeleton

Upon oxidation of hydroquinone fused with 1,2,5-thiadiazole by using FeCl3, a novel dibenzofurano[1,2-c:6,7-c']bis[1,2,5]thiadiazole-type quinone was obtained whereas the similar dimerization-condensation process did not occur for the pyrazine-fused hydroquinone under the similar conditions. Quinones consist of an important class of compound undergoing facile electron transfer. The heterocyclic analogues are also attracting from the viewpoint of biological avtivity1 as well as the important charge-transporting material in the field of organic materials chemistry.2 The redox reactions of quinones are characterized by the reversible interconversion with the corresponding hydroquinones. However, oxidation of hydroquinone is sometimes accompanied by further reaction of the resulting quinone to give the product with a condensed stucture.3 Here we report the formation and crystal structure of heterocyclic quinone 1, the first member of dibenzofurano[1,2-c:6,7-c']bis[1,2,5]thiadiazole derivative, which was generated upon oxidation of benzo[1,2-c][1,2,5]thiadiazole-4,7-diol 2a.4,5 Previously, the oxidation of hydroquinone 2a was reported to give the corresponding quinone (4H,7H-benzo[1,2-c][1,2,5]thiadiazole-4,7-dione) 3a4,5 when Ag2O was used as an oxidant. We found here that the reaction of 2a with FeCl3•6H2O gave a different product6 as a dark orange crystalline solid (Scheme 1), which shows C=O stretching frequency characteristic to quinones. Its MS spectral data as well as the elemental analysis indicated that the product has the dimerized structure of 3. Its structure was finally determined by X-ray analysis7 to be the five-ring-fused quinone 18 with a hitherto unknown heterocyclic skeleton (Figure 1, Table 1).