Regiochemistry of the reaction between dibenzothiophene radical cation and nucleophiles or nitrogen dioxide

The regiochemistry of nucleophile or radical attack upon dibenzothiophene radical cation (2(.+)) has been studied, using the photolysis of 2-tetranitromethane, the NO2 or nitrous acid catalyzed (NAC) nitration of 2 and the silver(II) mediated acetoxylation of 2 as test reactions in all likelihood mediated by 2(.+). The results are discussed in relation to theoretical predictions based on the VBCM model. The photolysis of the charge-transfer complex of 2 and tetranitromethane in dichloromethane gives mainly dibenzothiophene sulfoxide (3) and minor amounts of 2-nitrodibenzothiophene (4), r-1-hydroxy-c-4-trinitromethyl-1,4-dihydrodibenzothiophene (5), t-2-nitro-r-1-trinitromethyl-1,2-dihydrodibenzothiophene (6) and c-1-nitro-r-4-trinitromethyl-1,4-dihydro-dibenzothiophene 7. In acetonitrile the major product is the sulfoxide 3 together with 2-nitrodibenzothiophene (4) and 4-nitrodibenzothiophene (9). The photolysis in 1,1,1,3,3,3-hexafluoropropan-2-ol yields only the sulfoxide 3 and minor amounts of the 2-nitroarene 4. Nitration of 2 by NO, in dichloromethane gives largely 3 and the 2-nitro isomer 4, as does NAC nitration in acetic acid. The oxidation of 2 by an Ag-II complex of 2,2'-bipyridine gave mainly 1- and 4-acetoxydibenzothiophene and a minor proportion of the 3-acetoxy isomer, no sulfoxide 3 being formed. The X-ray crystal structure is reported for compound 5.