Abstract
A detailed nuclear magnetic resonance study of 4,6-dinitro-2,1,3-benzoxadiazole N-oxide (DNBF), which includes a 2D INADEQUATE analysis and experiments with samples 15N-labelled at the NO2 groups, is described. The results lead to an unambiguous assignment of the carbon and proton chemical shifts in various solvents. Some 1H and 13C data pertaining to 4,6-dinitro-2,1,3-benzoxadiazole (DNBZ), the deoxygenated analog of DNBF, are also reported, allowing the influence of the N-oxide group on the proton and carbon chemical shifts to be discussed. It is shown that all of the 1H nmr evidence so far reported for the characterization of DNBF σ adducts as arising from nucleophilic addition to the 7-position, i.e., as structure 2a, has no significance. That complexation actually occurs at this position is, however, convincingly demonstrated by 15N data. These also allow us to establish that this process is both kinetically and thermodynamically favored. Keywords: nitrobenzofuroxans, Meisenheimer complexes, 13C nmr of benzofuroxans, 2,1,3-benzoxadiazole N-oxides, nitrobenzofuroxan σ-adducts.
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