Interaction of nitroalkane anions with superelectrophilic 4,6-dinitrobenzofuroxan: σ-adduct formation and vicarious heteroaromatic substitution of hydrogen

Abstract

Carbanions of nitromethane, nitroethane, 1-nitropropane and 2-nitropropane readily add to 4,6-dinitrobenzofuroxan (DNBF) to form carbon-bonded σ-adducts (5a–d) which are very stable and isolable as crystalline alkali salts. In the case of the nitroethane and 1-nitropropane systems, the complexation results in the formation of two chiral centres, so that diastereoisomeric σ-adducts are formed. Structural features are discussed on the basis of the collected ¹H and ¹³C NMR parameters. Contrary to expectation, addition of base (MeO^–, CF₃CH₂O^–, NEt₃) to dimethyl sulfoxide solutions of 5a–c did not result in further ionization of the nitroalkane moiety and formation of dianions 7a–c. Instead, it promotes a base-catalysed β-elimination of nitrous acid. This process also occurs and is especially clean with the 2-nitropropane adduct 5d, an example of the rare occurrence of a nucleophilic aromatic substitution of an hydrogen atom assisted by the departure of a vicarious nitro group in the exocyclic moiety. The result is re-aromatization of the carbocyclic ring of 5a–d with formation of carbanions (8a–d) which are the conjugate bases of the corresponding 7-R-4,6-dinitrobenzofuroxans (R = Me, Et, Prⁿ, Prⁱ). Despite their lack of stability, NMR characterization of these carbanions could be made. Rate and equilibrium data for DNBF complexation by the four nitroalkane anions studied have been obtained in aqueous solution. The results show that the thermodynamic stability of the adducts 5a–d is 10⁵–10⁸ times greater than that of the analogous σ-adducts of 1,3,5-trinitrobenzene, the common reference aromatic electrophile in σ-complexation processes. Such stability differences emphasize the extremely high electrophilic character of DNBF. Analysis of the rate data reveals that nitrocarbanions exhibit especially low intrinsic reactivities in σ-adduct forming reactions.