INTERACTION OF NITROALKANE ANIONS WITH SUPERELECTROPHILIC 4,6-DINITROBENZOFUROXAN - SIGMA-ADDUCT FORMATION AND VICARIOUS HETEROAROMATIC SUBSTITUTION OF HYDROGEN

Carbanions of nitromethane, nitroethane, 1-nitropropane and 2-nitropro pane readily add to 4,6-dinitrobenzofuroxan (DNBF) to form carbon-bond ed sigma-adducts (5a-d) which are very stable and isolable as crystall ine alkali salts. In the case of the nitroethane and 1-nitropropane sy stems, the complexation results in the formation of two chiral centres , so that diastereoisomeric sigma-adducts are formed. Structural featu res are discussed on the basis of the collected H-1 and (13)CNMR param eters. Contrary to expectation, addition of bask (MeO(-), CF3CH2O-, NE t(3)) to dimethyl sulfoxide solutions of 5a-c did not result in furthe r ionization of the nitroalkane moiety and formation of dianions 7a-c. Instead, it promotes a base-catalysed beta-elimination of nitrous aci d. This process also occurs and is especially clean with the 2-nitropr opane adduct 5d, an example of the rare occurrence of a nucleophilic a romatic substitution of an hydrogen atom assisted by the departure of a vicarious nitro group in the exocyclic moiety. The result is re-arom atization 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-dini trobenzofuroxans (R = Me, Pt, Pr-n, Pr-i). Despite their lack of stabi lity, NMR characterization of these carbanions could be made. Rate and equilibrium data for DNBF complexation by the four nitroalkane. anion s studied have been obtained in aqueous solution. The results show tha t the thermodynamic stability of the adducts 5a-d is 10(5)-10(8) times greater than that of the analogous sigma-adducts of 1,3,5-trinitroben zene, the common reference aromatic electrophile in sigma-complexation processes. Such stability differences emphasize the extremely high el ectrophilic character of DNBF. Analysis of the rate data reveals that nitrocarbanions exhibit especially low intrinsic reactivities in sigma -adduct forming reactions.