The feasibility of carrying out nucleophilic displacement of hydrogen from highly electron-deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a series of nitrobenzofuroxans 3a-i and two related heterocycles 5 and 6, with the 2-nitropropenide anion. Although this series corresponds to a large modulation in the electrophilic properties of the six-membered ring, all reactions first lead to the quantitative formation of the sigma-adducts C-3a-i, C-5 and C-6 arising from covalent addition of the nucleophile to the C-7 carbon. With the exception of the 4-aza substituted member C-5, all the adducts have been isolated as pure and very stable alkali salts. Measurements of the oxidation potentials by cyclic voltammetry reveal that the ease of subsequent hydrogen substitution at carbon-7 strongly decreases with increasing electron deficient character of the six-membered ring. The measured E0 values are in the range 0.5-0.6 V (vs. SCE) for the 4-nitro-benzofuroxan and -benzofurazan adducts (C-3e, C-3i) but they go up to 1.20-1.33 V for the 4,6-dinitro- and 4-nitro-6-trifluoromethanesulfonylbenzofuroxan adducts (C-3a,d) in acetonitrile. Consistently with these E0 values, only very powerful oxidants such as the Ce4+/Ce3+ or the MnO4-/Mn2+ couples can successfully oxidize the adducts leading to the expected substitution products in moderate to good yields (35-72%). Interestingly, the rearomatization of the 4-nitro substituted benzofuroxan adducts proceeds with a partial Boulton-Katritzky rearrangement of the resulting products. Another noteworthy result is that the 4-nitrobenzofuroxan and 4-nitrobenzofurazan molecules suffer competitive addition of the (CH3)2C-NO2- anion to the 5- and 7- positions under some experimental conditions. This represents the first well-defined example of isomeric addition of a carbon nucleophile to these heterocycles.

A detailed thermodynamic and kinetic investigation of the reactions of 4,6-dinitrotetrazolo[1 ,5-a]pyridine (DNTP) with water and methanol has been made in the corresponding solvents. In aqueous solution, covalent addition of water to DNTP occurs with the exclusive formation of an anionic hydroxy sigma-complex C-4a which is half-formed in a 0.4 M HCl solution (pKa(sigma) = 0.4). This corresponds to a 3 pK-units jump in thermodynamic stability from the most stable hydroxy sigma-complex known so far, i.e. the hydroxy adduct C-3a of 4,6-dinitrobenzofuroxan (DNBF). DNTP forms similarly a very stable methoxy sigma-complex C-4b in methanol (pKa(sigma) = 2.64). Interestingly, the addition of methanol to DNTP also results in the partial formation of a neutral carbinolamine-type adduct (C-5b) at low pH. Rate and equilibrium constants pertaining to most of the reaction pathways involved in the interactions have been determined. In particular, the following rate constants k1(H2O) and k1(MeOH) for formation of C-4a and C-4b have been measured: k1(H2O) = 1.93 s(-1); k1(MeOH) = 3.50 s(-1), to be compared with k1(H20) = 0.035 s(-1); k1(MeOH) = 0.030 s(-1) for sigma-complexation of DNBF under similar experimental conditions. Altogether, the results obtained reveal that DNTP is a considerably more powerful electrophile than DNBF, the common reference as to whether an electron-deficient aromatic or heteroaromatic substrate may be accorded superelectrophilic properties in addition or substitution processes.