The Diels–Alder reactivity of nitrobenzofuroxans: mono- and di-adducts of isoprene and 2,3-dimethylbutadiene. New convenient precursors to naphtho- and phenanthreno-furoxanic and -furazanic structures

The Diels-Alder reaction of 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene: a case example of a stepwise cycloaddition

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.

Superelectrophilicity of the Nitroolefinic Fragment of 4-Nitrobenzodifuroxan in Michael-Type Reactions with Indoles: A Kinetic Study in Acetonitrile

The kinetics of the coupling of 4-nitrobenzodifuroxan (NBDF) with a series of indoles 8 a-e to give the expected Michael-type adducts 9 a-e have been investigated in acetonitrile solution. No significant influence of the nature of the isotopic substitution at C-3 of the indole ring has been found, indicating that the NBDF addition step is the rate limiting step of the SEAr substitution of the indole moiety. This implies that the measured second-order rate constants (k) for the reactions are identical to the second order rate constants (k1NBDF) associated to the C--C coupling step. By using the known N and s parameters characterizing the nucleophilicity of indoles, the k1NBDF rate constants are found to fit nicely to the three parameters equation logk1=s(N+E) introduced by Mayr to describe the feasibility of nucleophilic-electrophilic combinations. Based on this, the electrophilicity parameter E of NBDF could be determined as E=-6.15. This corresponds to a positioning of the reactivity of the nitroactivated double bond of NBDF in the domain of superelectrophilicity previously defined for nitrobenzofuroxans, in accord with the finding that the rates of coupling of 8 a-e with NBDF are only one order of magnitude lower than those for the coupling of these indoles with 4,6-dinitrobenzofuroxan (DNBF). The theoretical scale of electrophilicity introduced by Domingo et al. on the basis of the global electrophilicity index omega defined by Parr is also a very useful tool to discuss the relative reactivities of NBDF, DNBF, and a number of differently activated C==C double bonds.

Nitrobenzoxadiazoles and related heterocycles: a relationship between aromaticity, superelectrophilicity and pericyclic reactivity

A study of the dual electrophilic and pericyclic reactivity of 4,6-dinitrobenzofurazan (DNBZ, 2), 4,6-dinitro-2,1,3-benzothiadiazole (DNBS, 3), 4,6-dinitro-2,1,3-benzoselenadiazole (DNBSe, 4) is reported. Kinetic and thermodynamic measurements of the ease of covalent hydration of 2-4 to give the corresponding hydroxy sigma-adducts C-2-C-4 have been carried out over a large pH range in aqueous solution. Analysis of the data has allowed a determination of the rate constants k1(H2O) pertaining to the susceptibility of 2-4 to water attack as well as the pKa values for the sigma-complexation processes. With pKa values ranging from 3.92 for DNBZ to 6.34 for DNBSe to 7.86 for DNBS, the electrophilic character of the three heteroaromatics is much closer to that of the superelectrophilic reference, i.e. 4,6-dinitrobenzofuroxan (DNBF, 1; pKa = 3.75), than that of the standard Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB, pKa = 13.43). Most importantly, water is found to be an efficient nucleophile which contributes strongly to the formation of the adducts C-2 and C-4. This confirms a previous observation that a pKa value of ca. 8 is a primary requirement for having H2O competing effectively as a nucleophile with OH- in the formation of hydroxy sigma-adducts. On the other hand, 2-4 are found to exhibit dienophilic and/or heterodienic behaviour on treatment with isoprene, 2,3-dimethylbutadiene, cyclopentadiene or cyclohexadiene, affording Diels-Alder mono- or di-adducts which have all been structurally characterized. A major finding is that the order of Diels-Alder reactivity follows clearly the order of electrophilicity, pointing to a direct relationship between superelectrophilic and pericyclic reactivity. This relationship is discussed.

Tautomerism and 1H, 13C and 15N NMR spectral assignments of some nitro derivatives of malonic acid diamide

Because of its reactivity, malonic acid diamide (1) was initially identified as an alternative precursor for the development of a new class of high-density insensitive energetic materials possessing low sensitivity to thermal decomposition and detonation by impact. Nitration of 1 was studied under different conditions and led to three different tautomeric forms (2-4) of nitromalonic acid diamine. Using stronger oxidation conditions the oxadiazole 5 was generated in one step. We report the full 1H, 13C and 15N NMR structural characterization of these compounds in DMSO together with thermal, infrared, mass spectrometric and x-ray analysis. Experimental data obtained for 4 are compatible with an enol-imine form. Our interpretation is consistent with calculated 1H and 13C NMR spectra (ACD).

Sigma-complex formation and oxidative nucleophilic aromatic substitution in 4-nitro-2,1,3-benzoxadiazoles

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.

High Brønsted beta nuc values in SNAr displacement. An indicator of the SET pathway?

Nucleophilic substitutions of 4-chloro-7-nitrobenzofurazan (NBD-Cl) and 3-methyl-1-(4-nitrobenzofurazanyl)-imidazolium ions (NBD-Im+) with a series of 4-X-substituted anilines have been kinetically investigated in 70-30 (v/v) and 20-80 (v/v) H2O-Me2SO mixtures. The rate-limiting step in these reactions is nucleophilic addition with formation of Meisenheimer-type sigma-adducts followed by fast expulsion of the leaving group (Cl- or Im). The reactions are characterized by a notable sensitivity to basicity of the aniline nucleophiles, with Hammett rho values of -2.68 and -3.82 in 30% and 80% Me2SO, respectively, for NBD-Cl and even more negative values, -3.43 and -5.27, respectively, for NBD-Im+. This is consistent with significant development of positive charge at the nitrogen atom of the zwitterionic sigma-adduct. Unexpectedly, the Brønsted-type plots reveal abnormally high beta nuc values, ca. 1.0 and 1.3-1.4, respectively. Satisfactory correlations between the rates of the reactions and the oxidation potentials of the respective anilines support a SET mechanism for this process, i.e. initial (fast) electron-transfer from the aniline donor to the nitrobenzofurazan acceptor moiety and subsequent (slow) coupling of the resulting cation and anion radicals within the solvent cage with formation of the sigma-adduct. An alternative possible explanation of the high beta nuc values being related to the strong--I effect exerted by the negatively charged 4-nitrobenzofurazanyl structure, which would induce a greater positive charge at the developing anilinium nitrogen atom in the sigma-adduct-like transition state as compared with the situation in the reference protonation equilibria of anilines, is considered less probable. It is thus proposed that obtention of abnormal beta nuc values may be an indicator of electron-transfer in nucleophilic aromatic substitution and highlights the transition from the polar (SNAr) to the single electron-transfer (SET) mechanism.