42239-06-5Relevant academic research and scientific papers
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
Goumont, Régis,Sebban, Muriel,Sépulcri, Patricia,Marrot, Jérome,Terrier, Fran?ois
, p. 3249 - 3262 (2002)
The reactions of a series of differently substituted nitrobenzofuroxans with isoprene and 2,3-dimethylbutadiene have been investigated. A variety of mono- and di-adducts resulting from normal electron demand Diels-Alder condensations involving the activated C6C7 and/or C4C5 double bonds of the carbocyclic ring as the dienophile contributors have been identified and structurally characterized. The regioselectivity of the reactions is found to be strongly dependent on the substitution pattern of this ring. In the 4-nitro-6-X-series, the diene molecule first adds to the C6C7 double bond if X is a strong electron-withdrawing substituent (X=NO2, SO2CF3) but to the nitroactivated C4C5 double bond if X is a moderately activating substituent (X=CN, CF3). Subsequent addition of a second molecule of diene occurs to give highly stereoselective diadducts in the 6-cyano, 6-trifluoromethyl and 6-nitro systems. Contrasting with this behavior, only monoadducts corresponding to the addition of diene to the nitroactivated C6C7 double bond were obtained in the 4-X-6-nitro-series (X=CN, CF3). 4,6-Dinitrotetrazolo[1,5-a]pyridine reacts similarly to 4,6-dinitrobenzofuroxan, i.e. highly stereoselective diadducts are formed on the reaction with isoprene and 2,3-dimethylbutadiene. A most significant finding is that the treatment of some of the isolated mono- and di-adducts by a strong base like t-BuOK results in a facile β-elimination of nitrous acid. Concomitant oxidative rearomatization of the resulting cyclohexadiene moieties then occurs spontaneously to afford otherwise difficultly available naphtho- or phenanthreno furoxanic or furazanic structures as well as azaphenanthrenotetrazoles.
Nitrobenzoxadiazoles and related heterocycles: A relationship between aromaticity, superelectrophilicity and pericyclic reactivity
Lakhdar, Sami,Goumont, Regis,Boubaker, Taoufik,Mokhtari, Malika,Terrier, Franois
, p. 1910 - 1919 (2006)
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 σ-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 pertaining to the susceptibility of 2-4 to water attack as well as the pKa values for the σ-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 pK a value of ca. 8 is a primary requirement for having H2O competing effectively as a nucleophile with OH- in the formation of hydroxy σ-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. The Royal Society of Chemistry 2006.
