New reactivity patterns in the Diels–Alder reactivity of nitrobenzofuroxans †

Abstract

The reaction of cyclopentadiene with 4-nitro-6-trifluoromethylsulfonylbenzofuroxan 5 in dichloromethane or chloroform proceeds stereoselectively at 0 ЊC to afford a single compound 6, which is shown to result from an inverse electron-demand Diels-Alder condensation involving the carbocyclic ring of 5 as the diene contributor. However, the adduct 6, whose X-ray structure could be obtained, is not the thermodynamically stable product of the interaction. Keeping a solution of 6 at room temperature results after a few days in the isolation of a new adduct 7a which arises from a regioselective and stereoselective normal electron-demand Diels-Alder condensation involving the C(4)C( 5) double bond of 5 as the dienophile contributor. The carbodienic behaviour of 5 as well as the preferred dienophilic reactivity of the C(4)C( 5) rather than of the C(6)C( 7) double bond, represent two new reactivity patterns in the chemistry of the nitrobenzofuroxans.

NMR Spectroscopy: A Useful Tool in the Determination of the Electrophilic Character of Benzofuroxans - Case Examples of the Reactions of Nitrobenzofuroxans with Dienes and Nucleophiles

“…For example, for an IEDDA adducts, the 1 H NMR spectra show a deshielded signal at 6 ppm typical of H 11 and a multicoupled signal at 4 ppm typical of H 10 (see Figure 3) while 13 C NMR spectra show that the signals pertaining to carbon 7, 10, 14 are found to be at higher field in the case of IEDDA adducts than in the case of NEDDA adducts. For these latter adducts, the 1 H NMR spectra exhibited broad signals at 3.5 ppm pertaining to H 10 …”

“…On further 15 The 13 C NMR spectra of benzofuroxans A-F show some characteristic features. The complete 13 C NMR assignment of these compounds has been obtained using one-and twodimensional NMR techniques including HMQC and HMBC experiments. So, the two resonances pertaining to C 5 and C 7 are readily determined while the resonances of the C 9 and C 8 appear to be the key features of the 13 C NMR spectra of benzofuroxans.…”

“…[13][14][15][16][17][18] More importantly, DNBF, A reacts quantitatively at room temperature with such weak carbon -nucleophiles, as benzenoid aromatics (phenols, anilines) or -excessive heteroaromatics (indoles, pyrroles, thiophenes) to afford stable anionic C-bonded -adducts which are formally the products of S E Ar substitution on the benzene or hetarene ring. Coupling to weakly activated enolic double bonds is also a process that is readily achieved with DNBF, A.…”

“…Crucial evidence for this relationship has been the discovery that the nitro-activated double bonds of this ring behave similarly to nitroalkene fragments in a variety of Diels-Alder processes, acting as dienophiles or heterodienes depending upon the reaction partner and the experimental conditions at hand. [10][11][12][13][14][15][16] A first illustrative sequence refers to the reaction of ANBF with cyclohexadiene. Reflecting the potential 1-oxide/3-oxide interconversion of benzofuroxans through the intermediacy of an o-dinitroso intermediate, 15 diadducts 5 resulting from normal electron-demand Diels-Alder (NEDDA) processes involving the N=O double bonds of such intermediates as the dienophile contributors have also been isolated.…”

“…On the other hand, the preferred formation of the unsymmetrical IEDDA-NEDDA adduct 12 implies a greater dienophilic reactivity of the remaining nitroolefinic moiety in the IEDDA adduct 11 than in the NEDDA adduct 10. [10][11][12][13][14][15][16] 3.2 Elucidation of the reaction of DNBF with 2,3-dimethylbutadiene.…”