Abstract
The reactions of 4,6-dinitrobenzofuroxan (DNBF) and 4,6-dinitrobenzofurazan (DNBZ) with isoprene and 2,3-dimethylbutadiene afford stable diadducts (10a-d), which are shown to result from two consecutive and highly stereoselective normal electron-demand Diels-Alder (NEDDA) condensations. These condensations involve the two nitro-activated double bonds of DNBF and DNBZ as the dienophile contributors. Evidence that the first molecule of diene adds to the C6-C7 rather than the C4-C5 double bond has been obtained through 15 N-labelling of the 4-NO2 group of DNBF and NMR characterization of the corresponding short lived monoadducts (9a-d). An unprecedented finding is that these monoadducts undergo subsequent addition of the second molecule of diene to give not only the stable symmetrical NEDDA-NEDDA diadducts 10a-d but also the unsymmetrical diadducts 11a-d resulting from inverse electron demand Diels- Alder (IEDDA) condensations in which the O4N4C4C5 fragment of 9a-d acts as a heterodiene moiety. Although 11a-d are not thermodynamically stable, undergoing a slow conversion to 10a- d in solution, a pure sample of the isoprene-DNBF NEDDA-IEDDA diadduct 11a could be isolated as a crystalline solid and characterized by X-Ray crystallography.
Highlights
In the last two decades, much evidence has been accumulated showing that nitro-substituted2,1,3-benzoxadiazoles and related 1-oxides, commonly referred to as nitrobenzofurazans and nitrobenzofuroxans, respectively, are neutral 10-π electron-deficient heteroaromatic substrates which, in many processes exhibit extremely high electrophilic character.[1,2,3,4,5,6,7,8,9] As a best illustration for this behavior is the finding that 4,6-dinitrobenzofuroxan (DNBF), the reference compound in this family, behaves as a stronger electrophile than the 4-nitrobenzenediazonium cation.[10]
This has led to many analytical applications with the use of DNBF as a suitable probe to assess the reactivity of extremely weak carbon nucleophiles such as benzenoid aromatic or π-excessive heteroaromatics with large negative pKa values, e.g. 1,3-dimethoxybenzene,10c 3methoxythiophene10b or aniline.4b In all of the above processes, covalent addition of the carbon nucleophile takes place at C-7 of the carbocyclic ring of DNBF to give stable anionic σ-complexes, e.g. 1 or 2, as it does in all reported interactions of DNBF with oxygen, sulfur or nitrogen nucleophiles.[1]
The results obtained in the present study reveal that the interactions of DNBF and DNBZ with isoprene and 2,3-dimethylbutadiene proceed through the initial formation of the monoadducts 9a-d, resulting from a regioselective normal electron-demand Diels-Alder (NEDDA) process involving the nitroactivated C6-C7 double bonds of the parent heterocycles as the dienophile contributors
Summary
In the last two decades, much evidence has been accumulated showing that nitro-substituted. It has been convincingly recognized that the exceptional electrophilic character of nitrobenzofuroxans is closely related to the low aromaticity of the carbocyclic ring Crucial evidence for this relationship has been the discovery that the nitro-activated double bonds of this ring behave 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.[11,12] A first illustrative sequence refers to the reaction of DNBF with cyclopentadiene. The reaction of this compound with 1-trimethylsilyloxybuta-1,3-diene 6 gives rise quantitatively to the monoadduct 7 in its racemic form, resulting from a regioselective and diastereoselective NEDDA process involving the C6-C7 double bond of DNBF as the dienophile contributor.[15] Interestingly, 7 was inert to further reaction with 6 but not with vinyl ethyl ether In this instance, the dihydrooxazine N-oxide 8 was obtained in 92% yield, arising from a highly diastereoselective IEDDA condensation involving the O4N4C4C5 fragment of 7 as the heterodiene contributor. The stereochemistry of 8, which is overall the result of an “unsymmetrical” NEDDA- IEDDA reactivity sequence, was firmly attributed, as that of the IEDDA-NEDDA diadduct 5 in Scheme 1, by X-ray crystallography.[15]
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