The multifaceted Diels-Alder reactivity of 4,6-dinitrobenzofuroxan and 4,6-dinitrobenzofurazan towards isoprene and 2,3-dimethylbutadiene

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-C 5 double bond has been obtained through 15N-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 O 4N4C4C5 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.