The mechanism of the Diels–Alder interaction of 4‐nitrobenzodifuroxan (NBDF) with cyclopentadiene (Cp), resulting in the highly stereoselective formation of the stable endo [2+4] adduct has been elucidated by combining density functional theory (DFT) and experimental studies. Calculations at the B3LYP/6‐31G* level reveal that this adduct does not derive from a direct normal electron demand cycloaddition process. Instead, the evidence is that the interaction proceeds initially through a very polar inverse electron demand process to afford the intermediate [4+2] cycloadduct. Then, this species undergoes a complete conversion into the more stable isomeric endo [2+4] adduct via a [3+3] Claisen‐type sigmatropic shift. The lifetime of the [4+2] intermediate was sufficient to allow its full 1H NMR characterization at −10 °C. Viewing the results in the general context of the Diels–Alder reactivity of nitrobenzofuroxans, a noteworthy feature is that the similar behavior of NBDF and 4‐aza‐6‐nitrobenzofuroxan (ANBF) goes along with a similar positioning of the two compounds on the general electrophilicity scale of Parr et al. Copyright © 2008 John Wiley & Sons, Ltd.

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New quinoxaline 1,4‐dioxide derivatives have been synthesized from novel fluorinated benzofuroxans such as 4‐fluorobenzofuroxan, which is prepared for the first time. Furthermore, the preparation 4,6‐difluorobenzofuroxan has been revisited because we were unable to reproduce the reported synthetic method. Several synthetic pathways have thus been investigated, and the optimal way to prepare this disubstituted benzofuroxan was from the 3,5‐difluoro‐2‐nitroaniline. The various synthetic attempts have allowed the isolation of interesting new compounds such as hydroxybenzotriazole‐like heterocycles or benzoquinone 2‐diazide. In the latter case, our study reveals some interesting features in the mechanism of their formation. The first structural elucidation of benzoquinone 2‐diazide through an X‐ray crystallographic study is also reported. This study has unambiguously shown that benzoquinone 2‐diazide can be involved in Diels–Alder reactions. The first case of a reaction involving a nitro‐substituted benzoquinone 2‐diazide and cyclopentadiene is also reported. To understand better the influence of the fluorine atoms on the reactivity of benzofuroxans and benzofurazans, a wide array of new fluorinated heterocycles were synthesized together with some already known compounds. This has enabled an extensive investigation of their electrophilic behavior to be undertaken through a theoretical and an electrochemical study. From these studies, it could be deduced that the replacement of nitro groups by fluorine atoms results in a significant decrease in the electrophilic character of benzofuroxan. Nevertheless, these compounds could undergo SNAr processes, leading to new functionalized heterocycles. The first examples of aromatic nucleophilic substitution of fluorine with these compounds are also described.

The Diels-Alder reaction of 4-nitrobenzodifuroxan (NBDF) with 1-methoxy-3-trimethylsilyloxy-1,3-butadiene has been investigated experimentally and theoretically. Treatment of NBDF with excess diene in chloroform at room temperature was found to afford a single product that contained a carbonyl functionality. Based on an X-ray structure and NMR spectroscopic data, the product appeared to be a result of the hydrolysis of the OSiMe(3) moiety of the thermodynamically more stable endo [2+4] cycloadduct, characterized by a cis arrangement of the MeO and NO(2) functionalities. In situ NMR investigations of the interaction were carried out at room temperature in CDCl(3) and at -40 °C in deuterated acetonitrile. Calculations at the B3LYP/6-31G* level in the gas phase and in acetonitrile were carried out under the assumption that the most stable cis conformation of the diene is also the most reactive in the interaction. The analysis revealed the NBDF/cis diene interaction involves the formation of a zwitterionic intermediate. Importantly, this intermediate is formed in two preferred conformations, which correspond to the endo and exo modes of approach of the reagents. Cyclization of these two identified conformations afforded the experimentally characterized endo and exo [2+4] cycloadducts. According to the calculations, the interconversion of the two conformers can either take place through a return to the pre-reaction complexes or it can occur by rotation through an intermediate conformation of lesser stability. In view of the stepwise character of the interaction, the possibility that the intermediate zwitterion is the result of the interaction between NBDF and the trans diene could not be excluded. Calculations carried out with the most stable and more populated s-trans conformer confirmed this idea and supported the role of the zwitterion in the overall interaction.