A theoretical and experimental study of the polar Diels–Alder cycloaddition of cyclopentadiene with nitrobenzodifuroxan

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 1 H 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.