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 H-1 NMR characterization at -10 degrees 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 (C) 2008 John Wiley & Sons, Ltd.