Received: 16 June 2008, Revised: 29 September 2008, Accepted: 3 October 2008, Published online in Wiley InterScience: 26 November 2008 A theoretical and experimental study of the polar Diels–Alder cycloaddition of cyclopentadiene with nitrobenzodifuroxan Dmitry V. Steglenko a , Mikhail E. Kletsky a , Sergey V. Kurbatov a , Artem V. Tatarov a , Vladimir I. Minkin a * , Re ´ gis Goumont b * and Franc ¸ois Terrier b * The mechanism of the Diels–Alder interaction of 4-nitrobenzodifuroxan (NBDF) with cyclopentadiene (Cp), resulting in the highly stereoselective formation of the stable endo [2R4] 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 [4R2] cycloadduct. Then, this species undergoes a complete conversion into the more stable isomeric endo [2R4] adduct via a [3R3] Claisen-type sigmatropic shift. The lifetime of the [4R2] intermediate was sufficient to allow its full 1 H NMR characterization at S10 -C. Viewing the results in the general context of the Diels–Alder reactivity of nitrobenzofur- oxans, 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 ß 2008 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: polar Diels–Alder cycloaddition; superelectrophilic olefins; nucleophile–electrophile combination; nitrobenzo- furoxans; DFT calculations INTRODUCTION The high susceptibility of nitro-substituted 2,1,3-benzoxadiazoles and related 10-p-electron heteroaromatic substrates to undergo covalent nucleophilic addition or substitution processes has long attracted considerable attention. [1–25] Quantitative evaluation of thermodynamic reactivity for these Meisenheimer electrophiles is nicely provided by a comparison of the pK a values for water addition. The pK a values for hydration of the prototype 4,6-dinitrobenzofuroxan (DNBF, 1) and 4-aza-6-nitrobenzofur- oxan (ANBF, 2) structures to yield the s-complexes C-1 and C-2 are 3.75 and 4.06, respectively, in aqueous solution, as compared with a pK a value of 13.43 for formation of the analogous adduct C-3 of 1,3,5-trinitrobenzene (TNB, 3). [8,26,27] Based on these figures, neutral compounds such as 1 and 2 have been accorded superelectrophilic properties. [26,27] This has led to numerous synthetic, analytical and biological applications. [10,11,28–33] Of equal interest, however is that compounds like 1 and 2 have also been found to undergo a variety of Diels–Alder reactions, a behavior which is in itself evidence that the six-membered ring of these superelectrophilic heterocycles has a poor aromatic character relative to TNB. [26,34–39] As illustrated in Scheme 1, DNBF can formally behave as do nitroalkenes, [40,41] being susceptible to act as a dienophile in normal electron-demand (NED) Diels–Alder reactions as well as a heterodiene in inverse electron-demand (IED) Diels–Alder reactions. [42,43] In Scheme 1, the reaction of DNBF with cyclopentadiene (Cp) affords initially a mixture of the two stereoselective NED and IED adducts, i.e. the [2þ4] and [4þ2] adducts 5 and 6, respectively. [37] Because the remaining nitroolefinic fragment of the monoadduct 6 is also very reactive, diadduct formation subsequently occurs, proceeding with high stereoselectivity to give the highly functionalized structure 7 as the thermodynamically stable product of the reaction. [37] The reaction of ANBF with Cp has also been studied, yielding a mixture of the two [2þ4] and [4þ2] monoadducts 8 and 9 at 20 8C. Raising the temperature to 0 8C led to conversion of 9 into the more stable adduct 8 (Scheme 2). [26] Density functional theory (DFT) calculations have been recently carried out which have fully confirmed the reactivity patterns depicted in Schemes 1 and 2. [43,44] More importantly, however, these calculations have demonstrated that the formation of the [2þ4] and [4þ2] DNBF adducts 5 and 6 proceeds through the initial formation of the zwitterionic (www.interscience.wiley.com) DOI 10.1002/poc.1469 Research Article * Correspondence to: R. Goumont, Institut Lavoisier de Versailles (ILV), UMR 8180, University of Versailles, 45 Etats-Unis Ave., 78035 Versailles Cedex, France. E-mail: goumont@chimie.uvsq.fr a D. V. Steglenko, M. E. Kletsky, S. V. Kurbatov, A. V. Tatarov, V. I. Minkin Department of Chemistry, Southern Federal University, 7 Zorge St., 344090 Rostov-on-Don, Russian Federation b R. Goumont, F. Terrier Institut Lavoisier de Versailles (ILV), UMR 8180, University of Versailles, 45 Etats-Unis Ave., 78035 Versailles Cedex, France J. Phys. Org. Chem. 2009, 22 298–307 Copyright ß 2008 John Wiley & Sons, Ltd. 298