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

Abstract

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.

The Diels–Alder Reaction of 4,6‐Dinitrobenzofuroxan with 1‐Trimethylsilyloxybuta‐1,3‐diene: A Case Example of a Stepwise Cycloaddition

“…4.5) and DN max values, which leaves little doubt that the related cycloadditions will exhibit a large polar character. [44][45][46][47] Consistent with this anticipation, the experimental evidence reported in this work is that the reaction of DNBF with 8 (Dw = 4.73) proceeds stepwise through a relatively stable zwitterionic intermediate. Interestingly, a DFT study of the reaction of DNBF with cyclopentadiene (Cp) has been made that suggests that decreasing the electron-rich character of the diene on going from 8 to Cp or other common dienes, such as 2,3-dimethyl-butadiene, should not change the mechanism of the DNBF cycloadditions, even though the intermediates are no longer stable enough to be experimentally observed.…”

“…For the best evaluation of the Diels-Alder reactivity of the aforementioned electron-deficient olefinic structures, it is worth noting that the corresponding w values are all considerably high compared with that of the reference nitroole- 1-trimethylsilyloxy-1,3-butadiene 0.73 0.52 [45] 1,1-dimethoxy-1,3-butadiene 0.83 0.58 [b] 1-methoxy-1,3-butadiene 0.77 0.548 [45] cyclopentadiene 0.83 0.55 [45] cyclohexadiene 0.90 0.59 [47] Isoprene 0.94 0.57 [45] 2,3-dimethylbutadiene 0.97 0.58 [47] 3,5-diphenyl-1,2,4-triazine 2.09 1.00 [b] nitroethylene 2.61 0.98 [40] trans-b-nitrostyrene 2.70 1.11 [47] 3-(p-nitrophenyl)-5-phenyl-1,2,4-triazine 2.80 1.19 [b] 2,4,6-tris(trifluoromethyl)-1,3,5-triazine 3.14 1.04 [b] 1,1-dinitro-2,2-diphenylethylene 3.16 1.27 [47] tetrazine 3.38 1.36 [46] nitroethylene/BH 3 4.33 1.55 [40] 4-nitrobenzodifuroxan (NBDF) 4.80 1.65 [47] 4-aza-6-nitrobenzofuroxan (ANBF) 4.81 1.64 [47] 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) 4.92 1.8 [b] 4,6-dinitrobenzofuroxan (DNBF) 5.46 1.85 [40] tetracyanoethylene (TCNE) 5.96 1.695 [45] [a] w is defined by Equation (7), in which m and h refer to the electronic chemical potential and the hardness of the substrates, respectively (see ref.…”

“…The calculated values of w and DN max of relevance for our comparison of the DNBF-8 system with well-established stepwise cycloadditions are collected in Table 5, together with reported data for a few reference cycloadditions. [45][46][47] All structures quoted in Table 5 are shown in Chart 1 in the Supporting Information.…”

“…[44][45][46] Recent investigations have used the w and DN max values to classify dienes and dienophiles on a unique electrophilicity scale. [44][45][46][47]51] According to the model, the polar character of a Diels-Alder interaction can be assessed from the difference in the global electrophilicities of the two reagents, Dw, and the DN max values for the system at hand. Thus, it is anticipated that reactions involving a diene and a dienophile located at the ends of the w scale will proceed with an especially strong polar character.…”

“…This can be done by using the approach recently developed by Domingo et al on the basis of two reactivity indexes defined within the DFT theory and found to be very useful for predicting the feasibility of many nucleophileelectrophile combinations and the more-or-less polar character of Diels-Alder reactions. [44][45][46][47] A first key parameter is the global electrophilicity index, w, introduced by Parr and defined in Equation (7). [48][49][50] In this equation, the electronic chemical potential m and the chemical hardness h of a substrate are two parameters that were evaluated in terms of the one-electron energies of the frontier molecular orbitals (FMO) HOMO and LUMO at the ground state of the molecules.…”