Department of Chemistry, Southern Federal University, 7 Zorge St., 344090 Rostov-on-Don, Russia.
Chemistry. 2011 Jun 27;17(27):7592-604. doi: 10.1002/chem.201003695. Epub 2011 May 17.
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.
硝呋草醚（NBDF）与 1-甲氧基-3-三甲基甲硅氧基-1,3-丁二烯的 Diels-Alder 反应已通过实验和理论进行了研究。在室温下，将 NBDF 与过量二烯在氯仿中处理，发现得到了一种含有羰基官能团的单一产物。基于 X 射线结构和 NMR 波谱数据，该产物似乎是热力学上更稳定的内型 [2+4] 环加成物的 OSiMe(3) 部分水解的结果，其特征是 MeO 和 NO(2) 官能团的顺式排列。在室温下在 CDCl(3)中和在-40°C 下在氘代乙腈中进行了反应的原位 NMR 研究。在气相中和在乙腈中在 B3LYP/6-31G*水平上进行了计算，假设二烯的最稳定的顺式构象也是反应中最具反应性的构象。分析表明，NBDF/顺式二烯相互作用涉及两性离子中间体的形成。重要的是，该中间体以两种优选构象形成，这两种构象对应于试剂的内型和外型接近方式。这两种鉴定出的构象的环化得到了实验表征的内型和外型 [2+4] 环加成物。根据计算，两种构象的互变可以通过返回到预反应复合物发生，也可以通过通过较小稳定性的中间构象发生旋转来发生。鉴于相互作用的逐步性质，不能排除两性离子中间体是 NBDF 与反式二烯相互作用的结果。用最稳定和更普遍的 s-顺式构象进行的计算证实了这一想法，并支持两性离子在整个相互作用中的作用。
