Lakhdar Sami, Goumont Régis, Boubaker Taoufik, Mokhtari Malika, Terrier François
Laboratoire SIRCOB, Institut Lavoisier, UMR CNRS 8180, Université de Versailles, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France.
Org Biomol Chem. 2006 May 21;4(10):1910-9. doi: 10.1039/b602002j. Epub 2006 Apr 13.
A study of the dual electrophilic and pericyclic reactivity of 4,6-dinitrobenzofurazan (DNBZ, 2), 4,6-dinitro-2,1,3-benzothiadiazole (DNBS, 3), 4,6-dinitro-2,1,3-benzoselenadiazole (DNBSe, 4) is reported. Kinetic and thermodynamic measurements of the ease of covalent hydration of 2-4 to give the corresponding hydroxy sigma-adducts C-2-C-4 have been carried out over a large pH range in aqueous solution. Analysis of the data has allowed a determination of the rate constants k1(H2O) pertaining to the susceptibility of 2-4 to water attack as well as the pKa values for the sigma-complexation processes. With pKa values ranging from 3.92 for DNBZ to 6.34 for DNBSe to 7.86 for DNBS, the electrophilic character of the three heteroaromatics is much closer to that of the superelectrophilic reference, i.e. 4,6-dinitrobenzofuroxan (DNBF, 1; pKa = 3.75), than that of the standard Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB, pKa = 13.43). Most importantly, water is found to be an efficient nucleophile which contributes strongly to the formation of the adducts C-2 and C-4. This confirms a previous observation that a pKa value of ca. 8 is a primary requirement for having H2O competing effectively as a nucleophile with OH- in the formation of hydroxy sigma-adducts. On the other hand, 2-4 are found to exhibit dienophilic and/or heterodienic behaviour on treatment with isoprene, 2,3-dimethylbutadiene, cyclopentadiene or cyclohexadiene, affording Diels-Alder mono- or di-adducts which have all been structurally characterized. A major finding is that the order of Diels-Alder reactivity follows clearly the order of electrophilicity, pointing to a direct relationship between superelectrophilic and pericyclic reactivity. This relationship is discussed.
报道了对4,6 - 二硝基苯并呋咱（DNBZ，2）、4,6 - 二硝基 - 2,1,3 - 苯并噻二唑（DNBS，3）、4,6 - 二硝基 - 2,1,3 - 苯并硒二唑（DNBSe，4）的双亲电和亲环反应性的研究。在水溶液中的较大pH范围内，对2 - 4进行共价水合生成相应的羟基σ - 加合物C - 2 - C - 4的动力学和热力学测量。对数据的分析使得能够确定与2 - 4对水进攻的敏感性相关的速率常数k1(H2O)以及σ - 络合过程的pKa值。DNBZ的pKa值为3.92，DNBSe为6.34，DNBS为7.86，这三种杂芳烃的亲电特性比标准的迈森海默亲电试剂1,3,5 - 三硝基苯（TNB，pKa = 13.43）更接近超亲电参考物，即4,6 - 二硝基苯并呋咱（DNBF，1；pKa = 3.75）。最重要的是，发现水是一种有效的亲核试剂，对加合物C - 2和C - 4的形成有很大贡献。这证实了先前的观察结果，即在形成羟基σ - 加合物时，约8的pKa值是使H2O作为亲核试剂与OH - 有效竞争的主要要求。另一方面，发现2 - 4在用异戊二烯、2,3 - 二甲基丁二烯、环戊二烯或环己二烯处理时表现出亲双烯体和/或杂双烯体行为，得到了均已进行结构表征的狄尔斯 - 阿尔德单加合物或多加合物。一个主要发现是狄尔斯 - 阿尔德反应性的顺序明显遵循亲电性顺序，表明超亲电反应性和亲环反应性之间存在直接关系。对这种关系进行了讨论。
