Lakhdar Sami, Goumont Régis, Terrier François, Boubaker Taoufik, Dust Julian M, Buncel Erwin
Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles, 45 Avenue des Etats-Unis, 78035-Versailles Cedex, France.
Org Biomol Chem. 2007 Jun 7;5(11):1744-51. doi: 10.1039/b702060k. Epub 2007 Apr 30.
We report on the dual reactivity, i.e. anionic Meisenheimer sigma adduct formation and Diels-Alder adduct formation, of a series of heteroaromatic super-electrophiles, including 4,6-dinitro-benzofuroxan, -N-arylbenzotriazoles (4), -benzothiadiazole and -benzoselenadiazole. Measured pK(a)(H(2)O) values for sigma adduct formation provide a quantitative measure of super-electrophilic reactivity with a satisfactory correlation between the Mayr E electrophilicity parameter and pK(a)(H(2)O): E = -0.662 pK(a)(H(2)O) (or pK(R+) -3.20 (r(2) = 0.987). The most highly electrophilic, pre-eminent super-electrophile is 4,6-dinitrotetrazolopyridine (E = -4.67, pK(a)(H(2)O) = 0.4), which supercedes the reference Meisenheimer super-electrophile, 4,6-dinitrobenzofuroxan (E = -5.06, pK(a) = 3.75), having itself an E value superior by 8 orders of magnitude compared to 1,3,5-trinitrobenzene as the benchmark normal Meisenheimer electrophile (E = -13.19, pK(a)(H(2)O) = 13.43). (For relevant kinetic parameters as well as E and pK(a) values, see .) In a parallel study we have investigated Diels-Alder (normal and inverse electron demand) reactivity of this series of heteroaromatic electrophiles and have shown that Mayr E values are valid predictors of whether DA adducts will form and how rapidly. The observed order of pericyclic reactivity corresponds to E = -8.5 as the demarcation E value, in close agreement with sigma complexation; thus pointing to a common origin for the two processes, i.e. an inverse relationship between the degree of aromaticity of the carbocyclic ring and ease of sigma complexation, or DA reactivity, respectively.
我们报道了一系列杂芳基超亲电试剂的双重反应性，即阴离子型迈森海默σ加合物的形成和狄尔斯-阿尔德加合物的形成，这些超亲电试剂包括4,6-二硝基苯并呋咱、-N-芳基苯并三唑（4）、苯并噻二唑和苯并硒二唑。测量得到的σ加合物形成的pK(a)(H₂O)值提供了超亲电反应性的定量度量，迈尔E亲电参数与pK(a)(H₂O)之间具有令人满意的相关性：E = -0.662 pK(a)(H₂O)（或pK(R⁺) - 3.20，r² = 0.987）。亲电性最强、卓越的超亲电试剂是4,6-二硝基四唑并吡啶（E = -4.67，pK(a)(H₂O) = 0.4），它取代了作为参考的迈森海默超亲电试剂4,6-二硝基苯并呋咱（E = -5.06，pK(a) = 3.75），其E值本身比作为基准普通迈森海默亲电试剂的1,3,5-三硝基苯（E = -13.19，pK(a)(H₂O) = 13.43）高8个数量级。（有关相关动力学参数以及E和pK(a)值，见……）在一项平行研究中，我们研究了这一系列杂芳基亲电试剂的狄尔斯-阿尔德（正常和逆电子需求）反应性，并表明迈尔E值是DA加合物是否会形成以及形成速度的有效预测指标。观察到的周环反应性顺序对应于E = -8.5作为分界E值，与σ络合密切一致；因此表明这两个过程有共同的起源，即碳环的芳香性程度与σ络合或DA反应性的难易程度分别呈反比关系。
