Terrier F, Sebban M, Goumont R, Halle JC, Moutiers G, Cangelosi I, Buncel E
Laboratoire SIRCOB, ESA CNRS 8086, Batiment Lavoisier, Universite de Versailles, 45, avenue des Etats-Unis, 78035 Versailles Cedex, France.
J Org Chem. 2000 Nov 3;65(22):7391-8. doi: 10.1021/jo0005114.
In investigating the reactivities of aza analogues of super-electrophile 4,6-dinitrobenzofuroxan (DNBF, 1), we have found that the nitro-substituted pyridofuroxan 2 gives a remarkably stable hydrate 3 in aqueous solution (as evidenced by the requirement of ca. 50% H(2)SO(4) (H(0) approximately -3) for complete recovery of 2). The equilibrium constant K(H)()2(O) for hydration of 2 is estimated to be >/= 100, being comparable only with the K(H)()2(O) values reported for hydration of highly activated neutral polyazaaromatics such as 2- and 6-hydroxypteridines or 7-azapteridine. Interestingly, the NH group of 3 undergoes ionization at rather low pH (p = 5.79), affording an anionic hydroxy sigma-adduct 4 which is thermodynamically 10(8) times more stable than the related sigma-adduct of pteridine. The experimental evidence is that 4 is slightly more stable than the hydroxy sigma-adduct of DNBF, indicating not only that 2 ranks among the most electrophilic heteroaromatics known to date but also than an aza group may in fact be as efficient as a nitro group in promoting sigma-complex formation. 2 is also found to be a versatile Diels-Alder reagent, as a result of the low aromatic character of its six-membered ring. Upon treatment of 2 with cyclopentadiene and 2,3-dimethylbutadiene, various reactivity patterns have been observed. These led to different cycloadducts arising from normal as well as inverse electron-demand condensations involving the pyridine ring as the dienophile or the heterodiene contributor. Altogether, the results reveal major differences between the reactivity of 2 and that of DNBF, with in particular a remarkable tendency of the pyridofuroxan adducts to undergo covalent hydration, resulting in the formation of stable carbinolamines. Also noteworthy is the characterization of a diadduct which results from a Diels-Alder trapping of the o-dinitroso intermediate involved in the exchange of the 1-oxide and 3-oxide tautomers of 2.
在研究超亲电试剂4,6-二硝基苯并呋咱（DNBF，1）的氮杂类似物的反应活性时，我们发现硝基取代的吡啶并呋咱2在水溶液中能生成一种非常稳定的水合物3（这可通过需要约50%的硫酸（H(0)约为-3）才能使2完全回收得以证明）。2水合的平衡常数K(H)()2(O)估计大于或等于100，仅与报道的高活性中性聚氮杂芳烃（如2-和6-羟基蝶啶或7-氮杂蝶啶）水合的K(H)()2(O)值相当。有趣的是，3的NH基团在相当低的pH值（p = 5.79）下发生电离，生成一种阴离子羟基σ-加合物4，其热力学稳定性比蝶啶的相关σ-加合物高10(8)倍。实验证据表明4比DNBF的羟基σ-加合物稍稳定，这不仅表明2是迄今为止已知的最亲电的杂芳烃之一，而且表明氮杂基团实际上在促进σ-络合物形成方面可能与硝基一样有效。由于其六元环的低芳香性，2还被发现是一种通用的狄尔斯-阿尔德试剂。用环戊二烯和2,3-二甲基丁二烯处理2时，观察到了各种反应模式。这些导致了不同的环加成产物，它们源于涉及吡啶环作为亲双烯体或杂二烯体的正常以及逆电子需求缩合反应。总之，结果揭示了2和DNBF反应活性之间的主要差异，特别是吡啶并呋咱加合物有显著的共价水合倾向，导致形成稳定的氨基醇。同样值得注意的是对一种双加合物的表征，它是由2的1-氧化物和3-氧化物互变异构体交换过程中涉及的邻二亚硝基中间体的狄尔斯-阿尔德捕获反应产生的。
