Trication species stabilized by heteroazulenes: synthesis and properties of 1,3,5-tris[bis(heteroazulen-3-yl)methyliumyl]benzenes
作者:Shin-ichi Naya、Masatoshi Isobe、Yukiko Hano、Makoto Nitta
DOI:10.1039/b107962j
日期:2001.11.29
A general synthesis and properties of a novel type of heteroazulene analogues of fairly stable trimethyliumylbenzenes (14a–i·3BF4−) bearing 1,3,5-trimethyliumyl groups substituted with six 2H-cyclohepta[b]furan-2-one 8a, six 1,2-dihydro-N-phenylcyclohepta[b]pyrrol-2-one 8b, six 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 8c, and their related compounds are reported. The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 8a, 8b, and 8c with 1,3,5-triformylbenzene 9, mono- and diformylbenzene having di-and monoheteroazulene-substituted methyl groups to afford the corresponding 1,3,5-trimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HBF4 solution in Ac2O. In spite of their tricationic nature, 14a–i exhibited high stability with large [pKR+] values due to the stabilizing effect of the heteroazulene units. In the case of trications 14b, three methyliumyl-units were neutralized stepwise at the pH of 10.4, 11.5, and 13.0. However, we could not determine pKR+, pKR++, and pKR+++ values separately in the cases of other trications 14a
and 14c–i. Thus, some [pKR+] values were obtained as the average values of pKR+++ and pKR++ values as well as of pKR++ and pKR+ values. The electrochemical reduction of most of the trications exhibits irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the basis of a comparison with those of the related monocation and dication species to clarify the reduction process of trications 14a–i. The reduction waves of 14a–e,h,i were irreversible, while those of 14f,g
seem to be reversible; this feature would be ascribed to their large steric constraints.
一种新型杂氮烯类似物的一般合成方法和性质,这种类似物是一种相当稳定的三甲基苯(14a--i-3BF4-),带有 1,3、报告了一种新型杂氮杂环烯类似物的合成和性质,这些类似物具有相当稳定的三甲基铵基苯(14a-i-3BF4-),其中含有 1,3,5-三甲基铵基,被六个 2H-环庚烷并[b]呋喃-2-酮 8a、六个 1,2-二氢-N-苯基环庚烷并[b]吡咯-2-酮 8b、六个 1,2-二氢-N-甲基环庚烷并[b]吡咯-2-酮 8c 以及它们的相关化合物取代。合成方法是在 TFA 催化下,杂氮环戊烯 8a、8b 和 8c 与 1,3,5-三甲酰苯 9、具有二和单杂氮环戊烯取代甲基的单和二甲酰苯发生单步亲电芳香取代反应,得到相应的 1,3,5-三甲基苯衍生物,然后用 DDQ 氧化取氢,再用 Ac2O 中的 HBF4 溶液交换反阴离子。尽管 14a-i 具有三芳基化性质,但由于杂氮烯单元的稳定作用,它们表现出很高的稳定性,[pKR+] 值很大。在三聚物 14b 中,三个甲基单元在 pH 值为 10.4、11.5 和 13.0 时逐步被中和。然而,我们无法分别确定其他三反应 14a 和 14c-i 的 pKR+、pKR++ 和 pKR+++ 值。因此,一些 [pKR+] 值是作为 pKR+++ 值和 pKR+++ 值以及 pKR+++ 值和 pKR++ 值的平均值得出的。在循环伏安法(CV)下,大多数三酸酯的电化学还原表现出不可逆的波和较低的还原峰电位;在与相关的一价物和二价物进行比较的基础上讨论了这些值,以澄清三酸酯 14a-i 的还原过程。14a-e,h,i 的还原波是不可逆的,而 14f,g 的还原波似乎是可逆的;这一特征可归因于它们的巨大立体约束。