Ito Shunji, Kikuchi Shigeru, Kobayashi Hiroki, Morita Noboru, Asao Toyonobu
Department of Chemistry, Faculty of Science, Tohoku University, Kawauchi, Aoba-ku, Sendai 980-77, Japan, and Faculty of Education, Iwate University, Ueda, Morioka 020, Japan.
J Org Chem. 1997 Apr 18;62(8):2423-2431. doi: 10.1021/jo961820q.
Acid-catalyzed condensation of azulenes 8a-c with 4-hydroxy- and 3,5-di-tert-butyl-4-hydroxybenzaldehyde leads to substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methanes 7a- f, which are easily converted into substituted di(1-azulenyl)(4-hydroxy- and 3,5-di-tert-butyl-4-hydroxyphenyl)methyl cations 5a-f by oxidation with DDQ. The spectroscopic data are consistent with the protonated cationic structures of 5a-f. The electrochemical reduction of 5a-f showed a reversible wave at -0.74 to -0.86 V (V vs Ag/Ag(+)) upon cyclic voltammetry (CV), although 5d and 5e showed an irreversible wave at -0.79 V. The relatively high reduction potentials of 5a-f, compared with those of di(1-azulenyl)phenylmethyl cations 2a-c, exhibit the stabilization by 4-hydroxy substituents on the phenyl groups. These salts (5a-f.PF(6)(-)) bearing 4-hydroxyl groups on the phenyl rings have been converted by treatment with bases to alpha,alpha-di(1-azulenyl)-1,4-benzoquinone methides 6a-f, which revert to 5a-f.PF(6)(-) upon reprotonation with HPF(6). These quinone methides (6a-f) are highly polarized by the extreme-electrodonating properties of 1-azulenyl groups. The highly polarized properties of 6a-f reflected to the high pK(a) values of their conjugate acids (5a-c, 6.5-7.1, and 5d-f, 3.4-3.8). The strong solvatochromic effects also provide strong evidence of a large contribution of dipolar forms (6') in the ground state. The relatively low oxidation potentials of 6a-f (+0.35 to +0.47 V vs Ag/Ag(+)) reflected facile formation of phenoxy radical cations, stabilized by two 1-azulenyl groups.
薁类化合物8a - c与4 - 羟基以及3,5 - 二叔丁基 - 4 - 羟基苯甲醛在酸催化下缩合，生成取代的二(1 - 薁基)(4 - 羟基 - 和3,5 - 二叔丁基 - 4 - 羟基苯基)甲烷7a - f，通过用DDQ氧化，它们很容易转化为取代的二(1 - 薁基)(4 - 羟基 - 和3,5 - 二叔丁基 - 4 - 羟基苯基)甲基阳离子5a - f。光谱数据与5a - f的质子化阳离子结构一致。在循环伏安法（CV）中，5a - f的电化学还原在 - 0.74至 - 0.86 V（相对于Ag/Ag(+)）处显示出可逆波，尽管5d和5e在 - 0.79 V处显示出不可逆波。与二(1 - 薁基)苯基甲基阳离子2a - c相比，5a - f相对较高的还原电位表明苯基上的4 - 羟基取代基起到了稳定作用。这些在苯环上带有4 - 羟基的盐（5a - f.PF(6)(-)）通过用碱处理已转化为α,α - 二(1 - 薁基)-1,4 - 苯醌甲基化物6a - f，在用HPF(6)重新质子化后又恢复为5a - f.PF(6)(-)。这些醌甲基化物（6a - f）由于1 - 薁基基团的极强给电子性质而具有高度极化性。6a - f的高度极化性质反映在其共轭酸（5a - c，6.5 - 7.1，以及5d - f，3.4 - 3.8）的高pK(a)值上。强烈的溶剂化显色效应也为基态中偶极形式（6'）的巨大贡献提供了有力证据。6a - f相对较低的氧化电位（相对于Ag/Ag(+)为 + 0.35至 + 0.47 V）反映了由两个1 - 薁基基团稳定的苯氧基自由基阳离子的容易形成。
