The chemical oxidation of azulene-1-azoarenes with FeCl3 in benzene was studied. For unsubstituted azo compounds, a 4′,4′ coupling takes place yielding biarylic derivatives 4 or 6. 4′-Substituted azulene-1-azobenzenes react depending on the nature of the substituent. Electron-withdrawing substituents such as NO2 increase the oxidation potential of the starting azo compounds and, as a consequence, they hinder the oxidative coupling. On the contrary, electron-releasing substituents such as 4′-methoxy (compound 1b) activate the position 3 in the azulene moiety and coupling in this position takes place as the main pathway; products with three and four azulene moieties coupled in the 1 and 3 positions (11 and 12) were also separated and characterized. The coupling product for the oxidation of compound 1c with a 4′-CH3 substituent results in 15% yield only; large amounts of oligomers and polymers with substituted methyl detected in the reaction mixture prove that in 1c the benzylic position is also a reactive one. A mechanism with a radical cation intermediate generated by the single-electron transfer oxidation of the starting azo compounds is proposed.
研究了苯中的 FeCl3 对偶氮-1-唑烯的
化学氧化作用。对于未取代的偶氮化合物,会发生 4′,4′偶联反应,生成双芳基衍
生物 4 或 6。4′-取代偶氮-1-苯的反应取决于取代基的性质。吸电子取代基(如
NO2)会增加起始偶氮化合物的氧化电位,从而阻碍氧化偶联。相反,释放电子的取代基,如 4′-甲氧基(化合物 1b)激活了偶氮薁分子中的 3 号位,在这个位置的偶合成为主要途径;在 1 号位和 3 号位偶合了三个和四个偶氮薁分子的产物(11 和 12)也被分离出来并进行了表征。化合物 1c 与 4′-
CH3 取代基的氧化偶联产物的产率仅为 15%;在反应混合物中检测到的大量甲基取代基低聚物和聚合物证明,在 1c 中,苄基位置也是一个反应位置。提出了一种机理,即起始偶氮化合物的单电子转移氧化生成了一个自由基阳离子中间体。