Syntheses of polycyclic compounds by oxidative coupling of azulene-1-azoarenes

Abstract

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.