Ziegler‐Hafner Azulene Synthesis

Abstract The versatile synthesis of azulene derivative by the condensation of a cyclopentadienyl anion with the intermediate arising from the nucleophilic addition of dimethylamine onto an activated pyrinium salt is generally known as the Ziegler–Hafner azulene synthesis, Ziegler–Hafner synthesis, or Ziegler–Hafner method. Moreover, the more reactive isoquinoline can be activated simply by methyl iodide, as indicated in the reaction between 2‐methylisoquinolinium iodide and cyclopentadienyl anion to form dihydroisoquinoline, which undergoes ring opening and electrocyclization to form benz[f]azulene in low yield. In this reaction, isoquinoline has been used to form benz[f]azulene. In addition, trifluoromethanesulfonic anhydride has been used for the activation of the pyridine moiety at the occasion when 2,4‐dinitrochlorobenzene is not applicable due to the low basicity of the pyridine moiety. This reaction has been used for the preparation of azulene derivatives, especially for those with substituents at the seven‐membered moiety.