1-Fluoro- and 1,3-difluoroazulenes were synthesized by the reaction of azulene with N-fluoropyridinium salts in acetonitrile. Methyl azulene-1-carboxylates were also fluorinated to give the corresponding 3-fluoro derivatives. On the basis of the UV-visible absorption spectra, the fluorine atom acts as an electron-donating group to the azulene π-system due to the +Iπ effect.
3-difluoroazulenes were synthesized for the first time by the electrophilic fluorination of azulenes with N-fluoro reagents. Selective preparation of 1-fluoroazulenes were performed by the fluorination of methyl azulene-1-carboxylates, followed by demethoxycarbonylation in 100% H3PO4. 2-Substituted azulenes were fluorinated in higher yields. In the 1H NMR of 1-fluoroazulene, long-range JFH values were
2‐Formylazulene derivatives have been obtained in good yields by the reactions of 2‐methylazulenes with N,N‐dimethylformamide dimethyl acetal, followed by oxidative cleavage of the intermediately formed enamines with NaIO4. The 2‐formylazulenes obtained were transformed into 2‐ethynylazulenes in good yields by a modification of the Seyferth–Gilbert reaction.