Synthesis and Properties of Fluoroazulenes. II. Electrophilic Fluorination of Azulenes with N-Fluoro Reagents

Article abstract of DOI:10.1246/bcsj.69.1645

1-Fluoro- and 1,3-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 ¹HNMR of 1-fluoroazulene, long-range .J_FH values were not observed at H-2 and H-8, in contrast to those for 1-fluoronaphthalene. The 1-fluorine atom causes significant bathochromic shifts in the visible absorption of azulene, due to the so-called +Iπ effect.