Generation of azulene radical cations from arylalkynes

Abstract

If a diarylalkyne 4-XC₆H₄CCR (R = Ph, 4-MeC₆H₄, or 4-Bu^tC₆H₄) or 1-phenylpropyne in trifluoroacetic acid containing mercury (II) trifluoroacetate is irradiated with u.v. light filtered through Pyrex glass, the e.s.r. spectrum of the corresponding azulene can be observed. The azulenes have been isolated and converted back into their radical cations by irradiation in trifluoroacetic acid, or in dichloromethane containing (4-BrC₆H₄)₃N⁺˙ or (2,4-Br₂C₆H₃)₃N⁺˙. Possible mechanisms by which the azulenes are formed from the alkynes are discussed.