Helicenes: Synthesis and Applications

Work done in the field of helicenes from 1900s to the early 2011 is reviewed. In 1967, Martin and co-workers reported the first photoinduced synthesis of heptahelicene. Katz et al. developed another impressive strategy using excess propylene oxide plus a stoichiometric amount of iodine in an inert atmosphere, which not only enhances the yields greatly compared with the traditional conditions for the photocyclization of stilbenes but also prevents photoreduction or photooxidative side reactions of the double bonds. A Ru-catalyzed double cyclization through a naphthoannulation procedure based on 1,1-diaryl-2,2-diethynylethylenes, was disclosed by Scott and Donovan in 2004. Hassine and coworkers utilized β-styrylpyridine as one aryl moiety to give two helicene isomers after Mizoroki-Heck coupling and a photocyclization. Staab and co-workers reported the use of Stille-Kelly coupling to synthesize 1,16-diaza[6]helicene in 52% yield in the presence of hexamethyldistannane.