One-Step Construction of Five Successive Rings by Rhodium- Catalyzed Intermolecular Double [~(2+)~(2+)2] Cycloaddition: Enantioenriched [9]Helicene-Like Molecules

摘要

Helicenes and helicene-like molecules have long attracted much attention because of their potential applications to optical or electronic functional materials. Therefore, flexible as well as convenient methods for their syntheses are highly desired for structural alterations and supply of sufficient quantities. The most frequently employed method for the synthesis of helicenes is the oxidative photocyclization of stilbene-type precursors, however this procedure cannot be conducted on a large scale because of the highly dilute reaction conditions.Several non-photochemical methods for the synthesis of [6]- and [7]helicenes and helicene-like molecules have been developed to date. As such, transition metal mediated intramolecular [2+2+2] cycloadditions of triynes are useful methods for the synthesis of [6]- and [7]helicene-like molecules through the formation of three successive rings. Stara and co-workers pioneered this strategy by using cobalt- or nickel-mediated or catalyzed [~(2+)~(2+)2] cycloadditions. Following this pioneering work, we recently reported a cationic rhodium(I)/chiral bisphosphine complex, which catalyzed the intramolecular [~(2+)~(2+)2] cycloadditions of 2-naphthol-linked triynes, leading to enantioenriched [7]helicene-like molecules (Scheme 1). Notably, non-photochemical methods that can furnish higher ordered (≥ [8]) helicenes and helicene-like molecules are rare.