Helicenes are a class of polycyclic aromatic hydrocarbons with helical core structures and inherent chirality. Due to their interesting optical and electronic properties, helicenes attract significant attention in fundamental and materials science. Recently, the pentagon-fused helicenes have been prepared by utilization of radical cascade synthetic approach.[1] Incorporation of a five-membered ring “defect” in the helicene framework partially disrupts aromatic conjugation and provides a site for selective deprotonation. Since no electron acquisition properties of helicenes have been reported, we initiated their broad investigation in stepwise chemical reduction reactions using a combination of experimental and theoretical methods. The reactivity of a double[5]helicene (C39H24, Scheme) with Group 1 metals has been investigated, revealing two distinctive reaction steps according to the UV-Vis spectroscopic study. First, the site-selective deprotonation afforded the C39H23– monoanion, which was crystallized with K and Cs counterions. The X-ray diffraction characterization confirmed the formation of the solvent-separated ion pairs (SSIP) in both cases, allowing the anion geometry evaluation without metal binding influence. Notably, deprotonation occurs at the five-membered ring, switches on conjugation and injects a charge in a helical π-system without adding a spin. Next, the dianionic species has also been isolated and crystallographically characterized as a SSIP with two Cs counterions. Intriguingly, its X-ray structural analysis revealed the core transformation through the double dehydrogenative ring closures that resulted in the formation of a new planarized decacyclic dianion, C39H192–. The structural consequences of deprotonation and reduction of a double[5]helicene core will be discussed.

Double[5]helicene: Taking advantage of a pentagonal “defect” and helicene strain for reductive annulation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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