Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations

• Published in: Angewandte Chemie International Edition Vol. 59; no. 3; pp. 1256 - 1262
• Main Authors: Zhou, Zheng, Kawade, Rahul Kisan, Wei, Zheng, Kuriakose, Febin, Üngör, Ökten, Jo, Minyoung, Shatruk, Michael, Gershoni‐Poranne, Renana, Petrukhina, Marina A., Alabugin, Igor V.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 13.01.2020
• Edition: International ed. in English
• Subjects: Anions; Aromaticity; benzannulation; Closures; Conjugation; Crystallization; Crystallography; cyclizations; Dehydrogenation; helicenes; Perturbation; reductive coupling; Switches; Time dependence; cyclizations; helicenes; benzannulation; reductive coupling; aromaticity
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201911319

Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K+(18‐crown‐6)(THF)} and {Cs+2(18‐crown‐6)3}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion. Flat as a pancake: Relief of helicene strain, avoidance of antiaromaticity, and increase in charge delocalization assist in dehydrogenative ring closures that convert an anionic fused bis‐[5]helicene into a planarized decacyclic dianion where nine hexagonal units are fused to a single pentagon.