Synthesis of the Unknown Indeno[1,2‐a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion

• Published in: Angewandte Chemie International Edition Vol. 56; no. 48; pp. 15363 - 15367
• Main Authors: Dressler, Justin J., Zhou, Zheng, Marshall, Jonathan L., Kishi, Ryohei, Takamuku, Shota, Wei, Zheng, Spisak, Sarah N., Nakano, Masayoshi, Petrukhina, Marina A., Haley, Michael M.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 27-11-2017
• Edition: International ed. in English
• Subjects: anions; antiaromaticity; Computer applications; Crystallography; Crystals; Data acquisition; Fluorene; radicals; Single crystals; structure determination; X-ray diffraction; X-ray diffraction; antiaromaticity; structure determination; radicals; anions
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201709282

Of the five possible indenofluorene regioisomers, examples of a fully conjugated indeno[1,2‐a]fluorene scaffold have so far remained elusive. This work reports the preparation and characterization of 7,12‐dimesitylindeno[1,2‐a]fluorene as a highly reactive species. Experimental and computational data support the notion of a molecule with pronounced diradical character that exists in a triplet ground state. As such, both NICS and ACID calculations suggest that the indeno[1,2‐a]fluorene scaffold is weakly Baird aromatic. Reduction of the unstable red solid with Cs metal produces the dianion of the title compound, from which single crystals could be obtained and X‐ray data acquired, thus fully corroborating the proposed indeno[1,2‐a]fluorene hydrocarbon core.
Not a matter of IF but when: Described is the preparation of the last elusive indenofluorene (IF) regioisomer. While the body of circumstantial spectroscopic data support formation of the highly reactive triplet diradicaloid, definitive structural proof is provided by X‐ray diffraction analysis of crystals of the dianion, obtained by reduction of the indeno[1,2‐a]fluorene with Cs metal.