Synthesis of Dicyano-Substituted Benzo[c]fluorenes from Tetraaryl[3]cumulenes

• Published in: European journal of organic chemistry 2016-06, Vol.2016 (17), p.2919-2924
• Main Authors: Gawel, Przemyslaw, Halabi, Elias A., Schweinfurth, David, Trapp, Nils, Ruhlmann, Laurent, Boudon, Corinne, Diederich, François
• Format: Article
• Language: English
• Subjects: Chromophores; Conjugation; Cumulenes; Hydrocarbons; Polycycles; Spectrum analysis
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201600470

We have developed a one‐step synthesis of 5,6‐dicyano‐7‐(diphenylvinyl)‐7H‐benzo[c]fluorenes, starting from easily accessible tetraaryl[3]cumulenes and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). In the proposed mechanism, this transformation is initiated by single‐electron transfer from the cumulene to DDQ with subsequent ring closure. In the next step, [4+2] cycloaddition of an additional DDQ molecule is followed by transformation of the formed cycloadducts, which provides new 5,6‐dicyanobenzo[c]fluorene derivatives in fair to good yields. Isolated intermediates and time‐resolved EPR spectroscopy measurements support our mechanistic proposal. Furthermore, X‐ray crystallographic proof for the postulated structures as well as for the reaction intermediates is provided. The optoelectronic properties of these new chromophores were derived from UV/Vis spectroscopy, electrochemical, and computational studies. Additionally, the aromaticity of the new polycyclic aromatic backbone was analyzed by using nucleus‐independent chemical shift (NICS)‐XY‐scans. Dicyano‐substituted benzo[c]fluorenes are prepared from tetraaryl[3]cumulenes by single‐electron transfer from the cumulene to 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) followed by a reaction cascade for which the mechanistic proposal is supported by EPR spectroscopy and X‐ray analysis of isolated intermediates. Insight into the π conjugation in these new polycyclic systems is obtained.