title: A Nonaromatic thiophene-fused heptalene and its aromatic dianion.
abstract: Heptalene, a nonaromatic, bicyclic 12 π-electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two-electron oxidized or reduced forms. The synthesis of thiophene-fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4, and its solid-state structure, which was confirmed by X-ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)(+) ]2 5(2-) . The X-ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5. (1) H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5(2-) , which was corroborated by nucleus-independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π-electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan).
authors.forename: Hiroya
authors.initials: H
authors.lastname: Oshima
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan). aiko@chem.nagoya-u.ac.jp.
authors.forename: Aiko
authors.initials: A
authors.lastname: Fukazawa
authors.affiliation: Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011 (Japan).
authors.forename: Takahiro
authors.initials: T
authors.lastname: Sasamori
authors.affiliation: Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan). yamaguchi@chem.nagoya-u.ac.jp.|Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan). yamaguchi@chem.nagoya-u.ac.jp.
authors.forename: Shigehiro
authors.initials: S
authors.lastname: Yamaguchi
keywords: aromaticity; dehydroannulene; heptalene; transannular cyclization; π conjugation