A Nonaromatic Thiophene‐Fused Heptalene and Its Aromatic Dianion1. (12th May 2015)

• Record Type: Journal Article
• Authors: Oshima, Hiroya; Fukazawa, Aiko; Sasamori, Takahiro; Yamaguchi, Shigehiro
• Is Part Of: Angewandte Chemie. Volume 127:Number 26(2015)
• Page Start: 7746
• Page End: 7749
• Publication Date: 2015-05-12
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/ange.201501790
• Languages: English
• ISSNs: 0044-8249

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>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 <bold>5</bold> from the reductive transannular cyclization of bisdehydro[12]annulene <bold>4</bold>, and its solid‐state structure, which was confirmed by X‐ray crystallographic analysis, is presented. Chemical reduction of <bold>5</bold> readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂<bold>5</bold>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 <bold>5</bold>. ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion <bold>5</bold>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.</p> </abstract>