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A Nonaromatic Thiophene‐Fused Heptalene and Its Aromatic Dianion
Author(s) -
Oshima Hiroya,
Fukazawa Aiko,
Sasamori Takahiro,
Yamaguchi Shigehiro
Publication year - 2015
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201501790
Subject(s) - chemistry , aromaticity , thiophene , annulene , moiety , ring (chemistry) , chemical shift , crystallography , bicyclic molecule , cryptand , stereochemistry , computational chemistry , molecule , organic chemistry , ion
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.