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Superheteroaromaten mit Furan‐Bausteinen: Isomere antiaromatische Tetraepoxy[36]annulene(6.4.6.4) und aromatische Tetraoxa[34]porphyrin(6.4.6.4)‐Dikationen
Author(s) -
Märkl Gottfried,
Ehrl Robert,
Kreitmeier Peter,
Burgemeister Thomas
Publication year - 2000
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/(sici)1522-2675(20000216)83:2<495::aid-hlca495>3.0.co;2-3
Subject(s) - annulene , chemistry , porphyrin , hexa , furan , conformational isomerism , antiaromaticity , isomerization , stereochemistry , medicinal chemistry , wittig reaction , molecule , aromaticity , photochemistry , catalysis , organic chemistry
Superheteroaromatic Systems with Furan Building Blocks: Isomeric Antiaromatic Tetraepoxy[36]annulenes(6.4.6.4) and Aromatic Tetraoxa[34]porphyrin(6.4.6.4) Dications The title compounds are available by a twofold cyclizing Wittig reaction of ( all‐E )‐3,3′‐(hexa‐1,3,5‐triene‐1,6‐diyldifuran‐5,2‐diyl)bis[prop‐2‐enal] ( 4 ) with ( all‐E )‐(hexa‐1,3,5‐triene‐1,6‐diyl)bis(furan‐5,2‐diylmethylene)bis[triphenylphosphonium] dibromide ( 7 ). Two conformational isomers 2a / 2a ′ of ( Z , E , E , E , E , Z , E , E , E , E )‐tetraepoxy[36]annulene(6.4.6.4) are obtained. The oxidation of 2a / 2a ′ yields two ( E , E , Z , E , E , E , E , Z , E , E )‐tetraoxa[34]porphyrin(6.4.6.4) dications 3a / 3a ′, which are conformers, too. The oxidation of 2a / 2a ′ is accompanied by the isomerization of four ethen‐1,2‐diyl bridges. The reduction of the dications 3a / 3a ′ leads to the new ( E , E , Z , E , E , E , E , Z , E , E )‐tetraepoxy[36]annulene(6.4.6.4) ( 2b ) and ( E , E , E , Z , E , E , E , E , Z , E )‐tetraepoxy[36]annulene(6.4.6.4) ( 2c ). In 2b as well as in 2c , both 1,3‐butadiene‐1,4‐diyl bridges are rotating until −90°. The Δ δ values, i.e. , the maximum δ difference of the `inner' and `outer' perimeter protons of 3a / 3a ′ (26.62 and 25.32 ppm) are of the same size as the Δ δ value of the tetramethyl[34]porphyrin(5.5.5.5) dication ( 1 ; Δ δ =25.3 ppm); therefore, they might be called `superheteroaromatic' too. The Δ δ values of the tetraepoxy[36]annulenes(6.4.6.4) ( 2a – c ; Δ δ =2.3 – 3.3 ppm) establish that they are still paratropic; they represent the most expanded antiaromatic systems yet known.