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Konfigurations‐ und konformationsisomere paratrope, rotationsdynamische Tetraepoxy[32]annulene(6.2.6.2) und diatrope Tetraoxa[30]porphyrin(6.2.6.2)‐Dikationen : Nachweis eines Tetraepoxy[31]annulen(6.2.6.2)‐Radikalkations
Author(s) -
Märkl Gottfried,
Ehrl Robert,
Sauer Heinrich,
Kreitmeier Peter,
Burgemeister Thomas
Publication year - 1999
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(19990113)82:1<59::aid-hlca59>3.0.co;2-8
Subject(s) - annulene , chemistry , porphyrin , intramolecular force , stereochemistry , crystallography , proton nmr , photochemistry
Configurational and Conformational Isomeric Paratopic, Rotational Dynamics Tetraepoxy[30]annulenes(6.2.6.2) and Diatropic Tetraoxa[30]porphyrin(6.2.6.2) Dications: Detection of a Tetraepoxy[31]annulene(6.2.6.2)Radical Cation The synthesis of tetraepoxy[32]annulenes(6.2.6.2) ( 4 ) by a cyclizing twofold Wittig reaction of ( E , E , E )‐5,5′‐(hexa‐1,3,5‐triene‐1,6‐diyl)bis[furan‐2‐carbaldehyde] ( 6 ) and the corresponding bis‐phosphonium salt 7 is described ( Scheme 1 ). Contrary to the configuration of the educts, the obtained annulenes 4a and 4b are ( Z , E , E , E , Z , E , E , E )‐ and ( E , Z , E , E , E , Z , E , E )‐configurated, respectively. The 1 H‐NMR spectra establish the paratropic, antiaromatic character of 4 . The annulenes 4 are highly dynamic systems, the ( E )‐ethenediyl bridges rotate around the adjacent σ ‐bonds, these rotations are frozen at −80°. The McMurry condensation of dialdehyde 6 yields the ( E , E , Z , E , E , E , Z )‐4,5‐dihydrotetraepoxy[32]annulene(6.2.6.2) ( 13a ), where the configuration of the dialdehyde 6 – beside the hydrogenated double bond – is retained. As result of an intramolecular McMurry reaction of 6 , ( Z , E , Z , Z )‐dioxa[16]annulene(6.2) 14 is formed. By oxidation of the [32]annulenes(6.2.6.2) 4a and 4b , a mixture of the four stereoisomeric tetraoxa[30]porphyrin(6.2.6.2) dications 5a / 5a ′/ 5b / 5c is obtained; the configuration of the isomers is determined by COSY, NOESY, and NOE experiments. The Δ δ values (26.81, 25.83, and 21.11 ppm) underline the diatropic, aromatic character of the dications 5 , the Soret bands are shifted bathochromically to 550 nm, and the Q ‐bands are in the NIR region (896 – 1039 nm). The dihydroannulene 13a is dehydrogenated by p ‐chloroanil (tetrachloro‐1,4‐benzoquinone) to give the annulenes 4a and 4b , its oxidation with DDQ (=4,5‐dichloro‐3,6‐dioxocyclohexa‐1,4‐diene‐1,2‐dicarbonitrile) results in the same mixture of dications 5 . Entirely different results are obtained by reaction of the dihydroannulene 13a with DDQ. Here, the ( E , E , E , Z , E , E , E , Z ) tetraoxa[30]porphyrin(6.2.6.2) dication 5c – formed only in traces from 4a / 4b – is the main product. Beside 5c , a by‐product (3%) can be isolated, which turns out (ESR, conductivity) to be the ( E , E , E , Z , E , E , E , Z )‐tetraoxa[31]porphyrin(6.2.6.2) radical cation 16 , obviously the intermediate in the oxidation sequence of the annulene to the dication. This result leads to the conclusion that the reaction of the dihydro compound 13a with p ‐chloroanil and DDQ follows different reaction mechanisms. For all isolated stereoisomeric tetraepoxy annulenes and tetraoxaporphyrin dications, the Δ H f values are calculated by the semiempiric AM1 method. The results are in agreement with the experimental observations. All data confirm the antiaromaticity of the tetraepoxy[32]annulenes(6.2.6.2) 4 and the aromaticity of the tetraoxa[30]porphyrin(6.2.6.2) dications.