Maerkl, Gottfried and Ehrl, Robert and Sauer, Heinrich and Kreitmeier, Peter and Burgemeister, Thomas (1999) Configurational and conformational isomeric paratropic, rotational dynamic tetraepoxy[32]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. HELVETICA CHIMICA ACTA, 82 (1). pp. 59-84. ISSN 0018-019X,
Full text not available from this repository.Abstract
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 I). 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 H-1-NMR spectra establish the paratropic, antiaromatic character of 4. The annulenes 4 are highly dynamic systems, the (E)-ethenediyl bridges rotate around the adjacent sigma-bonds, these rotations are frozen at - 80 degrees. 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 Delta delta 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 Delta H-1, 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.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | OLEFINS; |
| Subjects: | 500 Science > 540 Chemistry & allied sciences |
| Divisions: | Chemistry and Pharmacy > Institut für Organische Chemie |
| Depositing User: | Dr. Gernot Deinzer |
| Date Deposited: | 29 Nov 2022 13:50 |
| Last Modified: | 29 Nov 2022 13:50 |
| URI: | https://pred.uni-regensburg.de/id/eprint/48659 |
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