Configurational and conformational isomeric antiaromatic [28]tetraoxaporphyrinoids(4.2.4.2) and aromatic [26]tetraoxaporphyrin(4.2.4.2) dications. A new type of molecular dynamics in macrocyclic systems

• Published in: Helvetica chimica acta Vol. 80; no. 1; pp. 14 - 42
• Main Authors: Markl, G, Stiegler, J, Kreitmeier, P, Burgemeister, T, Kastner, F, Dove, S
• Format: Journal Article
• Language: German
• Published: BASEL NEW SWISS CHEMICAL SOC 01.01.1997
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; PORPHYCENE
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19970800103

The [28]tetraoxaporphyrinoids(4.2.4.2)6 are synthesized by cyclizing Wittig reaction of (E,E)-5,5'-(buta-1,3-diene-1,4-diyl)bis[furan-2-carbaldehyde] (8) with (E,E)-{(buta-1,3-diene-diyl)bis[(furan-5,2-diyl)methylene]}bis[triphenylphosphonium] dibromide (9) and 3,3'-{[(E)-ethene-1,2-diyl]bis(furan-5,2-diyl)}bis[(E)-prop-2-enal] (22) with (E)-{(ethene-1,2-diyl)bis[(furan-5,2-diyl)methylene]}bis[triphenylphosphonium] dibromide (23). An alternative path to get 6 is the McMurry condensation of 8. Four different configurational isomers of 6 could be isolated and characterized by H-1-NMR spectroscopy. The (Z,EE,Z,EE)-isomer 6a is the first macrocyclic system where the inner and outer protons of the (E,E)-dienediyl bridges exchange by rotation around the adjacent single bonds. In the (Z,EE,E,EE)-isomer 6b, the (E)-ethenediyl bridge is rotationally active, while in the (E,ZE,E,EZ)-isomer 6c and in the (E,EZ,E,EZ)-isomer 6e, the rotation of both (E)-ethenediyl bridges is observed. When in the dynamic systems the rotation of the active (E)-double bonds at temperatures T < -90 degrees is frozen, all configurational isomers of 6 appear to be antiaromatic and paratropic. The oxidation of the [28]tetraoxaporphyrinoids 6c and 6e with DDQ yields the aromatic, diatropic [26]tetraoxaporphyrin(4.2.4.2) dications 21e/21e' both with (E,EZ,E,EZ)-configuration but different fixed conformations. (Z,EE,Z,EE)-Isomer 6;1 is oxidized to give the (Z,EE,Z,EE)-dication 21a, while the oxidation of 6b yields a mixture of 21a and 21e/21e'. The standard formation enthalpies of the obtained and expected [28]tetraoxaporphyrinoids 6 and [26]tetraoxaporphyrin dications 21 have been calculated with the AM1 method, showing good accordance with the experimental results.