Tetraepoxy[32]annulene(4.4.4.4) und `Tetraoxa[30]porphyrin(4.4.4.4)'-Dikationen

Abstract

mult. Siegfried Hünig zum 80. Geburtstag gewidmet Tetraepoxy[32]annulenes(4.4.4.4) and Tetraoxa[30]porphyrin(4.4.4.4) Dications Of the tetraepoxy[32]annulenes as well as the tetraoxa[30]porphyrin dications, hithertoo only the (8.0.8.0) and the (6.2.6.2) systems are known to exist in several geometric isomers and to possess antiaromatic and aromatic character, respectively. Here we describe the still missing symmetric member of the [32]annulenes, the tetraepoxy[32]annulene(4.4.4.4) 1 and the corresponding tetraoxa[30]porphyrin(4.4.4.4) dication 2.The cyclizing Wittig reaction of the dialdehyde 3 with the bis-phosphonium salt 7 at 708 yields the configurational isomers 1a (ZE,EE,EZ,EE), 1b (ZE,EE,EE,EE), and 1c (EZ,EE,EZ,EE). All isomers are antiaromatic; in 1a and 1c, the two (E,E)-buta-1,3-diene-1,4-diyl bridges rotate around the adjacent s-bonds; the rigidity of 1b with 3 (E,E) bridges prevents any dynamic character. The Wittig reaction of 3 with 7 at 208 only yields the kinetically controlled annulene 1c, and at 1208, an excess of the thermodynamically most stable isomer 1a is formed. The structure of 1 is elucidated mainly by COSY and NOESY experiments, and the dynamic character of 1a and 1c is established by temperature-dependent 1 H-NMR spectroscopy. The oxidation of the isomer mixture 1a ± c with 4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile (DDQ) gives two isomeric tetraoxa[30]porphyrin(4.4.4.4) dications 2' and 2'', which are frozen conformers with the same (EZ,EE,EZ,EE)-configuration and geometrically related to 1c. Semiempirical calculations of 1 and 2 are in full agreement with the experimental results.

Aromaticity Switching in Porphyrinoids

“…Further exploration of expanded porphyrin systems, deeply modified by introducing oxygen or sulfur instead of nitrogen, provides redox‐switchable hexaphyrin(2.1.1.2.1.1) 33 (Scheme ). The transoidal geometry of both C 2 bridges shows similarity to the approach introduced by Märkl and co‐workers for tetraoxatetraphyrins( m.n.m.n ), that is, 23 , but also for other tetraoxatetraphyrins, in which the flexibility of the carbon chain determines the electronic properties . The predicted 32 π‐electron delocalization and antiaromaticity of 33 have been confirmed by 1 H NMR spectroscopy, for which the perimeter signals are recorded in the δ =5.5–4.9 ppm region, whereas the protons pointed into the cavity resonate at δ =13.15 and 12.4 ppm.…”

Synthetic Expanded Porphyrin Chemistry

“…Another contribution from the Märkl research group describes the synthesis of the tetraepoxy [32]annulene(4.4.4.4). [41] As is true for the other furan-containing annulenes prepared by Märkl et al, tetraepoxy [32]annulene(4.4.4.4) exists as a mixture of at least three different isomers (for example, 28 a-c), all three of which display antiaromaticity as judged from 1 H NMR spectroscopic analyses. Exposure of this isomeric mixture to DDQ produces the expected mixture of corresponding tetraoxa [30] (30).…”

Synthesechemie expandierter Porphyrine

“…Ein weiterer Beitrag aus der Arbeitsgruppe von Märkl beschreibt die Synthese von Tetraepoxy[32]annulen(4.4.4.4) 41. Wie die anderen von Märkl et al hergestellten Furan‐Annulene besteht auch dieses aus einem Gemisch von mindestens drei Isomeren (z.…”