Neutral Aromatic Tetraepoxyannulenes: Tetraepoxy[26]annulenes(4.2.2.2) and Tetraepoxy[30]annulenes(4.4.4 .2) ± Systems with High Molecular DynamicsThe twofold cyclizing Wittig reaction of the bis-aldehyde 6 with the ylide of the bis-phosphonium salt 7 yields tetraepoxy[26]annulene(4.2.2.2) 4, which exists in the two isomeric forms 4a (EE,Z,E,Z) and 4b (EE,Z,E,E). Annulene 4a is a highly dynamic system down to À 808. Temperature-dependent 1 H-NMR spectra of 4a establish that the (E,E)-buta-1,3-dien-1,4-diyl as well as the (E)-ethen-1,2-diyl bridges rotate around the adjacent s-bonds in a synchronous manner. Isomer 4b, for steric reasons, is rigid. By Wittig reaction of the bisaldehyde 8 with the ylide of the bis-phosphonium salt 9, the tetraepoxy[30]annulene(4.4.4.2) 5 is obtained, which exists also in two isomeric forms, 5a and 5b. Only 5a (EE,ZE,EE,Z) can be isolated in pure form. Like 4a, 5a is highly dynamic, the (E,E)-buta-1,3-dien-1,4-diyl as well as the opposite (E)-ethen-1,2-diyl bridge being able to rotate down to À 808. The 1 H-NMR spectrum at À 808 indicates that 5a exists in the stable conformation 5a'. The 26-and 30-membered annulenes belong to the most expanded neutral annulenes known hitherto; their 1 H-NMR spectra confirm that they still have diatropic, aromatic character. 1. Einleitung. ± Während zahlreiche antiaromatische Tetraepoxy[m 2]annulene (m 22, 26, 30, 34, 38) bekannt sind, die zu den aromatischen Tetraoxa[m]porphyrindikationen oxidiert werden können, wurden bislang nur wenige neutrale, aromatische Tetraepoxyannulene beschrieben. Wir unterscheiden zwei Klassen neutraler aromatischer Annulene. Im Tetraepoxy[22]annulen(2.1.2.1) 1 [1] und im Tetraepoxy[22]annulen(3.0.3.0) 2 [2] setzt die Konjugation des Makrocyclus formal eine partielle Entaromatisierung der Furan-Ringe voraus, während im Tetraepoxy[22]annulen(2.2.2.0) (3) [3] eine makrocyclische Konjugation formal unter Beibehaltung der Furan-Konjugation möglich ist. Entsprechend diesen unterschiedlichen Bindungsverhältnissen sind die spektroskopischen Eigenschaften der Annulene 1 und 2 deutlich verschieden von denen von 3 (Tab. 1). Wir berichten hier über das Tetraepoxy[26]annulen(4.2.2.2) 4 und das Tetraepoxy-[30]annulen(4.4.4.2) 5, deren Bindungsverhältnisse denen von 3 entsprechen sollten.
15synchronen Rotation der (E)-Doppelbindungen wiirde 3 von der C2,,-in die C,,-Konformation iibergehen. Da hierbei die inneren Perimeterprotonen in eine starke sterische Wechselwirkung geraten, ist die nicht-synchrone Rotation wenig wahrscheinlich (Schema I ) . H OH \ \ \ H 1 Schema 1 Im Gegensatz zu 3 zeigt das [24]Porphyrinoid(4.0.4.0) 4 keine dynamischen Eigenschaften [5]. Modellbetrachtungen belegen, dass Rotationen um die (E,Z)-Butadiendiyl-Briikken aus sterischen Griinden nicht moglich sind, Rotationen ausschliesslich um die (E)-Ethendiyl-Einheiten werden ebenfalls nicht beobachtet. Wir stellten uns nunmehr die Frage, ob es sich bei der Rotationsdynamik von 3 ~ bei geeigneter Molekiilgeometrieum ein allgemeines Phanomen handelt. Die nachsthoheren Homologen von 3 und 4 sind die [28]Porphyrinoide. Das [28]Tetraoxaporphyrinoid(3.3.3.3) 5 ist nicht bekannt, wahrend das zu 5 gehorige [26]Tetraoxaporphyrin(3.3.3.3)-dikation bereits 1993 von E. Vogel und Mitarbeitern [2] beschrieben wurde. Modellbetrachtungen lassen erwarten, dass 5 nicht rotationsaktiv ist. Von dem zu 3 hoheren homologen (Z,EE,Z,EE)')-[28]Tetraoxaporphyrinoid(4.2.4.2) 6 sind dynamische Eigenschaften zu erwarten, wahrend das zu 4 hohere Homologe 7 wiederum rotationsinaktiv ist [26]. Wir beschreiben hier Synthese und Eigenschaften von 6.') Urn eine eindeutige Definition der (E,Z)-Isomerie von 6 bzw. 21 zu gewahrleisten, werden Kommata nur dann gesetzt, wenn die Doppelbindungsabfolge durch Furan-Einheiten unterbrochen wird.
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.
We describe the synthesis of tetraepoxy[22]annulene(2.2.2.0) 4, the first aromatic annulene of type B by a cyclizing twofold Witrig reaction of 2,2'-bifuran-5,5'-dicarbaldehyde (5) and the bis[phosphonium] salt 8. The configuration of 4, mainly determined by NMR spectroscopy is (Z,€,Z). According to the UVjVIS spectrum and the 'H-NMR data, the electronic situation in 4 is quite different from that of tetraepoxy[22]annulene(3.0.3.0) 2 and tetraepoxy[22]annulene(2.1.2.1) 3. According to variable-temperature 'H-NMR spectroscopy, 4 is a highly dynamic system, where the (€)-double bond rotates around the adjacent a-bonds. At ca. -130", this dynamic process is frozen, and 4 appears as a diatropic aromatic system; the free activation energy of the rotation dG* is ca. 8.75 kcdljmol.
are synthesized by Wittig reaction or McMurry condensation and characterized by 1H NMR. Isomer ( I) represents the first macrocyclic system where the inner and outer protons of the (E,E)-dienyl bridges exchange by rotation around the adjacent single bonds. In (II), the (E)-ethenediyl bridge is rotationally active, while in (III) and (IV), the rotation of both (E)-ethenediyl bridges is observed. When in the dynamic systems the rotation of the active (E)-double bonds at ¡ -90 • C is frozen, all configurational isomers of (I)-(IV) appear to be antiaromatic and paratropic. The oxidation of (I)-(IV) with DDQ yields dication such as (V) from (I). The standard formation enthalpies of the title compounds are calculated by the AM1 method, showing good agreement with the experimental results. -(MAERKL, G.; STIEGLER, J.; KREITMEIER, P.; BURGEMEISTER, T.; KASTNER, F.; DOVE, S.; Helv.
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