Abstract
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 ...
Abstract
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 'tetraoxii[30]porphyrin(4.4.4.4)' dication 2. The cyclizing Wittig reaction of the dialdehyde 3 with the bis-phosphonium salt 7 at 70 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 la and 1c, the two (EE)-buta-1,3-diene-1,4-diyl bridges rotate around the adjacent sigma -bonds the rigidity of lb with 3 (EE) bridges prevents any dynamic character, Thc Wittig reaction of 3 with 7 at 20 only yields the kinetically controlled annulene le, and at 120, an excess of the thermodynamically most stable isomer la is formed. The structure of I is elucidated mainly by COSY and NOESY experiments, and the dynamic character of la and Ic is established by temperature-dependent H-1-NMR spectroscopy. The oxidation of the isomer mixture la-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 I and 2 are in full agreement with the experimental results.
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