Neutrale aromatische Tetraepoxyannulene: Tetraepoxy[26]annulene(4.2.2.2) und Tetraepoxy[30]annulene(4.4.4.2) – Systeme mit hoher molekularer Dynamik

   Neutral Aromatic Tetraepoxyannulenes: Tetraepoxy[26]annulenes(4.2.2.2) and Tetraepoxy[30]annulenes(4.4.4.2) – Systems with High Molecular Dynamics The 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 −80°. 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 σ ‐bonds in a synchronous manner. Isomer 4b , for steric reasons, is rigid. By Wittig reaction of the bis‐aldehyde 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 −80°. The 1 H‐NMR spectrum at −80° 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.