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: Tetraepoxy26]annulenes(4.2.2.2) and Tetraepoxy30]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 tetraepoxy26]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 ¹H‐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 tetraepoxy30]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 ¹H‐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 ¹H‐NMR spectra confirm that they still have diatropic, aromatic character.