Popis: |
The synthesis of diepoxy[16]annulene(6.2) 3 is described. The dialdehyde (E,E)-3,3′-([2,2′-bifuran]-5,5′-diyl)bis[2-propenal] 4 undergoes an intramolecular as well as an intermolecular as well as an intermolecular McMurry coupling to give the diepoxy[16]annulene 3 beside the tetraepoxy[32]annulene(6.2.6.2) (5). According to the spectroscopic data 3 has the Z,Z,E,Z-configuration and it turns out to be a strongly paratropic, highly dynamic system, where the E-ethenediyl-1,2-bond rotates around the adjacent single bonds. According to VT-1H-NMR spectroscopy, this rotation is even not yet entirely frozen at −130°C. The calculated free activation energy of the rotation is about 24 kJ/mol (5,74 kcal/mol), the smallest value observed ever for the rotation of E-ethenediyl-1,2-bonds in annulenes. |