A FIRST EXAMPLE OF A PORPHYRINOID FULVALE NE - SYNTHESIS, STRUCTURE, ESR AND ELECTROCHEMICAL INVESTIGATIONS

The macrocyclic tetraepoxy-1H-[21]annulen-1-one 4 can by synthesized b y the cyclizing Wittig reaction of 5,5'-carbonylbis[furan-2-carbaldehy de] (11) and the bisphosphonium salt 12, obtained from 2,2'-bifuran-5, 5'-dicarboxaldehyde (13). According to an X-ray structure analysis, th e annulenone 4 is not planar in the crystal; the H-1-NMR spectra of 4 reveal an averaged planarity with respect to the NMR timescale. The Mc Murry reaction of 4 yields fulvalene 3 in 43 % yield as the most expan ded fulvalene hithertoo known. The X-ray structure analyses of 3 surpr ising establishes a 'syn'-orientation of the two rings with respect to the central C=C bond, thus forming a basket-like molecule. The H-1-NM R spectrum confirms the averaged planarity of both macrocycles in 3. C V and spectroelectrochemical measurements of 3 suggest a reversible tw o-electron reduction producing dianion 15 with two aromatic, anionic y dro-10H-21,22,23,24-tetraoxa-5a,15a-dihomocorrole (= tetraoxa[22]porph yrin(2.1.2.0)) ring systems containing 22 pi electrons each. The forma tion of 15 can also be achieved chemically by reaction of 3 with metal lic potassium. The dication 16 of 3 may be antiaromatic, but the exact electronic structure is dubious. ESR and ENDOR investigations on the radical cation and the radical anion of 3 indicate that the free elect ron is delocalized in the entire molecule.