Ein erstes Beispiel eines porphyrinoiden Fulvalens: Synthese, Struktur, ESR‐ und elektrochemische Untersuchungen

A First Example of a Porphyrinoid Fulvalene: Synthesis, Structure, ESR and Electrochemical Investigations The macrocyclic tetraepoxy‐1 H ‐[21]annulen‐1‐one 4 can by synthesized by the cyclizing Wittig reaction of 5,5′‐carbonylbis[furan‐2‐carboxaldehyde] ( 11 ) and the bisphosphonium salt 12 , obtained from 2,2′‐bifuran‐5,5′‐dicarboxaldehyde ( 13 ). According to an X‐ray structure analysis, the annulenone 4 is not planar in the crystal; the 1 H‐NMR spectra of 4 reveal an averaged planarity with respect to the NMR timescale. The McMurry reaction of 4 yields fulvalene 3 in 43% yield as the most expanded fulvalene hithertoo known. The X‐ray structure analyses of 3 surprising establishes a ‘ syn ’‐orientation of the two rings with respect to the central CC bond, thus forming a basket‐like molecule. The 1 H‐NMR spectrum confirms the averaged planarity of both macrocycles in 3 . CV and spectroelectrochemical measurements of 3 suggest a reversible two‐electron reduction producing dianion 15 with two aromatic, anionic 5a,15a‐didehydro‐10 H ‐21,22,23,24‐tetraoxa‐5a,15a‐dihomocorrole (= tetraoxa[22]porphyrin(2.1.2.0)) ring systems containing 22π electrons each. The formation of 15 can also be achieved chemically by reaction of 3 with metallic 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 electron is delocalized in the entire molecule.