Aromatische 10- und 14 π-Elektronensysteme

Abstract

Hückel’s molecular orbital theory predicts that monocyclic fully conjugated polyenes containing a closed shell of (4n+ 2) π-electrons should be aromatic provided their carbon skeleton can attain a reasonably planar configuration. In the case of the elusive benzene homolog [10]annulene (two trans double bonds) severe steric interactions of the two internal hydrogen atoms exist in a planar arrangement. Consequently, this molecule cannot be expected to be aromatic. Molecular models, however, indicate that replacement of the internal hydrogen atoms in [10]annulene by a methylene group, by oxygen or by an imino group should allow the C₁₀-skeleton to become nearly planar. The resulting bridged [10]annulenes may therefore be considered to be suitable models to test Hückel’s rule for the neutral 10 π-electron system. 1.6-Methano-[10]annulene (I), 1.6-oxido-[10]annulene (XXXIII), and 1.6-imino-[10]annulene (XXXVII) were synthesized from 1,4,5,8-tetrahydronaphthalene and shown to be aromatic molecules according to both chemical and physical criteria. The existence and aromaticity of these molecules suggested the idea of a homologous series of bridged annulenes (XXXVIII, XXXIX, XL etc.) which like the corresponding acenes contain (4 n + 2) π-electrons. From an inspection of Stuart-Briegleb models of XXXIX and XL it follows that substantial planarity of the carbon perimeter is only possible in the syn- and all-syn forms and with oxygen atoms or imino groups as bridges. The validity of this concept is dramatically demonstrated by the synthesis of aromatic syn-bisoxido-[14]annulene and olefinic anti-bismethano-[14]annulene.