Donor‐Acceptor‐Substituted Cyclic π‐Electron Systems—Probes for Theories and Building Blocks for New Materials

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Abstract
Donor and acceptor substituents stabilize (4n)π‐electron systems and destabilize those with (4n+2)π electrons. The same is true for the transition states of pericyclic reactions, which explains the appearance of dipolar intermediates in symmetry‐allowed cycloadditions and sigmatropic rearrangements. Donor‐acceptor‐substituted semibullvalenes undergo rapid Cope rearrangement, as do tetraazabarbaralanes. In contrast, tetraazasemibullvalenes can not be isolated, since the isomeric tetrazocines always result. The usefulness of the donor‐acceptor concept in preparative chemistry is demonstrated by numerous stable cyclic (4n)π‐electron systems, like donor‐acceptor‐substituted cyclobutadienes, tetraaminobenzene, and p‐benzoquinone dications, benzodiazepinyl anions, and donor‐acceptor‐substituted cyclopentadienyl cations and their heteroatom‐containing analogues. The new compounds are of interest in the fields of organic metals and ferromagnets, nonlinear optics, and dyestuffs, among others.