The Norcaradiene Problem

Compounds having the norcaradiene structure are stable if one of the double bonds of the norcaradiene also belongs to a benzene ring or if C atoms 1 and 6 are fixed in position by a bridge. A similar effect that favors the norcaradiene form is produced by the incorporation of nitrogen atoms in positions 3 and 4. Cycloheptatriene and most of the simply substituted tropilidenes exist exclusively in the monocyclic triene form. It has been possible in some cases to detect a valence‐tautomeric equilibrium between the norcaradiene and the cycloheptatriene ring system. Non‐classical stabilization of compounds having norcaradiene or cycloheptatriene structure in the sence of a pseudoaromatic “homobenzene” is only very slight, if it occurs at all.