Valence Isomerizations in Compounds with Strained Rings

Among the many molecular rearrangements that may occur in compounds with strained rings, valence (or valence bond) isomerizations have recently received special attention. Relief of strain in energy‐rich small or medium‐sized rings is the driving force behind these predominantly thermal (pyrolysis) reactions. In many instances, valence isomerizations must be considered as multicenter rearrangements (with six‐ or four‐membered cyclic transition states). This mechanism can be regarded as firmly established for Cope rearrangements, as well as for intracyclic diene syntheses and diene cleavages of strained rings, with which this paper deals.