From Small Carbocyclic Rings to Porphyrins: A Personal Account of 50 Years of Research

摘要

Among the numerous molecular rearrangements, which have been observed in the domain of three- and four-membered carbocyclic rings, the reaction type "valence isomerization" has attracted special attention in modern times. The driving force of these usually thermally induced processes is the release of strain from small or medium-sized rings. It is a characteristic mechanistic feature of valence isomerizations that neither radical nor ionic intermediates can be detected in their course, and that the structural change rests exclusively on a concerted reorganization of the α- and π -electrons of their molecular skeleton, associated with corresponding changes of bond lengths and angles. When a dynamic equilibrium is established between the components of such a rearrangement it is termed valence tautomerism.