Valence Isomerism in Selected Heterocyclic Systems

Valence isomerization and the phenomenologically related principle of valence tautomerism, as they apply to several nitrogen‐containing heterocyclic systems, are discussed herein. Attention is called first to 1H‐azepines and the reluctance of such substances to isomerize to azanorcaradienes. The role of homo‐1H‐azepines in this context is then briefly outlined. Azocines (azacyclooctatetraenes) are seen to occupy an interesting position in the sense that pronounced shifts in the position of equilibrium (from almost entirely monocyclic to virtually completely bicyclic) occur with substitutional alterations of various types. The capability of azabullvalenes for degenerate valence isomerization is the next presented, and contrast is made with the properties of benzazabullvalenes and an azasemibullvalene derivative.