Cross-Conjugation at the Heart of Understanding the Electronic Theory of Organic Chemistry

The present status of what has been known and understood about the concept of cross-conjugation is reviewed first and it has been shown how the structure-activity relationship in acyclic and monocyclic conjugated systems can be understood without elaborated formalism but by gathering the knowledge of the basic HMO of linear polyenes. Then it is shown that the mode of electron flow caused by hetero-atom substitution to these hydrocarbons as depicted by the curly arrows on the structural formula is correlated well with the topological structure of cross-conjugated system. Mathematical relations between the conventional HMO and resonance theories especially related to the Kekule structure of conjugated hydrocarbons are explained. The least amount of graph-theoretical techniques, such as the topological index, Z, is also explained. It will be understood how the global trend of the direction and extent of the π-electron flow in acyclic conjugated hydrocarbon networks can be predicted by the diagrammatic method of organic electron theory and how cross-conjugation is an important concept in organic chemistry. Finally several new research trends and observations are introduced and discussed from other than organic chemistry, such as “quantum interference” in physics.