Chapter 1 – Semi-Conjugated Heteroaromatic Rings: A Missing Link in Heterocyclic Chemistry∗

A systematic analysis of the connectivity of heteroatoms in heterocyclic mesomeric betaines (HMBs) reveals three main classes: conjugated HMBs, cross-conjugated HMBs, and the previously unrecognized semi-conjugated HMBs. The defining features of each class are discussed with particular emphasis on semi-conjugated systems.