The Chemistry of Heterocyclic Mesomeric Betaines

The heterocycles discussed in this section are classified by reference to the size and structure of their isoconjugate lOAH anions. Heterocyclic mesomeric betaines isoconjugate with fourteen different lOAH anions are known to the author. These AH skeletons (37-50) are shown in Table I. Table II gives examples of corresponding mesomeric betaines (51-64). 

Among the structural types that are within the scope of this review are many heterocyclic iV-oxides, A -imides and A -ylides (e.g., 36). The chemistry of these systems, which is extensive, has already been the subject of comprehensive reviews and a discussion of their chemistry is not included here. Similarly the structure and chemistry of prototropic tautomers has also been reviewed recently and will not be duplicated here. 

The following nomenclature has been used in this review The ring system is named as the appropriate heterocation and the exocyclic group carrying the negative charge is named as the appropriate anion (i.e., olate, thiolate, aminide, or methylate). Thus, compounds of type 9 are named as 

Ochiai, Aromatic Amine Oxides. Elsevier, Amsterdam, 1967. 

Katritzky and J. M. Lagowski, The Chemistry of Heterocyclic A-Oxides. Academic Press, New York, 1971. 

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This series of three reviews has surveyed the chemistry of the major types of heterocyclic mesomeric betaine. Examples of other heterocyclic betaines that do not fall into any of the categories that have been discussed are known. These include, for example, compounds 498 and 499, ° but so far systems of this type have received little attention. 

Heterocyclic betaines and other unusual structures have been the subject of extensive investigation [88AHC(44)269 92AHC32]. The specific focus of this report is to survey recent progress in the chemistry of heterocyclic betaines and molecules with a betaine character of general type 1 (Scheme 1), and should serve to complement other reviews that deal with heterocyclic betaines of alternant hydrocarbons [80AHC(26)1] and heterocyclic mesomeric betaines (85T2239). 

This chapter describes heterocyclic systems which are isoconjugate with the pentalenyl dianion (13) and which cannot be represented by classical Kekule structures. Molecules of this type (e.g. 9-12) can only be represented by dipolar structures, or possibly by structures involving tetracovalent sulfur atoms, and are described as conjugated mesomeric betaines or non-classical heteropentalenes. The structure and chemistry of this type of heterocycle have been the subject of two earlier reviews (77T3203, 77HC(30)317) and their classification as conjugated mesomeric betaines and relationship to other dipolar heterocycles has also been discussed (83UP43700). 

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