Publisher Summary This chapter discusses the chemistry of a major group of heterocyclic mesomeric betaines, those which are isoelectronic with odd alternant hydrocarbon anions. The structural types of the heterocycles include N -oxides, N -imides, and N -ylides. The chemistry of these systems with seven conjugated atoms, with eleven conjugated atoms, with thirteen conjugated atoms, with fifteen conjugated atoms, with seventeen conjugated atoms, and with nineteen conjugated atoms is reviewed. The electronic structure of the mesomeric betaines is presented through the perturbation molecular orbital (PMO) model. Frontier orbitals of several betaines are calculated using the Huckel method and the electroselectivity is correctly predicted on the basis of the orbital symmetry. The pericyclic reactions of mesomeric betaines are also presented which include valence tautomerism, dimerization, and cycloadditions. The chemistry of heterocyclic betaines derived from even alternant hydrocarbon dianions is discussed for (1) mesomeric betaines, and (2) cross-conjugated betaines.

Heterocyclic Betaine Derivatives of Alternant Hydrocarbons

Formation of β-hydroxyketones from α,β-epoxyketones by photoinduced single electron transfer reactions

The Knoevenagel condensation between aldehydes and substrates with active methylene groups was applied to synthesise a series of 3-(4-substituted phenyl)-2-arylacrylonitriles (aryl = phenyl or pyridyl). Chloro-, fluoro-, or dimethylamino-substituted aryls and a cyano group attached to the double bond of acrylonitrile were studied. Previous studies showed that the condensation products were E isomers. The compounds synthesised were: 3-(4-chlorophenyl)-2-phenylacrylonitrile, 3-(4-chlorophenyl)-2-(pyridin-2-yl)acrylonitrile, 3-(4-chlorophenyl)-2-(pyridin-3-yl)acrylonitrile, 3-(4-chlorophenyl)-2-(pyridin-4-yl)acrylonitrile, 3-(4-fluorophenyl)-2-phenylacrylonitrile, 3-(4-fluorophenyl)-2-(pyridin-2-yl)acrylonitrile, 3-(4-fluorophenyl)-2-(pyridin-3-yl)acrylonitrile, 3-(4-fluorophenyl)-2-(pyridin-4-yl)acrylonitrile, 3-(4-dimethylaminophenyl)-2-phenylacrylonitrile, 3-(4-dimethylaminophenyl)-2-(pyridin-2-yl)acrylonitrile, 3-(4-dimethylaminophenyl)-2-(pyridin-3-yl)acrylonitrile, and 3-(4-dimethylaminophenyl)-2-(pyridin-4-yl)acrylonitrile. Structures were confirmed by IR, MS, and NMR spectral data. Molar absorption coefficient, absorbance, and fluorescence emission spectra were compared in order to evaluate the effects of substituents on phenyl and the position of nitrogen in pyridine moiety on the electronic properties of acrylonitrile derivatives prepared.

Synthesis, optical, and spectroscopic characterisation of substituted 3-phenyl-2-arylacrylonitriles

Photolysis of 2,3-diphenylnaphthoquinone oxide resulted in carbon–carbon bond cleavage to give a seven-membered mesoionic benz[d]oxepine-1,5-dione intermediate, which was trapped by N-phenylmaleimide and norbornadiene.

Photolysis of 2,3-diphenylnaphthoquinone oxide, trapping of a benz(d)oxepine-1,5-dione, a seven-membered mesoionic system

This review provides a general survey of the chemistry of known type A and type B mesoionic heterocycles up to September 1974 with an appendix extending the literature coverage to the end of 1975. The extensive chemistry of sydnones and sydnone imines is not included as this is reviewed elsewhere. The difference between type A and type B mesoionic rings is demonstrated and the number of possible mesoionic rings of each type is discussed. The preparation and reactions of known systems are then systematically reviewed. Additional sections cover general properties of mesoionic rings including valence tautomerism, photochemistry, X-ray crystallography, molecular orbital calculations, pharmacological activity and the interconversion of mesoionic isomers. In a final section, the case for restricting the term mesoionic to five-membered heterocycles of type A and type B is emphasised. Examples of other dipolar heterocycles that have previously been described as mesoionic are discussed and it is recommended that these be described as mesomeric betaines. 

Meso-ionic compounds

PHOTOLYSIS OF 2,3-DIPHENYLNAPHTHOQUINONE OXIDE: TRAPPING OF A BENZ[d]OXEPINE-1,5-DIONE, A SEVEN-MEMBERED MESOIONIC SYSTEM

The N,N′-dimethylpiperazine molecule in the title crystals is bound by two chiral acetylenic alcohol molecules through the strong O–H···N hydrogen bonds. The symmetric 1:2 complex of N,N′-dimethylpiperazine with (+)- and (−)-1-(o-chlorophenyl)-1-phenyl-2-propyn-1-ol(CPPOH), which is useful for the optical purification of partially resolved CPPOH, has the denser packing of molecules in the crystal than the asymmetric 1:2 complex of N,N′-dimethylpiperazine with two (−)-CPPOH.

Hiroshi Tezuka

Papers

PHOTOLYSIS OF 2,3-DIPHENYLNAPHTHOQUINONE OXIDE: TRAPPING OF A BENZ[d]OXEPINE-1,5-DIONE, A SEVEN-MEMBERED MESOIONIC SYSTEM

The Crystal and Molecular Structures of 1:2 Complexes of N,N′-Dimethylpiperazine with 1-(o-Chlorophenyl)-1-phenyl-2-propyn-1-ol

Photolysis of 2,3-diphenylnaphthoquinone oxide, trapping of a benz(d)oxepine-1,5-dione, a seven-membered mesoionic system