Publisher Summary This chapter discusses aromaticity of heterocycles. At a time when the validity of the concept of aromaticity is questioned, increasingly, it seems appropriate to review various definitions in common use and to survey their applications in heterocyclic chemistry. From a qualitative viewpoint the characteristics of an aromatic compound are easy to define. A compound is said to be aromatic if it is cyclic and unsaturated with an enhanced stability over simple olefinic compounds. The characteristic aromatic reactivity of the type described is clearly a matter of degree and is difficult to assess quantitatively. From a simple consideration of pyridine, which reacts less readily with electrophiles but more readily with nucleophiles at the ring carbon atoms than does benzene, it soon becomes apparent that attempts to discuss even relative aromatic character using kinetic criteria is fraught with difficulties; this is apart from the fundamental objection that chemical reactivity is not solely a function of ground-state stability.

Aromaticity of Heterocycles

Publisher Summary This chapter discusses the synthesis of peri-annealated (peri-condensed) heterocyclic systems. peri-annelated (peri-condensed) heterocyclic systems imply structures 1 (where X is a heteroatom; Y, Z are heteroatoms or a hydrocarbon fragment), which contain a tricyclic nucleus where the heterocycle is linked to two other rings at the contiguous sides. Peri-heterocyclic systems with four to seven-membered heterorings are discussed. Since the chemical nature of a heterocyclic system is determined by the valent state of the heteroatom, the names used for a lot of periheterocycle series cannot be considered to be very suitable. Peri-heterocycles serve as dyes, luminophores, energochromophores, organic conductors, and bioactive compounds. The chapter also discusses synthesis of peri-annelated heterocycles such as peri-heterocycles with four membered heteroring, peri-heterocycles with five membered heteroring and single heteroatom, and peri-heterocycles with five-membered heteroring and two heteroatoms.

Synthesis of Peri-Annelated Heterocyclic Systems

Palladium catalyzed C-C coupling reactions of 3,5-dichloro-4H-1,2,6-thiadiazin-4-one.

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

Substitution at C-4 in 3,5-disubstituted 4H-1,2,6-thiadiazin-4-ones

The preparation and properties of some members of a new group of heterocycles derived from acenaphthylene (acenaphtho[5,6-de]triazines etc.) are described. Their electronic structure is compared with that of the non-benzenoid aromatic hydrocarbon cyclohept[fg]acenaphthylene (acepleiadylene). An attempt to generate 5,6-dehydroacenaphthylene is recorded.

peri-Naphthotriazines and related compounds. Part II. Some acenaphthylene derivatives; their preparation and properties

Novel heteroaromatic compounds related to acepleiadylene

Peri-naphthotriazine und verwandte verbindungen 2. mitt. darst. und verhalten einiger acenaphthylen-derivate 3. mitt. photochemische rk., synth. von 1-methyl-acenaphtho(5,6-bc)pyridin

M. J. Perkins

Papers

peri-Naphthotriazines and related compounds. Part II. Some acenaphthylene derivatives; their preparation and properties

Novel heteroaromatic compounds related to acepleiadylene

Peri-naphthotriazine und verwandte verbindungen 2. mitt. darst. und verhalten einiger acenaphthylen-derivate 3. mitt. photochemische rk., synth. von 1-methyl-acenaphtho(5,6-bc)pyridin