Nonaromatic compounds, ring systems

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

Several aspects of aromaticity have been studied <2002JOC1333> using statistical analyses of quantitative definitions of aromaticity. ASEs, REs, magnetic susceptibility exaltation (A), nucleus-independent chemical shift (NIGS), the harmonic oscillator model of aromaticity (HOMA), (/j) and (Aj), evaluated for a set of 75 five-membered 7t-electron systems and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations between the various aromaticity criteria, provided the whole set of compounds is used. The data in Table 9 have been found for arsole (AsH) 1 (E = As, R = H), its anion (As ), and protonated species (AsH2 ). 

The other four contributions all deal with topics that have not previously been reviewed in the series. Sammes and the series editor have written on Isopyrroles, the nonaromatic isomers of pyrroles. Two contributions deal with aspects of thiophene reactions Barker has summarized Dithienylmethane chemistry and Klemm has provided a comprehensive account of Condensed Thiophenes. Finally, Albert has contributed Compounds Containing a Fused Pyrimidine Ring, a chapter that includes many ring systems of pharmaceutical interest. 

Organic compounds that contain benzene rings as part of their molecular structure are called aromatic compounds. Nonaromatic hydrocarbons such as alkanes, alkenes, and alkynes are called aliphatic compounds. Some aromatic compounds contain two or more benzene rings fused together. Examples include naphthalene and anthracene. In these compounds, electrons are shared over all of the carbon atoms in the fused ring system. 

As already mentioned, on passing from the aromatic system of isoxazoles to the nonaromatic ones of isoxazolines and isoxazolidines, the N—O bond becomes more labile. In these compounds the ring is extremely readily cleaved. Many such reactions are useful to determine the structure of reduced isoxazole derivatives and are also of preparative value. 

In Section 3.4.1.1 the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later, in Sections 3.4.1.23.4.1.10 these reactions are reconsidered in more detail. Reactions of nonaromatic five-membered compounds with more than one heteroatom are considered in Section 3.4.2 and reactions of substituents attached to aromatic azoles are covered in Section 3.4.3. The reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acidbase equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 2.4. Acidbase equilibria are considered in Section 3.4.1.3 of the present chapter. 

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