Aromatic properties

COT is prepared by the polymerization of ethyne at moderate temperature and pressure in the presence of nickel salts. The molecule is non-planar and behaves as a typical cyclic olefin, having no aromatic properties. It may be catalytically hydrogenated to cyclo-octene, but with Zn and dil. sulphuric acid gives 1,3,6-cyclooclairiene. It reacts with maleic anhydride to give an adduct, m.p. 166 C, derived from the isomeric structure bicyclo-4,2,0-octa-2,4,7-triene(I) ... 

Thiophene 1,1-dioxide (61) is too unstable to isolate and dimerizes with loss of S02 to give 3a, 7a-dihydrobenzothiophene 1,1-dioxide (172) in 34%113. However, alkyl-substituted thiophene 1,1-dioxides can serve as dienes in the Diels-Alder reaction, since the aromatic properties of the thiophene nucleus are lost completely and the n-electrons of the sulfur atom are used for forming the bond with oxygen. The sulfones 173-178 are found to react with two moles of maleic anhydride at elevated temperature to give bicyclic anhydrides114. Thus, at high reaction temperature, S02 is split off to give cyclohexadiene... 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

The redox properties of cyclic polysilanes are interesting because they resemble those of aromatic hydrocarbons. For instance, cyclic polysilanes can be reduced to anion radicals or oxidized to cation radicals. ESR spectra for both the cation and anion radicals indicate that the unpaired electron is fully delocalized over the ring [17,19,20]. The aromatic properties of the cyclic polysilanes are ascribed to a high energy delocalized HOMO and a relatively low energy LUMO. Because the HOMO and LUMO levels lie at similar level to those of benzene, cyclic polysilanes can serve either as electron donors or electron acceptors. 

Heterocycles with Arsenic, Antimony, and Bismuth. Arsoles, stilboles, and bismoles (Scheme 54) are five-membered heterocycles for which experimental data do not indicate aromatic properties, while semiempirical calculations (CNINDO or CNDO/ S) showed that the anions are aromatic. The chemistry and theoretical aspects in connection with their... 

From our discussions in the last section, we might expect pyridine to display properties associated with the nitrogen function and also with the aromatic ring. Not surprisingly, it turns out that the aromatic ring affects the properties of the amine but, more significantly, the aromatic properties are greatly influenced by the presence of the heteroatom. 

One key conclusion is that the entire molecule must be taken into account to understand the aromatic properties of icosahedral fullerenes. The 2(N -1-1) rule of spherical aromaticity also sufficiently describes the magnetic behavior of non-icosahedral fullerenes [128], homoaromatic cage molecules [129], and inorganic cage molecules [130],... 

FIGURE 1.35 Scheme of the bubble bursting mechanism responsible for the ejection of champagne aerosols overconcentrated with compounds showing both surface activity and aromatic properties (these compounds appear as red dots) below are displayed high-speed photographs of a bubble collapse, leading to the projection of a liquid jet which quickly breaks up into tiny droplets (bar = 1 mm). 

Reduced and partially reduced azepines are more common. Perhydroazepine (hexamethylenimine) was first prepared in 1905 and chemically it behaves as a typical secondary amine (B-67MI51600). Its 2-oxo derivative (caprolactam) is a bulk chemical and is of great industrial importance as an intermediate in the manufacture of nylon. Many oxo and dioxo derivatives of azepines and benzazepines have been prepared, often with difficulty, in a mainly fruitless search for aromatic properties in these azatropone and benz-azatropone systems (see Section 5.16.3.1.2). 

Most of the highly unsaturated monocyclic eight-membered heterocycles contain one or two nitrogen atoms and have been obtained by bond reorganization processes from strained bicyclic or polycyclic precursors. Although several of the less substituted compounds without stabilizing substituents are highly labile substances, 1,4-dihydro-1,4-diazocines qualify as dihetera[8]annulenes and display distinct aromatic properties. 

Huo Xiang is used as chief. It has the function of either expelling wind, damp and cold from the exterior, or eliminating damp-cold from the Middle-Jiao. These functions are derived from its warm, pungent and aromatic properties and the ability to enter the Lung, Stomach and Spleen meridians. 

Lian Qiao, with its pungent and aromatic properties, can disperse stagnation and accumulation, but is used here principally... 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

In the foregoing discussion of the structure of benzene the stability and characteristic aromatic properties of the substance have been attributed to resonance of the molecule between the two Kekul6 structures. A similar treatment, which provides a similar explanation of their outstanding properties, can be given the condensed polynuclear aromatic hydrocarbons. 

Thiophene S,N-ylides are comparable with the 1-mono- and 1,1-dioxides in that they exhibit diene rather than aromatic properties (86JCS(P 1)233). The S,C-ylides appear to be more sluggish in cycloaddition reactions and are therefore considered more aromatic . 

The electronic similarity suggests that unsaturated boron heterocycles and aromatic systems have 7r-electron transitions in common. In fact, such similarities are abundant, and are one main reason for the statement that certain boron heterocycles possess aromatic properties. The UV spectrum of 5,6-dihydrodibenz[c,e][l,2]azaborine (15) is reminiscent... 

Proton NMR shifts have been interpreted in terms of a ring current, and some coupling constants also indicate aromatic properties. Boron-11 shifts alone can apparently not be used as criteria of aromaticity, in the same way that carbon-13 shifts are not meaningful. 

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