Nonbenzenoid aromatic ions

A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. 

Reaction of aryl nitrilium ions (40) or imidoyl chlorides (41) with benzonitrile produced the nitrilium species (42), which underwent Friedel-Crafts reaction yielding the quinazoline system (43 Scheme 17). Generation and cyclization of nitrilium species using Friedel-Crafts reagents has since found widespread favor in heteroaromatic synthesis and such procedures have been reviewed in detail. An illustration of its use for a nonbenzenoid aromatic system is given in equation (31). ... 

Preparation of Aromatic Compounds via Dehydrogenation. Dihydroaromatic compounds are easily converted into the corresponding aromatic compound by treatment with triphenylcarbenium tetrafluoroborate followed by base. Certain a,a-disubstituted dihydroaromatics are converted to the 1,4-dialkylaromatic compounds with rearrangement (eq 1). Nonbenzenoid aromatic systems, e.g. benzazulene or dibenzosesquifulvalene, are readily prepared from their dihydro counterparts. Aromatic cations are also easily prepared by hydride abstraction, for example, tropylium ion (e.g. in the synthesis of heptalene (eq 2)), cyclopropenyl cation, and others, including heterocycUc systems. Some benzylic cations, especially ferrocenyl cations, can also be formed by either hydride abstraction or trityl addition. 

The delocalization of the electrons in the pi orbitals of benzene accounts for the properties of benzene and its derivatives, which differ from the properties of alkenes and other aliphatic compounds. The phenomenon is called aromaticity. A definition of aromaticity is that it occurs in compounds that obey the Hiickel rule-, i.e. that there should be a planar ring with a total of (An + 2) pi electrons (where n is any integer). Using this rule as a criterion certain nonbenzene rings show aromaticity. Such compounds are called nonbenzenoid aromatics. Examples are the cyclopentadi-enyl ion CjHj" and the tropyllium ion C7H7. Other compounds that have a ring... 

A number of conjugated heterocyclic polymers, viz., poly(pyrrole) [9], poly(p-phenylene) [10], poly(thiophene) [11], and poly(aniline) [12] are also electrically conducting and continue to be developed and studied for electrochromic devices [13-14 see also the companion chapter in this volume] and ion switching devices [15-16], among others. Polymer films with high electrical conductivity have been generated by electrochemical polymerization of benzenoid, nonbenzenoid and heterocyclic aromatics, in particular from the derivatives of pyrrole, thiophene, carbazole, azulene, pyrene, triphenylene and aniline. The electrochemical approach for making these films is very versatile and it provides a facile way to vary the properties of the films. The realization of the applications for each electroactive polymer depends on the control and particularly the enhancement of the... 

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