Aromatic compounds antiaromatic

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

Tlie tautomerism of heteroaromatic compounds is intimately related to the problem of aromaticity and antiaromaticity (87MI1,94MI1), this being especially true for the compounds of this chapter, which are borderline between these showing Fliickel behavior and polyenic compounds (nonaromatic). 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

We have chosen to examine the problem not by studying more aromatic compounds but instead by looking at antiaromatic species. This approach has been utilized very infrequently (27) because of the perception that antiaromatic species should be so reactive that they would be difficult to study. As we have discovered and will discuss below, antiaromatic dications are very amenable to study. 

In the first part of this review a critical analysis of various criteria of aromaticity and the indices quantifying aromatic or antiaromatic character is presented in Section II with an emphasis on application to heterocyclic compounds. Special attention is paid to the elucidation of general trends observed in the change of aromatic character on going from the parent... 

The contributions of the first three types are practically local in character they are close in value for two protons with similar structural environment, such as the ethylenic- and aromatic-type protons. It is only the last term in Eq. (35) that defines the values of the chemical shifts characteristic of aromatic or antiaromatic compounds. 

The estimation of aromatic stabilization (antiaromatic destabilization) energy based on thermodynamic characteristics of different reactions may yield for the same compound quite dissimilar values. As has already been pointed out, these discrepancies stem from the fact that the cyclic electron... 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

C. Aromaticity and Antiaromaticity of Heterocyclic Compounds in Which the Number of 77-Electrons Is Different from That in the Parent Hydrocarbon... 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

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)... 

We classify compounds as aromatic [(4n + 2) ir-electron systems] or antiaromatic (2n TT-electron systems), if there is continuous conjugation around the ring, and as non-aromatic. Aromatic compounds are further subdivided into those without exocyclic double bonds and those in which canonical forms containing exocyclic double bonds contribute. 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

Discussion of the subject matter centers primarily on such physicochemical properties as are deemed indicative of -electron mobility and the attendant development of aromatic or antiaromatic character. Although it is not entirely neglected, the description of synthetic procedure is limited for the most part to the crucial final step. It may also be well to note that, while a serious attempt has been made to provide as complete as possible coverage of the area, the main emphasis in this review is on proper representation rather than on exhaustive enumeration. Also, in order to achieve maximum effectiveness in the coverage of the literature, compounds belonging to a given size-class are described in terms of increasing molecular complexity rather than historical sequence. 

The electron spin resonance (ESR) spectra of the radical ions of 230 indicate there are no large deviations from the free-electron g value that would have been expected had the 3d orbitals of the sulfur atom played an important part in influencing the spin density of the molecule. Consequently, structure 230 may not be the main contributor to the electronic structure of the compound. Such stability in this compound could be attributed to the inertness of the NSN group and the presence of the aromatic naphthalene ring. However, the H-NMR chemical shifts (8 = 4.45 ppm) suggest the compound is antiaromatic. The compound is therefore referred to as an ambiguous aromatic compound (78JA1235). 

Silicon compounds exhibit aromaticity and antiaromaticity in analogy to the corresponding carbon compounds. However, in general, the degree of their aromaticity and antiaromaticity is smaller than that of the analogous carbon compounds. 

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