Antiaromaticity, concept

The special case of pericyclic reactions is an appropriate means of introducing the subject These reactions are very common, and were extensively studied experimentally and theoretically. They also provide a direct and straightforward connection with aromaticity and antiaromaticity, concepts that mm out to be quite useful in analyzing phase changes in chemical reactions. 

In spite of these difficulties, the aromaticity and antiaromaticity concepts are quickly advancing in inorganic chemistry. 

As multinuclear transition-metal clusters were found to be more catalyticaUy active that mononuclear complexes, we believe that catalysis also could be a new area of advancement of aromaticity and antiaromaticity concepts for explaining catalytic activity. Complex active sites of enzymes could be another area where aromaticity and antiaromaticity may be useful. 

Furthermore, natural bond orbital (NBO) analysis of the first-order density has also been used to quantify aromaticity [73,74]. More recently Boldyrev and Zubarev [75] developed the adaptive natural density partitioning (AdNDP) algorithm attempting to combine the ideas of Lewis theory and aromaticity. The results obtained by the application of the AdNDP algorithm to the systems with non-classical bonding can be readily interpreted from the point of view of aromatic-ity/antiaromaticity concepts. 

The simplest, qualitative, theoretical understanding of the nature of oxirene is provided by Breslow s concept of antiaromaticity. Whatever criticisms may be levelled at this notion (78JA6920), it does correctly predict that oxirene should be unusually unstable. 

The concept of relative hardness and topological resonance energy per electron as a measure of aromaticity, with a relative hardness value of zero as the border for antiaromaticity, failed for 1-benzothiepin,3 2... 

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. 

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. 

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)... 

For a balanced account of these concepts, see V. I. Minkin, M.N. GlukhovtsevandB. Ya. Simkin, Aromaticity and Antiaromaticity, Electronic and Structural Aspects, Wiley, New York, 1994. 

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years33, the two concepts are inextricably linked34. This is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states35 and the Huckel/Mobius concept31. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 136. 

We have introduced the concept of homoheteroaromaticity (the equivalent of homoaromaticity [18] for heterocycles) to describe the structure of some cations obtained by protonation of l//-azepine (4) and 5//-dibenz[/ ,/]azepine (5). B3LYP/6-311++G, GIAO, and NICS calculations together with some NMR experiments lead us to conclude that the neutral molecules are antiaromatic and that the cations 6 and 7 are homoaromatic [22],... 

The concept of aromaticity is vety important in understanding the chemical behavior of cyclic, conjugated compounds. It is most important with benzene and its derivatives, but it also has applications to many other types of compounds. Whenever a reactant, product, or intermediate contains a planar cycle of p orbitals, the effect of aromaticity (or antiaromaticity) on the reaction must be considered. 

The word aromaticity usually implies that a given molecule is stable, compared to the corresponding open chain hydrocarbon. For a detailed account on aromaticity, see, e.g., Reference [95], The aromaticity rules are based on the Hiickel-Mobius concept. A cyclic polyene is called a Hiickel system if its constituent p orbitals overlap everywhere in phase, i.e., the p orbitals all have the same sign above and below the nodal plane (Figure 7-23). According to HiickeTs rule [96], if such a system has 4n + 2 electrons, the molecule will be aromatic and stable. On the other hand, a Hiickel ring with 4n electrons will be antiaromatic. 

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