Aromaticity and antiaromaticity

From an aromaticity-antiaromaticity point of view, an intriguing representative of the four-membered heterocycles is undoubtedly the hitherto unknown azete (21 R1=R2 = R3 = H). Theory 

The existence of ring strain, nitrogen lone-pair interactions, or both seems to be crucial in the absence of aromaticity of 1,2-A3-diazetine (26). 

For large rings, aromaticity is possible where the conditions of planarity and Hiickel s rule are met, but the majority of fully unsaturated large heterocycles are not aromatic. 

Unlike the tub-shaped parent azocines which are antiaromatic, their 1 Orr-electron dianions (e.g. 30) are planar and aromatic in nature (7iJAi6i). The dianiones are formed by two-electron reduction of azocines. An intermediate radical-anion (29) was obtained from 3,8-dimethyl-2-methoxyazocine (28) (83JA6078) which has a strong tendency to disproportionate into dianion (30) and neutral azocine. 

4-Dioxocine (32 X = 0) behaves chemically as an alkene rather than as an aromatic compound. Thus, it is readily hydrogenated to 1,4-dioxocane and polymerizes readily upon standing (72AG(E)935 . 

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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. 

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. 

Ring Currents Aromatic and Antiaromatic Magnetic Resonance Imaging Spectra by the Thousands Gas Chromatography GC/MS and MS/MS... 

Ring Currents Aromatic and Antiaromatic Spectra by the Thousands... 

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). 

Minkin VI, Clukhoytsev MN, Simkin BY (1994) Aromaticity and Antiaromaticity, Electronic and Structural Aspects. Wiley, New York... 

Ronald Breslow (Co-Chair) is University Professor of Chemistry, Columbia University, and a founder of a new pharmaceutical company. He received his B.A. (1952), M.A. (1954), and Ph.D. (1955) from Harvard University. His research area is organic chemistry with specialization in biochemical model systems, biomimetic synthetic methods, reaction mechanisms, and aromaticity and antiaromaticity. He served as president of the American Chemical Society in 1996 and has authored a book for the general public, Chemistry Today and Tomorrow The Central, Useful, and Creative Science. He is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, and the American Philosophical Society. He received the U.S. National Medal of Science in 1991. 

Figure 7. Aromatic and antiaromatic systems in the ground state (GS) and the twin excited state (ES). The parameter E, is the coordinate that transforms A to B.

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 above analysis suggests that aromaticity or antiaromaticity in any system is the result of sequential aromatic and antiaromatic unions. The union which dominates and, thus, determines whether the system will be aromatic or antiaromatic, is the one which involves the greatest spatial overlap between the fragments. Accordingly, one can simplify the analysis by focusing exactly on this crucial union. However, exceptions to this generalization do exist and arise in a very predictable fashion. 

Structural indices constructed in this fashion are, in essence, phenomenological, and one is entitled to ask whether the specific features in the geometry of the aromatic and antiaromatic molecules are indeed determined, and if so, to what degree, by the cyclic electron (bond) delocalization. 

Distinguishing Characteristics in the Geometry of Aromatic and Antiaromatic Molecules... 

The question to be answered in the first place is whether the ring current model criterion is compatible with the chief energy criterion of aromaticity and antiaromaticity. The answer will be positive if a relationship is revealed between ring currents and resonance energies. 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

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