Aromaticity homoaromaticity

In the light of the appreciable puckering found in the seven-membered ring of [16], Childs (1984) recalculated the expected chemical shifts for the exo and endo H(8) protons of [12]. He calculated the difference in chemical shift (A6) to be 6.9 ppm which is in good agreement with the observed AS - 5.86 ppm. However, his calculations revealed that both the exo and endo protons are shielded. This surprising result is opposed to the accepted intuitive view that in an aromatic/homoaromatic system protons with the H(8)(exo) orientation should be deshielded and those with the H(8)(cndo) orientation shielded. This result closely parallels the analogous calculated data for the homocyclopropenyl cation [2] (Schindler, 1987). 

P. von R. Schleyer (Ed.), Chem. Rev. 2001, 101, 1115 (a themed issue reviewing several aspects of aromaticity, homoaromaticity and antiaromaticity). For a recent review of methods of measuring the degree of aromaticity, see I. Fernandez and G. Frenking, Faraday Discuss., 2007, 135, 403. 

There is no doubt that the term aromaticity is one of the most widely used terms in chemistry. At the same time, aromaticity may well be one of the most widely misused terms in chemistry, not by being attributed to compounds that do not qualify as aromatic, but by becoming so unspecified that it is applied to too many compounds that show widely different physico-chemical properties. Labels that are so broad that they apply to a multitude of compounds are bound to cause confusion, just as confusion may arise when there are too many Smiths, Browns, and Jones in the same locality. To reduce the latter confusion, people have been given two and even three names hence, it is understandable that similar attempts were made to discriminate among several types of aromaticity, like pseudo-aromaticity, homoaromaticity, quasi-aromaticity, etc. (Table 2). 

Predict whether the following systems would be expected to show strong (aromatic or homoaromatic) stabilization, weak stabilization by conjugation (non-aromatic) or destabilization (antiaromatic) relative to localized model structures. Explain the basis for your prediction. 

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

In order for the orbitals to overlap most effectively so as to close a loop, the sp atoms are forced to lie almost vertically above the plane of the aromatic atoms. In 107, Hb is directly above the aromatic sextet and so is shifted far upfield in the NMR. All homoaromatic compounds so far discovered are ions, and it is questionable as to whether homoaromatic character can exist in uncharged systems. Homoaromatic ions of 2 and 10 electrons are also known. 

Homoaromaticity may still result if the delocalization in an aromatic compound is interrupted by more than one saturated linkage. In this case a bis-, tris-, or tetra-, etc., homoaromatic compound results. In the notation of Winstein (1967) the size of the saturated linkage (e.g. -CH2- and -CH2CH2-) is not considered in classifying the degree of homoaromaticity. Only the number of interruptions to delocalization is taken into account. Thus, if cycloheptatriene [5] were homoaromatic, it would be monohomobenzene. Similarly, all m-l,4,7-cyclononatriene [8] could be named trishomobenzene if homoaromatic. 

As stated at the beginning of this Introduction, (homo)aromaticity refers to a special (thermodynamic) stability relative to some hypothetical reference state. It is therefore most attractive to use a thermochemical discriminator for the designation of homoaromaticity. However, such thermochemical methods suffer the same disadvantages when applied to homoaromaticity as they do in the case of aromaticity (see for example Garratt, 1986 Storer and Houk, 1992). There have been several recent studies using the heats of hydrogenation of potential homoaromatics in an attempt to classify these species (vide infra). Due, in the main, to the hypothetical nature of the localized model reference states there is some debate regarding these results (see Dewar and Holder, 1989 Storer and Houk, 1992). 

Another measure of the homoaromatic stabilization of the homotropylium systems can be gained from the heats of protonation of the ketones [17]—[20] (Childs et al., 1983). The difference in heats of protonation between [18] and [19] (A//I8 i9) is significantly larger than the corresponding difference between [17] and [18] (AHl7 ls). This increase in stabilization is associated with the homoaromaticity of [21]. Similarly, there is a large discontinuity between A//17 18 and A//18 2o which is associated with the aromaticity of the tropylium ion [22]. 

Hirsch s rule has more limited applicability than the Hiickel rule. However, the 2(n +1)2 concept has been used very successfully to interpret relative fullerene stabilities [66], and to suggest new systems, including neutral and charged non-fullerene carbon [67] and homoaromatic cages [68]. All these species have large NICS values in their centers and satisfy other criteria of aromaticity. 

The stabilization of 10B versus 10A of 38.1 kcal mol-1 is remarkably large for an uncharged homoaromatic. This demonstrates the power of 2e aromaticity 8B is 61.6 kcal mol-1 lower in energy than 8A. Less repulsion between the a skeleton electrons, the number of which is reduced by two in the 2n electron aromatics as compared with the classical isomers, certainly contributes to the huge energy differences. The additional stabilization by formation of BHB bridges is, however, of minor importance. Note that classical a skeletons are to be expected only for 8A2-and 10A2- [6, 20] (Scheme 3.2-6), the 2e reduction products of triborirane and triboretane. 

The first derivative of carba-nido-tetraboranes(7), 16a, was prepared by reaction of anionic 17 with iodomethane and characterized by NMR spectroscopy and by model computations (Scheme 3.2-10) [28]. The structurally analyzed 16b is obtained by deuteration of the dianion 10a2 [20] mentioned in Section 3.2.2.2. The results of an X-ray structural analysis of its dilithium salt are discussed in Section 3.2.8.3. The lithium cations are coordinated side-on to the B-B 2c2e bonds just as predicted for the aromatic U2B3H3 [6]. Obviously, (Li+)210a2 is a 2e homoaromatic. Since the positions of the lithium cations resemble those of the deuter-... 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

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],... 

One such problem is the possibility of the existence and aromatic character of analogues of five-membered heteroaromatics with one heteroatom and those of azoles in which one or several ring C atoms are substituted by a metal atom, e.g. structures (289)-(305) (M = Ge, Sn, Pb X = O, S, NR). The other specific problems involve antiaromaticity of 871-electron carbenoids (306) and homoaromaticity of 67t-electron systems with a tetravalent metal (307). 

Homoaromatic compounds. When cyclooctatetraene is dissolved in concentrated H2S04, a proton adds to one of the double bonds to form the homotropylium ion 94.247 In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The... 

We reiterate that both homoaromaticity and aromaticity are more pronounced in ions than in related neutrals. In the tour-de-force of computational theory, S. Sieber, P. v. R. Schleyer, A. H. Otto, J. Gauss, F. Reichel and D. Cremer [7. Phys. Org. Chem., 6,445 (1993)] document considerable homoaromatic stabilization of the cyclobutenyl cation. Yet the difference of the enthalpy of formation they calculate from their quantum chemical cations, 1021 kj mol-1, is only 54 kJmol-1 lower than that archivally recommended for the cyclopropenium ion [S. G. Lias,... 

J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 17 (1988), Supplement 1], This last number allows us to conclude that the aromatic stabilization of cyclopropenium ion exceeds the homoaromatic stabilization of cyclobutenyl cation by at least (160 - 54) = 106 kJ mol-1. Regrettably, inadequate time and space does not allow us to discuss the energetics of ions containing three-membered rings in the current chapter. 

Some authors have used the term antihomoaromaticity. We think that this term may be misleading since it implies either a system that, despite homoaromaticity, is destabilized or the anti form of a homoaromatic system, which may be considered as the aromatic form itself. 

FIGURE 2. Types of aromaticity and homoaromaticity. The preferred electron delocalization mode is given in each case according to Cremer13... 

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