Homoaromatic 2-electron system

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). 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

In line with the discussion given in Section II.E, three different structures are possible (Scheme 14), namely a classical bicyclic structure 45a that can benefit from cyclopropyl homoconjugation, then a classical monocyclic open structure 45c, that should possess normal conjugation of a cyclopolyene, and finally a non-classical no-bond homoaromatic structure 45b with a cyclic 67t electron system formed by 1,7 through-space interactions. 

For the magnetic susceptibility -y determined as a function of the interaction distance, a maximum value should be calculated for the homoaromatic system, i.e. the exaltation of the magnetic susceptibility should indicate homoaromatic electron delocalization. 

Roberts and coworkers, investigating the ionization reactions of cyclobutene derivatives, found that the resulting cyclobutenyl ions were unusually stable10-113. They suggested that rather than regarding these ions as simple allyl cations, their properties were consistent with a C(1 ),C(3) interaction and cyclic delocalization of the 7t-electrons. As such, these 27r-electron systems were considered to be the homoaromatic counterparts of the well established, aromatic cyclopropenium ions39. 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

Thiabenzene 256 and its benzologues 1-thianaphthalene 257, 2-thianaphthalene 258 and 9-thiaanthracene 259 are also potentially antiaromatic 8ji electron systems provided they are planar. However, they adopt a boat conformation in which the S atom lies above the plane of the C atoms, thereby creating a 671 electron homoaromatic system with ylidic character. Calculations show that the energy barrier to inversion at S increases in the order 259 < 257 < 256 < 258 and the calculated dipole moments indicate the greatest ylide character is found in 259. The relative stability is in the order 259 > 257 > 258 > 256 (Table 19) <2006HAC376>. 

In the case of 10, the strain of the six-membered ring is small, as is the strain energy due to ring annelation. Utilizing heats of formations for cw-1,3-butadiene, gauche-Vmy cyc o-propane" and norcaradiene ", a homoaromatic stabilization energy of 4 kcal mol is calculated in line with a description of 10 as a cyclopropyl homoaromatic 6n electron system. 

An X-ray crystal structure proved the symmetry of 27. The three six-membered rings exist in a boat conformation. 27 was expected to be homoaromatic but with the aid of photoelectron spectroscopy only a relatively small interaction between the rc-fragments could be proved. The term homoaromaticity describes systems in which a stabilized cyclic conjugated system containing (4n -I- 2) Tc-electrons is formed by bypassing one or more saturated atoms on the condition that a considerable interaction between the ir-fragments exists due to the proximity of p-orbitals [25]. 

The site of dihydroxylation in heterocycles depends on the nature of the heteroaromatic system (Scheme 9.31) usually, electron-rich heterocycles like thiophene are readily biooxidized but give conformationally labile products, vhich may undergo concomitant sulfoxidation [241]. Electron deficient systems are not accepted only pyridone derivatives give corresponding cis-diols [242]. Such a differentiated behavior is also observed for benzo-fused compounds biotransformation of benzo[b] thiophene gives dihydroxylation at the heterocyclic core as major product, while quinoline and other electron-poor systems are oxidized at the homoaromatic core, predominantly [243,244]. 

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. 

Another approach to evaluating homoaromaticity is to compute various reaction properties such as heats of reaction. A typical example of this approach is a recent paper by Storer and Houk (1992) using molecular mechanics calculations (MM2) of the heats of hydrogenation of triquinacene [118]. In this study they conclude that the anomalous heat of hydrogenation can be explained without invoking homoaromaticity. The use of this type of computational data suffers the same problems as experimentally measured values there is an ambiguity with regard to separation of structural and electronic effects and how to choose appropriate reference systems. 

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