Homoaromaticity

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example  

The cyclobutenyl cation is the homoaromatic analog of the very stable cycloprope-nium cation. This ion can be prepared from 3-acetoxycyclobutene with the use of superacid conditions  

The temperature-dependent NMR spectrum of the ion can be analyzed to show that there is a barrier (8.4 kcal/mol) for the ring flip that interchanges the two hydrogens of the methylene group. The C-NMR chemical shift is also compatible with the homoaromatic structure. MO calculations are successful in reproducing the structural and spectroscopic characteristics of the cation and are consistent with a homoaromatic structure.  

Clearly, stabilization in such systems will depend not only on the presence of a favorable electronic system, but also on the ability of the molecule to adopt a geometry favorable for overlap of the tt system. Ions such as 9 and 10 do appear, however, to gain considerable stability from such conjugation and to have other aromatic characteristics. For example, protons a and b exhibit sharply different chemical shifts  

Proton a is 5.8 ppm upfield of b, indicating the existence of an aromatic ring current. The 7T system also apparently contributes to imposing a high barrier to conformational inversion  

Perhaps the simplest example of a homoaromatic cation is the ion C4H5 , 11. 

However, homoaromaticity appears to be absent in homobenzene (cycloheptatriene) 1.30a and in trishomobenzene (triquinacene) 1.31a, even though transannular overlap looks feasible. In both cases, the conventional structures 1.30a and 1.30c, and 1.31a and 1.31c are lower in energy than the homoaromatic structures 1.30b and 1.31b, which appear to be close to the transition structures for the interconversion. 

Homoantiaromaticity is even less commonly invoked. Homocyclobutadiene 1.32b and the homocyclo-pentadienyl cation 1.33b are close to the transition structures for the interconversion of cyclopentadiene 1.32a and bicyclo[2.1.0]pentene 1.32c and of the cyclohexatrienyl cation 1.33a and the bicyclo[3.1.0]hex-enyl cation 1.33c. However, homoantiaromaticity does show up in these cases, in the sense that, unlike the interconversions in 1.30 and 1.31, neither of these interconversions is rapid. 

The concept of homoaromaticity and homoantiaromaticity is sound. The nature of the overlap in the aromatic and antiaromatic systems is not dependent upon the atoms being directly bonded by the 7 framework. The a framework in an aromatic system has the effect of holding the p orbitals close, 

The concept of aromaticity can also be extended to systems in which the conjugated system is interrupted, by a methylene group, or other insulating structural feature, provided that the overlap between the p orbitals of the conjugated systems can still take place through space. When such overlap has energy-lowering consequences, evident in the properties of the molecule, the phenomenon is called 

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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. 

Katritzky et al. have studied, using pK measurements, the tautomerism of aza[10]annulenone (2-azabicyclo[4.4.1]undeca-4,6,8,10-tetraen-3-one) 68 (93H2483). Comparison of the pX s of 68 and of model compounds 69 and 70 suggests that 68a is the dominant tautomer pKr = -0.70).Tliis implies a significantly diminished preference for the oxy form in compound 68 compared to 2-pyridone pKr = -3.0). This result indicated that aza[10]-annulenones 68a and 70 should be viewed as 677-homoaromatic species. NMR spectra support this conclusion (93H2483). 

Prolonation of thiepins leads to //-thiepinium ions, for which a homoaromatic character has been discussed.24 The 5//-1 -benzothiepinium ion can be regarded as a benzohomothiopyrylium ion, whereas the 3//-l-benzothiepinium ion is postulated as having a localized sulfonium ion structure.25... 

Biooxidation products originating from simple homoaromatic precursors are usually formed in high regio- and stereoselectivity. Lipophilic substituents are most readily converted and the incorporation of a halogen (R = Cl, Br, I) into biooxidation products of TDO is also exploited to give a more pronounced difference in reactivity... 

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]. 

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... 

H2—He at 8.5 5. This ion is an example of a homoaromatic compound, which may be defined as a compound that contains one or more sp -hybridized carbon atoms in an otherwise conjugated cycle. °... 

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. 

In contrast to the series of hydrocarbons, heterocyclic analogues, even of cyclonona-l,4,7-triyne 2 (u = 3), are known. This is because heteroatom linkers more easily adopt smaller bonding angles and thereby provide some relief of overall angle strain. Since heteroatoms have different electronic properties to carbon atoms, the perceived homoconjugative and homoaromatic effects might be expressed more pronouncedly in heterocyclic [n]pericyclines. 

Structural Features of [n]Pericyclines and the Quest for Homoconjugation as well as Homoaromaticity... 

Fig. 5. [5]Pericyclines with conceived cyclic homoconjugation and neutral homoaromaticity...

All this is in line with the most recent finding for triquinacene, for which the direct determination of its AHy(g) from its experimentally measured heat of combustion finally corroborated the results of the most advanced computational studies that triquinacene is not homoaromatic [35]. Evidently, heat of combustion measurements should also be carried out for some representative [nlpericyclines to finally settle the quest for their neutral homoaromaticity. 

Si4 revealed a distorted tetrahedral configuration, and the Sil-Si2 and Si2-Si3 bond distances of 2.240(2) and 2.244(2) A were intermediate between the Si=Si and Si-Si bond lengths of the precursor 19. This was explained by the delocalization of the positive charge over the Sil, Si2, and Si3 atoms, accompanied by the Sil-Si3 through-space orbital interaction, resulting in the overall homoaromaticity of 20. The hypothesis of homoaromaticity was further supported by the observation of an extremely low-field shifted signal of Si2, the central atom of the Sis homoaromatic system, at 315.7 ppm. 

The synthetic utility of cyclopentadienones is limited, however, because they are quite unstable. Exceptionally facile elimination of CO also takes place from 22, in which homoaromaticity can facilitate elimination. 

Isaacs, L., Wehrsig, A., and Diederich, F. (1993) Improved purification of C60 and formation of S- and 7i-homoaromatic methano-bridged fullerenes by reaction with alkyl diazoacetates. Helv. Chim. Acta 76, 1231-1250. 

High-spin organic molecules and spin alignment in organic molecular assemblies. 26, 179 Homoaromaticity, 29, 273... 

Relatedly, one would have expected 1,3,5-cyclooctatriene to have a more negative enthalpy of formation than tropilidene by the same —20.6 kJmol-1. By contrast, the difference for these enthalpies of formation of species 86 and 87 as derived from experimentally measured enthalpies of formation is ca +12 kJ mol-1. From this we may deduce that tropilidene enjoys considerable stabilization due to homoaromatic interactions. While this conclusion is not new64, nonetheless we find it encouraging to see it corroborated. 

For some homoaromatic carbocations the NICS values and chemical shifts have been calculated.105,106 IGLO-HF and GIAO-MP2 calculated 13C NMR chemical shifts for bishomoaromatic 7-norbornenyl 90 and 7-norbornadienyl cation 91 have also been reported.107... 

The cyclobutenyl cation (92) and the homotropylium cation, CgHcf 93 are both prototypes of homoaromatic systems. 

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