Nonaromatic double-bonded structures

The nonaromatic character of oxepins makes this structure susceptible to the reduction of one or more double bonds. 10-[(2-Aminoethyl)sulfanyl]-substituted dibenz[b,/]oxepins with various substituents in position 2 smoothly react to give the 10,1 l-dihydrodibenz[b,/]oxepin system 1 on treatment with magnesium in methanol.71 202 The advantage of this method is the possibility of reducing dibenz[b,/]oxepins with various heteroatoms such as sulfur, oxygen or nitrogen in the side chain. 

Bromine can also undergo addition to the nonaromatic double bond of dibenz[ft,/]oxe-pin.160,161 Oxygen-bridged annulenes that incorporate an oxepin structure have been shown to undergo 1,4-addition of bromine at — 78°C.159... 

In 1-benzoxepins the benzene oxide form is energetically unfavorable. Thus, the adducts 5 formed with dienophiles such as ethenetetracarbonitrile arise from the oxepin structure with the nonaromatic double bonds as diene fragment.233 The yields of these reactions arc almost quantitative. 

Cyclooctatetraene (or simply cyclooctatetraene) is a bright-yellow, nonplanar, nonaromatic compound (Section 21-9A). Apparently the resonance energy of a planar structure is insufficient to overcome the unfavorable angle strain the planar structure would have with its C-C-C bond angles of 135°. Cyclooctatetraene normally assumes a tub structure with alternating single and double bonds ... 

The use of hydrodesulfurization in the determination of the structures of unknown benzo[6]thiophenes is becoming increasingly recognized about 100 examples have now been recorded. Two sets of experimental conditions are commonly used the benzo[6]thiophene may be treated with Raney nickel alloy directly in alkaline medium (the method of Papa et al.7M), or it may be boiled in ethanol with Raney nickel of varying reactivity (commonly W-7). Even under mild conditions, nonaromatic double bonds are usually saturated, halogens are removed, and nitro groups are reduced. Raney cobalt has about one-tenth the activity of Raney nickel in the hydrodesulfurization of benzo [6]thiophenes.765... 

Moreover, this molecule is believed to have the ability of turning inside out via a transitional plane structure. At the same time it was demonstrated that in a plane cyclooctatetraene molecule the bond lengths do not get equalized, the double bonds retaining their distinctive features. Even if this molecule were plane, it would still be nonaromatic. 

The barrier to converting the s-trans conformation to the s-cis conformation contributes to the overall activation barrier for Diels-Alder reactions. Structural factors that increase the proportion of diene in its s-cis conformation increase the rate of the Diels-Alder reaction, and factors that increase the proportion of diene in its s-trans conformation decrease the rate of the reaction. Cyclopentadiene is one of the best dienes for the Diels-Alder reaction partly because it cannot rotate out of its s-cis conformation. In fact, cyclopentadiene undergoes [4 + 2] cycloaddition to itself so readily that it lasts only a few hours at 0 °C. o-Xylylenes are especially good dienes both because of their enforced s-cis geometry and because a nonaromatic starting material is transformed into an aromatic product. By contrast, dienes in which one of the double bonds is cis are poor substrates for Diels-Alder reactions because steric interactions between the in substituents in the s-cis conformation are particularly severe, and dienes whose s-trans conformation is enforced do not ever undergo the Diels-Alder reaction. 

The XH NMR spectrum shows bands in the region 4.0-6.0 ppm (See Table 6). Following the discussion of priority of paths of delocalization of aceheptylene dianion 232 and acenaphthylene dianion 82 also 332 and 342 show that specific paths of delocalization are favoured. While in the neutral structure 33 and 34 the competition is between aromatic and nonaromatic structures, in the respective dianions the competition is between nonaromatic and antiaromatic structures (Fig. 9). From the spectroscopic parameters, i.e., chemical shifts and coupling constants of the bridge protons it can be concluded that the neutral systems are best represented by structures with an aromatic skeleton connected to a virtually isolated double bond. In the charged systems, viz. 332 and 342 it seems that a nonaromatic path of conjugation is preferred to an antiaromatic path (Fig. 9). These considerations are also reflected in the carbon chemical shifts and in their HOMO-LUMO gap (AE) (vide infra) 122). It can be concluded from all these observations that there is a tendency of aromatic systems to remain so and to avoid as much as possible paratropic antiaromatic contributions. 

The equal sharing of the six electrons of the p orbitals results in a rigid, flat ring structure, in contrast to the relatively flexible, nonaromatic cyclohexane ring. The model also explains the unusual chemical stability of benzene and its resistance to addition reactions. The electrons of the tt cloud are said to be delocalized. That means they have much more space and freedom of movement than they would have if they were restricted to individual double bonds. Because electrons repel one another, the system is more stable when the electrons have more space to occupy. As a result, benzene is unusually stable and resists addition reactions typical of alkenes. 

Hydroxylation or oxidative cleavage of the allylic methyl group present on the carbocyclic nonaromatic ring increases affinity for both CB receptors, as exemplified by the structure of nabilone (27), which has been introduced into the market as antiemetic agent for cancer supporting care and has also recently been approved in USA for the treatment of neuropathic pain [53]. The position of the double bond seems to be not crucial, indeed A -THC is almost equipotent with A -THC however, aromatization of this ring, as occurs in CBN, causes a marked decrease in affinity. 

Schematic structures and color-coded NICS maps of finite length (5,5) and (6,6) SWCNTs. Hydrogen atoms are omitted for clarity. Chemical bonds are schematically represented by using single bond solidsingle line bond length >1.43 A), double bond solid double line bond length <138 A), single bond halfway to double bond solid-dashed line 1.43 A > bond length >138 A), and Clar structure (i.e., ideal benzene). NICS coding red, aromatic < -4.5 blue, nonaromatic > -4.5 (Reprinted with permission from ref. Matsuo et al. (2003). Copyright 2003 American Chemical Society)...

Spectral and structural data confirm the ordinary alkene nature of cyclooctatetraene. Thus, the NMR spectrum shows a sharp singlet at S = 5.68 ppm, typical of an alkene. The molecular-structure determination reveals that cyclooctatetraene is actually nonplanar and tub-shaped (Figure 15-17). The double bonds are nearly orthogonal (perpendicular) and not conjugated. Conclusion The molecule is nonaromatic. 

Sondheimer obtained the first dianion of a dehydroannulene by reducing 1,3,7,9,13,15,19,21-octadehydro[24]annulene (12) with K [89]. Earlier, only the reductions to radical anions had been reported, as is the case in the reduction of l,8-bisdehydro[14]annulene (13) by Wratten [90], and the reduction of 1 2,5 6, 9 10-tribenzo-3,4,7,8,11,12-tridehydro[12]annulene (14) by Staab and Graf [91]. Neutral 12 is nonplanar and nonaromatic, probably due to the presence of four cis double bonds and the rigidity imposed by the four 1,3-diyne units. The EPR pattern of the radical anion shows that it has a planar structure and that the k-electrons are delocalized. The NMR spectrum of the dianion, which contains 26 Ti-electrons, shows low-field signals, indicating that the Ti-system is delocalized over the planar system, i.e., an aromatic system. 

We have already pointed out in the Introduction (see above) that the first review article on radialenes is only a few years old1. In this first summary we have enclosed a comprehensive survey and discussion of the structural and spectroscopic properties of the radialenes. Since progress in this latter area has not been very rapid in the last few years, we do not address here again these aspects of the radialenes. Furthermore, nothing new can be added to the statement that all radialenes are nonaromatic and that they have localized endocyclic single bonds and exocyclic double bonds1 (for recent discussions of ji-ji interaction in [5]- and [3]radialene, see elsewhere903 109). 

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