10-annulene

The unusual properties of benzene and its Kekule structure go back to the beginnings of organic chemistry. In 1931 Hiickel published his famous 4n + 2 rule for aromaticity, which both rationalized the observed facts and also suggested that there would be a large series of hitherto unknown aromatic compounds possible. However, to be aromatic they had to be planar, or nearly so, which meant that actually [18]annulene was the next member of the series that was realistically a physical possibility. It was accordingly of great interest to chemists to see if in fact this compound was aromatic.  

Mislow proposed in 1952 that the internal hydrogens in the [18]annulene ring system would interfere with one another (from van der Waals interactions) to such an extent, that the molecule would probably not be planar (as is clearly the case with [lOJannu-lene). The compound was synthesized in 1962 by Sondheimer and co-workers and has been subjected to extensive smdies subsequently. In spite of numerous studies by a variety of physical methods, and extensive computations as well, it can only be said at this writing that the structure is still not completely agreed upon. A brief exposition of the situation here up to the present time follows. 

There are two principle structures that are possible for this molecule, and then there are some additional perturbations that are possible. But let us consider the general problem and the two principle structures first. 

One can easily write a general stracture for an 18-membered ring, which must be close to planar, and must contain a minimal amount of unfavorable van der Waals interactions. Let us initially assume the molecule is planar. There are two obvious possibilities, one of which would have De symmetry and the other would have Du, symmetry, as shown in Structure 9. 

So which is the structure The first detailed theoretical studies on the structure of [18]annulene were carried out by Longuet-Higgins and Salem. They predicted that there would be a substantial n electronic ring current for this molecule, resulting in chemical shifts of the inner and outer protons as a function of the degree of bond alternation. The observed NMR spectrum indicated that, insofar as this approximate treatment is valid, bond alternation is present in [18]annulene. 

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Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

The kekulene macrocycle consists of twelve anellated benzene rings and may be considered as an [iSlaanulene (inside) or a 30]annulene (outside). H. Staab (F. Diederich, 1978) called it a superbenzene , since it has the same symmetry as benzene. 

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

Use Frosts circle to construct orbital energy diagrams for (a) [lOjannulene and (b) [12]annulene Is either aromatic according to Huckel s rule ... 

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

As noted earlier planar annulenes with 4n tt electrons are antiaromatic A mem ber of this group [16]annulene has been prepared It is nonplanar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene... 

Most of the synthetic work directed toward the higher annulenes was earned out by Franz Sondheimer and his students first at Israel s Weizmann Institute and later at the University of London Sondheimer s research systematically explored the chemistry of these hydrocarbons and provided experimental venfication of Huckel s rule... 

Something interesting happens when we go beyond benzene to apply the aromatic ring current test to annulenes... 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Which would you predict to be more shielded the inner or outer protons of [24]annulene" 13 41 F IS the only isotope of fluonne that occurs naturally and it has a nuclear spin of j... 

Divide the heats of combustion by the number of carbons The two aromatic hydrocarbons (benzene and [18]annulene) have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons (cyclooctatetraene and [16]annulene) On a per carbon basis the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons... 

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