Aromaticity of annulenes

Cyclophanes [97], in which the aromatic units are linked at the 1,4 positions by unsaturated bridges, can be viewed as derivatized annulenes. They therefore provide a playground for the study of the aromaticity of annulenes. This family of carbon-rich systems behaves like rigid annulenes with well-defined ring configurations therefore their reduction is the choice for systematic study of the effect of the number of the 71-electrons on aromaticity. 

The aromaticity of annulenes and heteroannulenes can be described with the Hiickel rule. Due to their closed-shell structures, annulenes with 4N + 2 electrons are not distorted Dnh symmetry) and show strong diamagnetic ring currents while singlet 47V annulenes are often distorted and have paratropic character. However, the Hiickel rule cannot be applied in polycyclic systems, where, for example. 

State, as shown in Fig. 10.8 [65]. The "trefoil bonding state, previously proposed and predicted to exist in aromatic hydrocarbon annulene molecules, is finally encountered, albeit in an extended intermetallic network [65]. This also highlights the important role of interactions between incompletely filled lone pairs in the stabilization of low-dimensional anion structures. 

In agreement with the Hiickel rule those annulenes and dehydroannulenes which contain (4 n + 2) 77 electrons and a reasonably planar carbon skeleton appear to be aromatic. Aromaticity in annulenes is usually equated with positive resonance energy and the absence of bond alternation. The most direct method of measuring bond alternation is by single crystal X-ray diffraction. Unfortunately this method has been applied in only a few cases. 

Annulene was the first macrocyclic annulene containing (4n -j- 2) zr-electrons to be synthesized. The compound is of considerable interest, since it is the type of annulene that was predicted to be aromatic by Hiickel.10 It proved to be aromatic in practice, as evidenced from the proton magnetic resonance spectrum,8-11 the X-ray crystallographic analysis,18 and the fact that electrophilic substitution reactions could be effected.13... 

The role of a-electrons in the stabilization of aromatics has been repeatedly emphasized by Shaik and Hiberty48 and has recently received experimental confirmation after a study of the effects of deuterium substitution of annulenes with internal hydrogens, such as the anti-aromatic [16] annul ene and the aromatic [18]annulene.49 50... 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

Dihydro- 1,2-diazocines such as (183) have been referred to as l,2-diaza[8]annulenes (78AHC(23)55), and are of interest in connection with the potential aromaticity of the 107r-electron system provided by these rings (see Section 5.19.4.4). On the basis of UV and NMR data the dibenzodiazocine (183) was suggested to have a partially planar structure there is no direct evidence of aromatic character. 

Eight-membered rings with two O, S or N atoms or combinations of these heteroatoms in a 1,2- or 1,4-relationship and three double bonds possess conjugated tt-electron frameworks and can be designated as dihetera[8]annulenes (78AHC(23)55). These 1,2- or 1,4-diheterocins are isoelectronic with the cyclooctatetraene dianion, and if planar represent potentially aromatic IOtt-electron systems. Considerable interest has attached to the degree of aromaticity of these compounds from both theoretical and experimental standpoints. Most theoretical treatments have led to the conclusion that 1,2-diheterocins, because of... 

Our definition does not take account of lone pairs on heteroatoms in the ring which do not contribute to the -system we are normally concerned here with pyridinoid nitrogen atoms, and their influence on aromaticity is discussed in Section V,B. It is also noteworthy that for molecules possessing an unsubstituted pyrrole-type nitrogen atom, the aromaticity of the anion produced on deprotonation can be significantly greater than that of the protonated form. This has been found in a number of cases with aromatic annulenes,15-18 and we will meet similar results with certain mesoionic azapentalenes (Section IV,C,l,a). 

A simple consequence of the above is the large vertical resonance energy of the distorted benzene in Figure 6 and its aromatic magnetic behavior both computed and measured.32 169-171 In fact, as emphasized by Choi and Kertesz,87 even when the aromatic [nj-annulenes undergo bond alternating distortion, their magnetic properties are still those of an aro-... 

The Hiickel theory [21, 22], which has become the standard description of annulene aromaticity, is one of the cornerstones of physical organic chemistry. Unfortunately, in its qualitative version, from which the famous Hiickel rules are derived, the theory does... 

We will not concern ourselves here with the attempt to discern a true definition of aromaticity, nor with the largely unproductive " desire to rank-order relative aromaticity. Rather, we will focus on how computational chemistry has been used to help define two of the criteria of aromaticity. We will discuss how to compute the stabilization energy of an aromatic species and then how we can compute magnetic properties that can be related to aromaticity. We will then conclude with a discussion of how these measures have been employed to investigate the nature of annulenes and the MiUs-Nixon effect. 

It seems clear that definitive evidence of a synthesized aromatic Mobius annu-lene has not been produced. We note in passing that a few examples of aromatic Mobius hexaporphyrins have been reported. Castro and Kamey have suggested that an aromatic Mobius annulene has been prepared, but as a transition state. The optimized transition state 54 for the twist-coupled bond shifting process then connects the tri-frani-[12]annulene 53 with the di-fra i-[12]annulene... 

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