Resonance energy 18 -annulene

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. 

It is interesting that attachment of — ( 2)4— and —CH2CH=CHCH2— to benzene results in nearly the same enthalpy of formation change but it is not obvious how fortuitous this equality is we have reasons for considerable skepticism of its validity68. That formation of naphthalene from benzene is accompanied by a lessened enthalpy of formation increase than that of l,6-methano[10]annulene (yet another name for species 90) from tropilidene would appear to be more of a strain than a resonance derived effect. From Roth, we find the resonance energy increase on going from tropilidene to l,6-methano[10]annulene is 55 kJmol-1 and from benzene to naphthalene the increase is nearly the same, nearly 59 kJmol-1. By contrast, the l,5-methano[10]annulene (99) is less stable by 77 kJmol 1 than the species it appears most naturally to be compared with, namely the isomeric 90. 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

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-... 

Predicting planarity in large, floppy annulenes is often difficult. If the annulene is aromatic if planar, then the resonance energy (AH) favors planarity, but the entropy factor (-TAS) usually favors a more disordered conformation. Which factor prevails is hard to predict, and may depend on the temperature and solvent. 

If the annulene is antiaromatic if planar, then both the resonance energy and the entropy favor a more disordered, nonplanar conformation unless the molecule is somehow locked in a planar conformation. 

Should one ask for direct enthalpy of formation investigations on anti-Bredt enamines, such studies are currently limited to l,6-imino[10]annulene (14) with its gas phase enthalpy of formation of 367.2 7.0 kJ mol-1. How do we affirm the decrease in resonance energy accompanying its geometric constraints There are no enthalpy of hydrogenation measurements for this compound, nor for any other 11-azabicyclo[4.4.1]-undecane (30) derivative71. However, we do know the gas phase enthalpy of formation... 

Annulene (115) is diatropic the 12 outer protons are found at 8 = 9 and the 6 inner protons at 5 = — 3. X-ray crystallography shows that it is nearly planar, so that interference of the inner hydrogens is not important in annulenes this large. Compound 115 is reasonably stable, being distillable at reduced pressures, and undergoes aromatic substitutions. The C—C bond distances are not equal, but they do not alternate. There are 12 inner bonds of 1.38 A and 6 outer bonds of 1. 42 A. Compound 115 has been estimated to have a resonance energy of 37 kcal mol (155 kJ mol ), similar to that of benzene. ... 

More recently, Dewar and Gleicher 2o> have carried out SCF calculations for the annulenes (with experimental geometries). From the calculated resonance energies they conclude that the onset of bond alternation in the An + 2) series should begin with [26]annulene, which they predict to be non-aromatic (see Table 1 and Fig. 2). Note, however, that the calculated difference in resonance energy between [22] and [26]an-nulene only amounts to about 7 kcal/mole in such large molecules this could easily be overshadowed by steric requirements. 

Fig. 2. Calculateda) resonance energies as a function of annulene ring size (CnHn)2c>. Reprinted -with permission of the copyright owner from J. Am. Chem. Soc. 87, 685 (1965)...

Table 1. Calculateda) n-binding energies and resonance energies of the annulenes CnHn...

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