Triquinacene Homoaromaticity

We can ask is there any conjngation to speak of between the two double bonds There really isn t, but if we could somehow force the bonds closer together, can we create some If we can, we might call this homoconjugation. 

The compound triquinacene was an early candidate as a compound that might show this effect (Structure 10). In this particnlar case the effect could be referred to as homoaromaticity. 

The double-bonded carbons that are adjacent to one another, but in different bonds, are approximately 2.54 A apart in this molecule. This is much further apart than are the central atoms in butadiene (1.45 A), but it is much closer together than nonbonded carbon atoms normally come (the sum of the MM4 van der Waals radii is 3.92 A). Also, the dome-like shape of the molecule causes a larger overlap of the bottom sides of the p orbitals than would be found if the system were planar. [When 2p orbitals overlap at a given distance, the overlap is much greater if they are pointed at each other (o overlap) than if they are parallel to each other (jt overlap. The geometry here leads to a considerable portion of o overlap.] The molecule was prepared and first studied in 1964.  

The usual way to determine the aromaticity of a compound is to determine its heat of formation, and compare this with an idealized calculated value, where there is no resonance. The actual path followed in the case of triquinacene was much more convoluted. To describe this history, we first need to say a little bit about thermochemistry and heats of formation. These topics will be discussed in more detail in Chapter 11, we will just give a brief outline here. The ideas are quite straightforward in principle, and the difficulty comes at the practical level. It is easy to imagine doing these things with high precision, but that is not so easy experimentally. 

TABLE 5.5. Heats of Formation of Triquinacene Products (kcal/mol) 

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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. 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

Another approach to evaluating homoaromaticity is to compute various reaction properties such as heats of reaction. A typical example of this approach is a recent paper by Storer and Houk (1992) using molecular mechanics calculations (MM2) of the heats of hydrogenation of triquinacene [118]. In this study they conclude that the anomalous heat of hydrogenation can be explained without invoking homoaromaticity. The use of this type of computational data suffers the same problems as experimentally measured values there is an ambiguity with regard to separation of structural and electronic effects and how to choose appropriate reference systems. 

An example of the inherent complexities in assigning homoaromaticity in neutral hydrocarbons, even when accompanied by thermochemical, molecular mechanical and/or quantum chemical analysis, is shown by the competing studies of the energetics of triquinacene ... 

Several systems have been examined in the context of potential tris- and higher homoaromatic systems21. These include cis. cis, cis-1,4,7-cyclonon atriene (128), triquinacene (129), hexaquinacene and the cyclic polyacetylenes such as 130. The conformations of some of these systems are such that they could be considered to be examples of in plane homoaromatic systems266. 

One molecule that might be expected to be homoaromatic, if the phenomenon can exist in neutral species, is triquinacene (Fig. 9.7) the three double bonds are held rigidly in an orientation which appears favorable for continuous overlap with concomitant cyclic delocalization of six n electrons. Indeed, its potential aromaticity was one of the reasons cited for the synthesis of this compound [58], A measurement of the heat of hydrogenation of triquinacene found a value 18.8 kJ mol 1 lower than that for each of the next two steps (leading to hexahydrotriqui-nacene) [59]. This was taken as proof of homoaromaticity in the triene, i.e. that the compound was 18.8 kJ mol-1 (4.5 kcal mol 1) stabler than expected for an... 

Fig. 9.7 Homoaromaticity. Interposing a CH2 group between one pair of formal double bonds of benzene gives monohomobenzene. Is this delocalized like benzene, or is it just cycloheptatriene Is triquinacene, with a CH group interposed between each pair of formal double bonds, a trishomobenzene ...

Fig. 9.8 The heat of hydrogenation of a double bond in triquinacene is essentially the same as that of a double bond in dihdrotriquinacene and in tetrahydrotriquinacene, and is about the same as in cyclopentene, indicating that triquinacene is not homoaromatic...

The absence of homoaromaticity in triquinacene is presumably due to the three pairs of nonbonded carbons being too far apart, 2.533 A, from X-ray diffraction in the transition state (Fig. 9.9), in contrast, the nonbonded CC distance has been reduced to 1.867 A according to a B3LYP/6-311+G (Section 7.2.3.4c) calculation. Significantly, the measured C=C length, 1.319 A, is close to the normal C=C length (calculated and measured parameters of triquinacene are cited in [60]). 

However, homoaromatic stabilisation appears to be absent in neutral systems. Homobenzene (cycloheptatriene) 1.23 and trishomobenzene (triquinacene) 1.26, even though transannular overlap looks feasible, show no aromatic properties. In both cases, the conventional structures 1.23 and 1.24, and 1.26 and 1.27 are lower in energy than the homoaromatic structures 1.25 and 1.28, which appear to be close to the transition structures for the interconversion. 

Triquinacene is a hydrocarbon that might be stabilized by homoaromaticity. 

Do these data indicate homoaromatic stabilization of triquinacene Why or why not ... 

Triquinacene, 57, has been a great source of interest for many years due to the potential for it to dimerize to dodecahedrane 58. This compound was first synthesized by Woodward in 1964 and was the target of several groups. In addition, the potential for the three alkene moieties to participate in conjugation in the form of neutral homoaromaticity led to an entire workstream of several independent laboratories to confirm this phenomenon of theoretical interest. ... 

Our conclusion from the MM3/MM4 results is that the homoaromatic resonance energy of triquinacene is 0.4 + l.Okcol/mol. It seems likely that there is a small resonance energy, but it is too small to specify with any confidence. 

There are other ways to decide if a compound is aromatic, of which the magnetic ring current from NMR, and various other spectroscopic methods were all examined. From each of these methods, the result was the same, namely any homoaromatic resonance present in triquinacene was too small to measure. 

Thus, there may be a nonbonded conjugated interaction or homoaromaticity in triquinacene, but it is quite small because the distance between the double bonds is just too great. This has prompted chemists to try to prepare additional molecules that would show this interaction, but where the double bonds were closer together, the overlap was greater, and the interaction would be much stronger. This led to speculation over the years, and a number of compounds were suggested as potential structures that could be prepared that might show homoaromaticity. 

The four-membered rings in this molecule pull the double bonds much closer to one another than they are in triquinacene, about 1.87 A compared to 2.54 A. At this distance there is quite appreciable overlap between the orbitals of the non-bonded carbons, and homoaromatic interactions seem much more likely. 

Anionic homoaromatic compounds are quite few, such as the bis-diazene dianion in Fig. 4.3. However, whether or not neutral species can be homoaromatic is still a matter of debate. Some of neutral molecules used to be considered as homoaromatic, such as the fulleroid, 1,2-diboroetane and triquinacene in Fig. 4.3 but their homoaromaticity characters are either in question or denied. Thus, the establishment of experimental models for potential neutral homoaromatic molecules has long been an exciting pursuit in synthetic and theoretical chemistry. The central challenges remain the development of efficient synthesis, and the collection of detailed experimental data, in order to gain a deep insight into the structure-reactivity relationship. 

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