Triquinacenes structure

FIGURE 8. Structures of triquinacene (1) and hexaquinacene (2) (distances in A, angles in degrees)... 

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. 

W(CO)3 to give tricarbonyl(triquinacene)tungsten X-ray analysis has revealed 470 to possess the indicated structure. 

Following the preparation of 2,6-di(bromomethyl)triquinacene 471), dimercaptan 472 was synthesize conventionally. Coupling of these intermeiates produced a 3.5 to 1 mixture of anti- and syn-triquinacenophanes 473 and 474. These [somers were separate chromatographically and identities established by x-ray structure determination of 473... 

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. 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

Centropentaindane 10 is a high-melting compound bearing two residual bridgehead C-H bonds. Similar to tri-/hso-tetraindane 8, centropentaindane has a rigid molecular structure. The presence of two (mutually fused) tribenzo-triquinacene units in 10 gives rise to an almost perfect C2v-symmetrical conformation. From another viewpoint, the benzo bridge across the fenestrindane backbone of 10 fixes its conformation in an almost perfect D2d-symmetrical skeleton, as confirmed by X-ray structure analysis [102]. 

A) is rather too great for trishomoaromatic stabilization by n-n bonding overlap to be a real physical possibility. Semi-empirical MINDO/3 calculations have been employed to optimize the structures of the triquinacen-l-yl and -10-yl cations, and reveal the former to be about 23 kcal mol more stable, with Eg = 9 kcal mol for its transformation into the C-10 cation. The open-shell version of MINDO/3 shows the same stability sequence for the analogous radicals, the C-1 radical being about... 

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. 

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. 

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. 

Provide the structure of the triquinacene derivative that could be prepared from cyclohexan-l,2-dione using this strategy. (Unnatural Products-13)... 

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