Cycloheptatrienes homoaromaticity

Homoaromaticity may still result if the delocalization in an aromatic compound is interrupted by more than one saturated linkage. In this case a bis-, tris-, or tetra-, etc., homoaromatic compound results. In the notation of Winstein (1967) the size of the saturated linkage (e.g. -CH2- and -CH2CH2-) is not considered in classifying the degree of homoaromaticity. Only the number of interruptions to delocalization is taken into account. Thus, if cycloheptatriene [5] were homoaromatic, it would be monohomobenzene. Similarly, all m-l,4,7-cyclononatriene [8] could be named trishomobenzene if homoaromatic. 

A semiempirical predictor of homoaromaticity has been developed based on the interactions between atoms obtained from an energy partitioning scheme (Williams et al., 1988). This technique correlates the energy lowering two-centre interactions of two non-bonded atoms with homoaromaticity. A second part of the predictor is the demonstration of the necessity of including at least a minimal 2x2 configuration interaction (Cl) treatment. This semiempirical predictor has been verified by correctly interpreting the interactions in cycloheptatriene [5], 1,6-... 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

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. 

In summing up this section on neutral homoaromatic compounds we point out that a considerable number of neutral molecules have been identified as benefiting from homo-conjugative electron delocalization. These include cycloheptatriene as well as several bridged derivatives of these molecules. We anticipate that further work on these systems and the related homoantiaromatic bicyclo[2.1. OJpentene will prove rewarding. 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

Benzo-l,3-dioxole (XXIII) is presumably nonaromatic, other than in its benzene ring. That there are ten 7t electrons does not make this species aromatic because the conjugation is interrupted [40, 41], much as the six-7t cycloheptatriene may enjoy homoaromatic stabilization but not aromatic stabilization, per se [42], Accordingly, we are confident that 2-benzimidazolinone (XXI) with its ten n electrons is aromatic. 

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 ...

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. 

As typical of many other attempts to describe homoconjugative interactions with the help of bond orders, we mention here recent investigations of Williams, Kurtz and Farley ". These authors used various semi-empirical methods (MNDO, AMI, MINDO-CI, AMI-Cl) to study cycloheptatriene, l,6-methano[10]annulene, elassovalene and some other potentially homoaromatic compounds. For the 1,6 interactions in cycloheptatriene and l,6-methano[10]annulene, small bond orders <0.1 were calculated suggesting the absence of homoconjugative interactions although homoaromatic character is generally accepted in the case of the l,6-methano[10]annulene. The authors concluded from this that bond orders seem to be of no use as possible discriminators of homoconjugative interactions ". ... 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

It has been shown experimentally (heats of hydrogenation, heats of formation) that there is significant homoaromatic stabilization in cycloheptatrienes such as 1, 7, 8, 9, 12 (in the latter three there is additional classical Huckel aromaticity), as well as in norcaradienes such as 2, 3, 10, 11. In cases where other energetic factors of the pair of valence tautomers 1 2 are very similar, the relative homoaromaticity determines the equilibrium position. ... 

Childs. Cremer, and Elia maintain that there are several examples of neutral homoaromatics. They suggest that 1,2-dihydroborete (70) and some of its derivatives are neutral homoaromatics. This electron-deficient system is isoelectronic with the mono-homocyclopropenium cation (17). It has been investigated by both theory and experiment, and its classification as homoaromatic is justified. In addition to 70, Childs, Cremer, and Elia propose that certain semibullvalenes, barbaralanes, bridged an-nulenes, cycloheptatrienes, and norcaradienes are homoaromatic. They draw particular attention to the semibullvalenes 71-75 and consider that 74 and 75 are the best candidates for experimental work. 

Obviously, whether the cycloheptatrienes should be classified as homoaromatic or not is still being debated. In this reviewer s opinion, cycloheptatriene (13) enjoys very weak homoaromatic stabilization which can be easily overwhelmed by other factors. 

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