Norcaradienes homoaromaticity

This is a quantitative definition of homoaromaticity that is generally applicable and helps to specify exactly the point Rb in Figure 3, at which cyclopropyl homoconjugation starts. However, this definition is much more stringent than Winstein s definition because it excludes all those systems with 1,3-interactions that do not lead to a bond path (no-bond homoaromaticity). Hence, it describes homoaromaticity only for the case of cyclopropyl homoconjugation. For example, Kraka and Cremer have used this approach to describe cyclopropyl homoconjugation in norcaradiene (10)27 54. 

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. 

Kinetic studies on the decarbonylation of 113 and 114 are of interest in terms of the homoaromatic character of 105/106. It has been reported that the rate of decarbonylation of 113 was 1 x 10s times greater than that of 114247. It was suggested that this reactivity difference was due to partial opening of the cyclopropane in the transition state and overlap between the Walsh orbitals of the cyclopropane and the rehybridizing s-orbitals of the breaking bonds. Homoaromatic electron delocalization in a norcaradiene/cyclohepta-triene-like transition state is only possible with the anti-isomer 113. 

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. 

In the case of 10, the strain of the six-membered ring is small, as is the strain energy due to ring annelation. Utilizing heats of formations for cw-1,3-butadiene, gauche-Vmy cyc o-propane" and norcaradiene ", a homoaromatic stabilization energy of 4 kcal mol is calculated in line with a description of 10 as a cyclopropyl homoaromatic 6n electron system. 

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. 

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