Norcaradienes homoconjugation

When vinylcyclopropane is used as the key reference compound, the only examples of significant bond (electron) delocalization energies are cyclic homoconjugative systems with potentially aromatic electron ensembles (compare compounds 27-38 in Table 2). For example, norcaradienes 33 and 34 possess small, but significant bond delocalization energies RE of about 3 kcal mol1 (Table 2). Similarly, the RE values of the cyclohepta-trienes 30,31 and 32 are between 4 and 6 kcal moT1. 

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. 

FIGURE 14. CC bond orders n and bond ellipticities i of cyclopropyl homoconjugated molecules (a) norcaradiene, (b) bicyclo[2.1.0]pentene, (c) bicyclo[3.1. OJhexenyl cation. On the right, the preferred mode of electron delocalization is indicated by dashed lines. Also given is the number of delocalized electrons as calculated from topological bond orders. See text... 

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... 

Perhaps the most interesting mechanism stabilizing radical cations of type B involves homoconjugation. The interaction with the butadiene frontier molecular orbital (FMO) can lift the degeneracy of the cyclopropane in-plane e orbitals S,A) and favor the type B structure. This principle was shown to stabilize substrates such as norcaradiene and derivatives [110, 131] and, to a lesser extent. 

Figure 19. Structures of norcaradiene, 16, and two derivatives, 17,18, and schematic illustration of homoconjugation between the frontier molecular orbitals of butadiene (bottom left) and styrene (bottom right), respectively, with the anti-symmetrical cyclopropane HOMO.

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