Cyclopropyl cations homoconjugated

The direct observation of ion 46 is of particular interest in that it clearly does not involve a significantly opened cyclopropane ring, which could lead to the formation of an allylic cation. Thus, it must be considered as a bent cyclopropyl cation.164-170 It is, however, clear that the Ci-C6 ct-bond must to some degree interact with the empty p orbital at Cn and that homoconjugation between them becomes the important factor in stabilizing such a bent cyclopropyl species. 

Cremer and coworkers investigated a number of potentially homoconjugated cyclopropyl compounds such as the monohomotropylium cation203- 205, the 1,4- and 1,3-bishomotropylium cation208, the trishomotropylium cation209, the barbaralyl cation210 and the cyclobutenyl cation211. All these cations have the choice between a closed cyclopropyl structure (la), an open cyclopolyenyl structure (Ic) and an intermediate structure (lb) as demonstrated in the case of the monohomotropylium cation. 

For all cations considered, 13C NMR spectra have been measured in solution while direct structural information on the question as to whether structure Ia, lb or Ic corresponds to a minimum energy form was completely missing. Therefore, the structural problem was solved by utilizing the ab mirio/IGLO/NMR method. Since results of this work will be reviewed in another chapter of this volume (see Chapter 7), we refrain from discussing calculated NMR data for potentially homoconjugated cyclopropyl compounds at this point. 

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... 

We start with an examination of some examples of acyclic systems in which there is evidence or the possibility of cyclopropyl homoconjugation. We then move on to a broader examination of homoaromatic systems, treating cationic, neutral and anionic systems in separate sections. The results of experimental work and theoretical examinations are integrated so as to provide a cohesive overview of each system. In order to limit the size of the chapter, we refrain from reviewing in detail systems such as the bridged annulenes and radical species. The chapter concludes with a reflective section that seeks to draw together theory with experiment and point out new directions for future work. 

Cation 71 has a bridge which links the cyclopropyl carbons and thus maintains an appropriate orientation for homoconjugation. While such a linkage is not formally required for homoaromatic delocalization, it is an open question as to whether examples of bishomotropenylium ions exist which lack such a framework. 

The trishomocyclopropenium species 91 has been an important cation in terms of the development of the concept of homoaromaticity. It also provides an example of a different mode of cyclopropyl homoconjugation to that encountered in the systems discussed thus far. In 91, the cyclopropane is formally interacting with the remote positive charge in an edge-on manner. All of the previous examples discussed have involved a cyclopropyl carbinyl-type interaction with the conjugating group being joined to the cyclopropane. 

Bicyclo[2.1.0]pentene, 116, can be considered to be the prototype of a neutral homoantiaromatic molecule. The types of structural and bonding effects found for this molecule parallel in many respects those found for the bicyclo[3.1.0]hexenyl cation reported above. Further studies on both of these 4q systems will likely be rewarding in terms of fully understanding the nature of cyclopropyl homoconjugation and homoantiaromaticity. 

Most of the work reported in this area is limited to carbocyclic systems. The recent developments with the boron analogues of the cyclobutenyl/homocyclopropenium and norbomenyl/norbornadienyl cations point to the potential importance of cyclopropyl homoconjugation and homoaromaticity in a much wider sphere of organic systems. This will likely be an area where there will be considerable further work. 

---