Homoconjugation cyclopropyl

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. 

Since the terms homoconjugation, cyclopropyl-(homo)conjugation, homoaromaticity, etc. are used by different authors in different ways, a review on cyclopropyl homoconjugation requires some clarification of these basic terms. 

Situation 1 (42a) corresponds to cyclopropyl homoconjugation (cyclopropyl homoaromaticity), the subject of this review article. Situation 2 (42c) may be only interesting in... 

The largest contribution and variety in the family of polyenes is to be found in the group of bicyclic and polycyclic compounds. For this chapter we selected those compounds which represent the most important prototypes of different kinds of interaction, namely cyclopropyl-conjugation, spiroconjugation, hyperconjugation and homoconjugation. 

Cyclopropyl homoconjugation, homoaromaticity and homoantiaromaticity— Theoretical aspects and analysis... 

Cyclopropyl homoconjugation is a special case of homoconjugation as is indicated in Figure 1. Homoconjugation can be found in molecules 1 or 2, where X = CH2 and atoms... 

The chapter concludes with a reflective section that provides the link between this more theoretically oriented review on cyclopropyl homoconjugation and the following chapter, which will concentrate on specific examples of homoconjugation and homoaromaticity3. In addition, we will point out some directions for future work on cyclopropyl homoconjugation and homoconjugation in general. 

SCHEME 1. Difference between cyclopropyl conjugation and cyclopropyl homoconjugation... 

Cyclopropyl homoconjugation is a special case of homoconjugation. It will occur if X = CH2 and there are sufficiently strong interactions between the double bonds such that a bond is formed. 

In order to clarify the role of cyclopropyl homoconjugation within the concept of homo-conjugation, it is appropriate to classify the various types of homoconjugative interactions according to the following seven critera ... 

A further decrease of R will lead to the piont Rb (Figure 3) at which a weak bond is established between the interacting centres. At this point no bond homoconjugation turns into bond homoconjugation or, for the special case X = CH2, into cyclopropyl homoconjugation. 

Further reduction in R leads to the point Rc (Figure 3) at which the bond is fully formed and normal cyclopropyl substituent interactions occur. Homoconjugation ceases to be a relevant chemical factor at point Rc and the molecule can be adequately described in terms of a cyclopropyl substituted system. 

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