Bent cyclopropyl cations

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. 

The cation (13), prepared from ll-methyl-ll-bromotricyclo[4.4.1.0 ]-undecane in SbF5-S02ClF at — 120°C, is neither a symmetrical cyclopropyl cation nor an allylic cation, but is best considered as a bent cyclopropyl cation showing significant 2ir-homoaromatic nature. An excellent review, of the concept and experimental evidence for homoaromaticity includes sections on mono-, bis-, and tris-homocyclopropenium cations. ... 

This increased stability has been explained between the bent orbitals of cyclopropyl rings and the vacant p orbital of the cation carbon. The vacant p orbital lies parallel to C2-C3 bond of the cyclopropane ring and not perpendicular to it. Thus the geometry becomes similar to that of a cyclopropane ring conjugated with an oblefinic bond. 

Molecular orbital calculations using 4-3IG and 6-3IG basis sets led to the conclusion that the minimum energy structure for C4Ht was the bisected cyclopropylcarbinyl cation 61 (Table 12) with a plane of symmetry defined by the formally cationic carbonA second structure (64) was also found as an energy minimum that was subtly different from 61, but possessed an unsymmetrical cyclopropyl ring with one corner bent toward the... 

The methyl cation is increasingly stabilized if it is substituted by one, two or three cyclopropyl groups. This stabilization is a result of hyperconjugation between the bent orbitals of the 2,3- and 2,4-(r-bonds in the three-membered rings (7.132 A-7.132 C) (see X-ray structures of Childs et al., 1990, and earlier literature cited there) and the C(l)-atoms. This is the reason for the greater reactivity of cyclopropylmethyl derivatives. 

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