Cyclopropyl carbinyl cation

Aziridinocyclopropanes 163 derived from 2-phenylsulfonyl-l,3-dienes undergo BF3-induced rearrangement to bicyclic amines 165, which feature the skeleton of the tropane alkaloids. The reaction proceeds via cyclopropyl carbinyl cation 164, an intermediate also invoked in the analogous epoxide rearrangements. Trapping by fluoride ion is a competing pathway <96TL3371>. 

The unusual nature of the cyclopropyl carbinyl cation allows yet another mode of attack to form cyclobutane products. Because this mode of attack releases little strain, normally some special structural features are required to direct the reaction along this pathway. 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

Cyclopropyl carbinyl cation1". The most dramatic example for a 7i-acceptor/7i-attractor substituent effect is provided by the cyclopropyl carbinyl cation. In its bisected conformation, the vicinal CC bonds are considerably longer (1.65 A) and the distal CC bond considerably shorter (1.41 A) than the bonds in 1 (Table 13). The CH2+ group stabilizes 1 considerably more in the bisected form than in the perpendicular form (13 kcal mol1). Since the CC+ bond possesses partial double bond character, it is shorter (1.35-1.36 A, Table 13) than a normal CC+ bond (1.51 A). 

Alkylation reactions which introduce a double bond adjacent to the cyclopropane ring provide intermediates which may undergo useful rearrangements to the cyclobutanone system. As shown in Scheme 20, reactions of 1-vinylcyclopropanol (108) with acid, positive halogens, peracids or carbonium ions lead to cyclobutanones via the cyclopropyl carbinyl cation 109. 76>... 

A new sesquiterpenoid skeleton has been discovered in the form of lactarorufin A (193) isolated from Lactarius rufus. At the present time only the relative stereochemistry of the four asymmetric centres is known and it has been suggested that its biogenesis can be considered in terms of opening of the cyclopropyl-carbinyl cation (194) which has previously been proposed in the marasmic acid biogenesis. In another study of Lactarius species, Magnusson et have... 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

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