Cyclopropylmethyl cation

Cyclopropylmethyl cations are even more stable than the benzyl type. Compound 7 has been prepared by solution of the corresponding alcohol in 96% H2S04. Compounds 5, 6, and similar ions have been prepared by solution of the alcohols in FSO3H—SO2—SbFf." This special stability, which increases with each additional cyclopropyl group, is a result of... 

In contrast to the stability of cyclopropylmethyl cations (p. 222), the cyclopropyl group exerts only a weak stabilizing effect on an adjacent carbanionic carbon. ... 

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

Scheme 3 Rearrangement of [C4H6R] + cations, for R = H a threefold degenerate interconversion of bicyclobutonium ion (54) and cyclopropylmethyl cations (55).

In this connection the case of cyclopropylalkyl cation and substituted cyclopropyl cations is interesting. Cyclopropylmethyl cation has been found to be more stable than benzyl cation and the stability increases with each addition of cyclopropyl groups. Thus... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

All attempts of preparing the parent secondary dication (63), a t -tiicyclo(5.1.0.0 ) octa-2,6-diyl dication, were unsuccessful. The dication was originally anticipated to undergo the circumambulatory rearrangement, as in cyclopropylmethyl cation rearrangements However, it spontaneously rearranged into the thermodynamically more stable homotropylium cation (64). 

Over the past twenty years, the ring enlargement of cyclopropylmethyl cationic compounds has reached maturity and is now a major route for the construction of four-membered rings. 

Cyclopropylmethyl cations 1-5 may be generated from hydrocarbons through electrophilic cleavage of strained r-bonds or electrophilic attack on appropriately located ji-bonds and rearranged to cyclobutanes or cyclobutenes. 

This rearrangement, which accounts for the scrambling, is completely stereospecific.166 The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropylmethyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 55 or on the cyclobutyl cation intermediate.165 A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

For a summary of additional evidence for ihc symmetrical nature of cyclopropylmethyl cations, see Wibcrg Hess Ashe, Ref. 158. pp. 1300-1303. 

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