Cyclopropylmethyl carbocations

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... 

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. 

Bicyclobutonium ions, are bridged carbocations with a pentacoordinated y-carbon which were first discussed as short lived intermediates in the solvolysis reaction of cyclopropylmethyl and cyclobutyl compounds (56, 57, 58, 59, 60, 61). 

It should be noted that analogous secondary a-methylcyclopropylmethyl cations are observable species in superacidic media at low temperatures.21 Observation of the secondary cyclopropylmethyl carbocations under superacidic conditions indicates the superior charge delocalization into the cyclopropyl ring as compared to the cyclobutyl ring. 

The cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Scheme9.15. Reactivity of cyclopropylmethyl carbanions, radicals, and carbocations [55-59],...

Keating and Skell" and later Laurent and Thomalla" have found that a similar mixture of products was obtained in the anodic oxidation of either cyclobutanecarboxylic acid, allylacetic or cyclopropaneacetic acid. They accounted for these results by a rapid equilibria among the carbocations initially formed. A similar observation is that in the anodic oxidation of the corresponding iodo reactants in acetonitrile N-(cyclopropylmethyl)acetamide was found in each product mixture". It is noteworthy that the same identity in products also holds for the chemical reactions of solvolysis and... 

Such a homoallylic carbocation may rearrange to either a cyclopropylmethyl or a cyclobutyl carbocation, which leads to the formation of cyclopropane or cyclobutane derivatives. In addition, elimination or direct substitution of the homoallylic system leads to considerable amounts of acyclic alkenes. Therefore, this rearrangement has found little synthetic application as mixtures of products are generally obtained. ... 

The striking influence of y-substitution on product formation is readily explained in terms of the relative charge densities at the cyclopropylmethyl and homoallylmethyl centers in the incipient nonclassical carbocation intermediates. Starting from y-di- and monosubstituted homoallylic iodides such carbocations do have considerable cyclopropyl carbocationic character, which, as a consequence, leads to cyclopropylmethyl derivatives on substitution. In the nonsubstituted case, the intermediate carbocation has predominant homoallylic character which under these conditions results in elimination. 

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