Allyl cations cyclopropylmethyl

One case where C—C bonds are exceptionally effective in hyperconjugation is in the stabilisation provided by a cyclopropyl substituent to an empty p orbital. The cyclopropylmethyl cation is actually better stabilised than an allyl cation, as judged by the more rapid ionisation of cyclopropylmethyl chloride 2.19 than of crotyl chloride 2.20. In this case, hyperconjugation appears, unusually, to be better than n conjugation. 

Very recently Deno et al. (88,89) have established the existence of stable allylic and cyclopropylmethyl cations. Simple protonation of the olefin was proved by recovery of olefin a cryoscopic i-factor of 2.08 and by NMR, ultraviolet, and infrared spectra. The cation shown below survives 24-hr. exposure to concentrated sulfuric acid unscathed. 

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. 

The cyclopropylmethyl cation 519 can be generated from allylic, cyclobutyl, and cyclopropylmethyl precursors (Scheme 3.22). The 1H NMR spectrum is shown in Figure 3.24. 

The intermolecular reaction of oxocarbenium ions with simple alkenes, less reactive carbon nucleophiles, is generally slow and inefficient. A recent paper has described that the (la)-promoted intermolecular reaction of acetals with vinylcyclopropane (22) gives 3,6-heptadienyl ethers (23) with high E selectivity [74]. The allylation mechanism involves the formation of a cyclopropylmethyl cation intermediate and its ring opening (Scheme 9.21). Homoallylstannane (24) also adds to the oxocarbenium ion generated from benzaldehyde dimethyl acetal and (la) [75]. 

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