And cyclopropylmethyl cations

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

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. 

Fig. 3. Qualitative energy profiles, energy E) vs. reaction coordinate (RC) for the rearrangement of parent bieyclobutonium ions 1 and cyclopropylmethyl cations 2, and of 1-methylbicyclobutonium ions 3 and (r-methylcyclopropyl)metbyl cations 4.

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. 

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... 

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... 

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... 

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. 

No reactions of t with protic solvents have been reported however, its cyclic analogue 1,2-diphenylcyclobutene (7) reacts with the protic solvents methanol, acetic acid, and water, to yield adducts 85 and 86 (eq. 28). The proposed mechanism for the formation of 85 and 86 involves the formation of singlet exciplex followed by proton transfer to yield a cyclobutyl cation 87. Stereoselective nucleophilic capture of 87 by solvent from its less hindered side yields 85, while skeletal rearrangement of 87 yields the cyclopropylmethyl cation 88, which reacts with solvent to yield 86 ... 

The first cyclopropylmethyl cation directly observed was the tricyclopropyl-methyl cation 60 by Deno et al.197 Its H NMR spectrum in H2S04 consists of a single sharp line at 8 H 2.26.200 In the 300-MHz 1H NMR spectrum in SbF5-S02C1F solution, however, the methine and methylene protons are well-resolved201 (Figure 3.9). 

The cyclopropylmethyl cation has recently been generated in the gas phase from both homoallyl chloride and cyclopropylmethyl chloride and studied using collisional activated dissociation mass spectrometry.216 Interestingly, cyclobutyl chloride, which yields the cyclopropylmethyl cation in the condensed phase, gives a substantial amount of the bicyclobutonium ion in the gas phase. 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity), primary cyclopropylmethyl cations show completely o-delocalized nonclassical carbonium ion character (see Section 3.5.2.5). Also, some of the secondary cyclopropylmethyl cations undergo rapid degenerate equilibrium (see later discussion). 

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. 

---