Cyclopropylcarbinyl cations formation

The 2-aryl substituted cyclopropylcarbinyl cations have partial homoallylic character, whose contribution to the resonance hybrid increases when strong electron-withdrawing substituents (e.g. phenyl) are attached at the C2. Thus, 3-arylcyclobutyl tosylates on acetolysis give the homoallylic acetates predominantly, through the intermediate formation of the 2-arylcyclopropylcarbinyl cations (equation 21). 

The parent 3-homonortricyclyl cation [147] underwent three-fold degenerate rearrangements in superacids, as shown by its temperature dependent nmr spectra, but only at higher temperatures (—85°C to 20 C) than for the corresponding dehydrohomoadamantyl [144 R = H] and dehydroadamantyl cations [126 X = H], The lower rearrangement rate of [147] was explained by a less favourable formation of the puckered cyclobutyl cation intermediates [159] in this geometrically more constrained system. The assignment of a symmetrical cyclopropylcarbinyl cationic structure to [147] was confirmed by comparison of its and C-nmr spectra with the static counterparts [157]. 

In ionic polymerization, Equation 17, it can be assumed that there is a first stage comprising formation of an alkylbicyclobutonium ion, very similar to the cyclopropylcarbinyl cation studied by Roberts and Mazur (25). The unusual alkylbicyclobutonium ion is considered as a resonant hybrid of pyramidal structure that interconverts at different rates. The... 

From the above considerations, it follows that the observed formation of cyclobutanol (cyclobutylamine) and cyclopropylcarbinol (cyclopropyl-carbinylamine) requires that cyclobutyl and cyclopropylcarbinyl cations, or at least some cyclic precursor of these ions, such as a bicyclobutonium ion, must exist in the dilute gas state at least for the time necessary to encounter a molecule of nucleophile, with lifetimes in excess of 10 s. Accordingly, cyclic C4H7 ions must be regarded as fully legitimate ionic intermediates, characterized by significant minima on the C4H7" ... 

By the ab initio method Hehre and Hiberty have calculated different conformations of the homoallylic ions C H. and the isomeric bisected cyclopropylcarbinyl cation to show all the homoallylic ions I-IV being energetically less stable than ion V by 20-30 kcal/mole. The solvolysis products whose composition points to the intermediate formation of homoallylic rations can 1% attribute either to the fast trapping of unstable ions by counterions or to the change of the stability ratio of the isomeric ions 4 by solvation effects. 

The cyclopropane ring in both epimers occupies the same position with respect to the departing group both epimers form a stable bisected cyclopropylcarbinyl cation. The exo endo rate ratio can reflect the contribution ratio of steric factors on the formation of the unsubstituted 2-benzonorbomenyl cation out of the two epimers. The hydrolysis of compounds 269 and 270 has resulted in an exo endo rate ratio of 12 and agrees fairly well with the previous value. A comparison of these data with the value of 15000 for the unsubstituted compounds clearly shows the steric and electrostatic effects to have a low value for secondary systems and in the absence of the ring 7i-participation the exo-isomer solvolyzes faster than its endo epimer only 2- to 10-fold. 

The data is consistent with the initial formation of a cyclopropylcarbinyl cation which then undergoes rapid ring-opening to the allylcarbinyl cation from which the products are derived. 

It is of interest to note that artemisia alcohol (18) produced in the hydrolysis of 16-OPy I" is essentially completely racemic (>98%) (57). Apparently (18) is formed by nucleophilic capture of the acyclic allylic carbonium ion (29) rather than direct attack in the 3 position of the chrysanthemyl carbonium ion (28). Nucleophilic substitution upon cyclopropylcarbinyl cations to give homoallyl products occurs with inversion of configuration 74—75). In the case of (28), however, position 3 is highly hindered by the adjacent gem dimethyl groups thus collapse to the allylicly stabilized (29) is faster than direct substitution. The formation of a small amount of cw-chrysanthemol (27, 0.25%) is taken to indicate that allylic ion (29) recyclizes, at least in part, back to (28) and its cis isomer (30). 

Carbocations on Surfaces Formation of Bicyclobutonium Cation via Ionization of Cyclopropylcarbinyl Chloride over NaY Zeolite... 

Recent progress in preparation and study of alkylated fullerene cations RC6o+ and RC o+ as long-lived species are examined by T. Kitagawa in Chapter 12. Chapter 13 by C. J. A. Mota and co-workers examines the formation of the bicyclobutonium cation via cyclopropylcarbinyl chloride over solid acid catalysts. 

The rearrangement of the cyclopropylcarbinyl chloride in solution is well known in the literature (//). In polar solvents three products, arisen from the nucleophilic substitution of the solvent to the chloride, are usually detected, which are formed via nucleophilic substitution of chloride by solvent. This chemistry can be explained by the formation of the bicyclobutonium cation (C4H7+), which acts as a tridentated ion, generating the three products shown in scheme 3. 

When a gaseous flow of cyclopropylcarbinyl chloride is passed over NaY zeolite at room temperature, formation of cyclobutyl chloride and allylcarbinyl chloride was observed (scheme 4), as well as cyclopropylcarbinyl chloride (product and unreacted starting material). These data are consistent with formation of the C4H7+ cation with internal return of the chloride ion. 

These results are consistent with ionization of the cyclopropylcarbinyl chloride on the zeolite, with formation of the C4H7+ cation. Attack of the chloride ion (internal return) might then occur at the three possible positions, giving the rearranged alkyl chlorides. This hypothesis was supported by the data obtained with impregnation of the NaBr on the NaY zeolite. The observation of the three alkylbromides is consistent with a mechanism involving ionization and attack of the external bromide nucleophile. 

Rearrangement of the cyclopropylcarbinyl chloride takes place over NaY zeolite, indicative of the formation of the bicyclobutonium cation. Theoretical calculations show that the bicyclobutonium is an intermediate on the zeolite surface and might be in equilibrium with the alkyl-aluminumsilyl oxonium ion. 

The results of cyclopropylcarbinyl chloride rearrangement over NaY impregnated with NaBr suggest that there is an equilibrium between the bicyclobutonium cation and the alkyl-aluminumsilyl oxonium ion, explaining the preferred formation of the allylcarbinyl bromide in the rearranged products. It also suggests that zeolites may act as solid solvents, providing unsymmetrical solvation for the ions inside the cavities. 

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