Cycloalkenyl cation

Cyclic alcohols exhibit the simplest behaviour. Cyclic aliphatic alcohols and olefins immediately form alkyl cations which can abstract hydride from their equilibrium olefins giving the corresponding hydrocarbon and the corresponding cycloalkenyl cation (equations 11 and 12). 

Studied by Olah (Olah and Liang, 1972 and Olah et al., 1972a) were chosen—the methyl substituted cycloalkenyl cations, represented hy solid dots, and the substituted aryldimethylcarbonium ions, represented by open circles. The correlation to a line of slope 0 0146 is excellent for all but two of the points. If the Spiesecke-Schneider value of 160 p.p.m. is used for the linear charge dependence of the carbon-13 shifts, then the calculated value for the methyl shifts is... 

Sorensen and co-workers799-801 have studied the fate of observable camphene hydrocation 190 prepared from isoborneol 187, camphene 188, or tricyclene 189 in HSO3F acid medium (Scheme 5.74). The intermediate cycloalkenyl cation 191 can also be prepared by protonation of a-terpineol (192), sabine (193), and /3-pinene (194) (Scheme 5.75). 

Figure 10. Correlation of proton chemical shifts of methyl groups in carbonium ions with carbon-13 shifts of the adjacent trigonal carbon atoms. See Table 3 for data and references, o Aryldimethylcarbonium ions cycloalkenyl cations phcnylmethyl-carbonium ions.

The 13C—NMR spectra of most alkenyl and cycloalkenyl cations (Table 4) indicate little 1.3-interaction48. The deshielding of C—2 relative to reference olefins... 

Table 4. 13C-NMR data for some alkenyl and cycloalkenyl cations (6 in ppm from TMS)...

Bicyclic secondary alcohols open to cycloalkenyl cations. For example, fenchol (VIII) produces l-isopropyl-3-methylcyclohexenyl cation (IX) in over 90% yield, and the same cation forms from borneol and camphene, but in lower yield (Deno and Houser, 1963). 

These ring openings to cycloalkenyl cations do not involve oxidation-reduction. 

Bicyclic tertiary alcohols, like bicyclic secondary alcohols, generally ring-open to cycloalkenyl cations. 

The resultant cycloalkenyl carbenium ions, especially the cyclopentenyl cations, are very stable (103,104) and can even be observed as free cations in zeolites 105,106). These ions can oligomerize further and, within zeolites, irreversibly block the acidic hydroxyl groups. With liquid acids, the oligomers will dilute the acid and thus lower its acid strength. 

The convincing corroboration of Winstein s correct concept as to the monohomoaromatic structure of this type of ions are the data of the NMR spectra, in particular for the unsubstituted cyclobutenyl cation 454 - ) in the latter case the signals of C and resonate in a higher field than that of C does contrary to what is observed in the spectrum of cycloalkenylic ions, but in full accord with the concept of 1,3-orbital interaction. The PMR spectra show l a reversible temperatiu e dependence of the methyl proton signal shape which was atrributed to the nonplanar ion inversion with the intermediate formation of a planar cation. The barrier value of this inversion determined experimentally (AG = 8.4 kcal/mole) agrees... 

Facile cycloalkenylations of carbonyl groups have been carried out with cyclopropylphosphonium fluoroborates . Complex carbocyclic systems, such as the sesquiterpene a-cedrene, can be effectively constructed by cationic cyclization Epoxide cleavage which follows upon dissolving metal reduction of proximal cyclopropane rings makes possible the ready synthesis of functionalized strained ring compounds inaccessible by other methods... 

The cycloalkenyls (25) and (26) can be obtained by proton abstraction from the cationic cyclopentene (31) and cyclohexene (32) complexes, respectively. However, attempted deprotonation of the corresponding cycloheptene... 

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