Cycloalkyl cations

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

The two chair conformations of tertiary methylcyclohexyl cation 43 and 44 are in rapid equilibrium. 

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Electron density methods such as GIAO-DFT methods require much less computational resources in terms of cpu time, memory and disk space compared to wave-function methods such as GIAO-MP2. A systematic study of a set of 16 alkyl- and cycloalkyl cation (Scheme 1) was performed to investigate the performance of GIAO-B3LYP methods for prediction of 13C NMR chemical shifts for these types of carbocations.37... 

Scheme 1 A systematic GIAO-B3LYP NMR study of 16 alkyl- and cycloalkyl cations.

Cyclic enediynes, see under Enediynes Cycloalkyl cations, NMR spectroscopy, 142-144... 

Tertiary cycloalkyl cations, such as the 1-methylcyclopent-l-yl cation 28, show high stability in strong acid solutions. This ion can be obtained from a variety of precursors (Figure 3.7).143,144 It is noteworthy to mention that not only cyclopentyl- but also cyclohexyl-type precursors give 1 -methylcyclopent-1 -yl cation 28. This indicates that the cyclopentyl cation has higher stability, which causes isomerization of the secondary cyclohexyl cation to the tertiary methylcyclopentyl ion. 

Sorensen and co-workers150 have prepared tertiary cycloalkyl cations of different ring sizes, n = 4 (small ring), n = 5-7 (common rings), n = 8-11 (medium rings),... 

A series of aryl-substituted cycloalkyl cations 38 and 39 and other aryl-substituted cyclic systems (40,41,42) have been studied in connection with the application of the tool of increasing electron demand.67,153-156... 

The rearrangement of [13] bears a close resemblance to the transannular reactions observed in medium sized rings that have been reviewed by Prelog and Traynham (1963) and Cope et al. (1966). Recently Sorensen and coworkers have studied medium sized cycloalkyl cations under stable ion conditions in non-nucleophilic media and demonstrated that their structures are ji.-hydrido-bridged. The bonding situation in these ions contrasts sharply with that in the ions described above and rather corresponds to that of transition states (or intermediates) for intramolecular hydride transfer in these ions. 

The correspondence in the spectroscopic properties of 102 with those of 101 suggests that the parent ion (101) can also be best presented in the same way. The 1-ethyl and 1-propyl analogs of 102 are similarly nonclassical but rearrange irreversibly upon warming to cycloalkyl cations. ... 

In contrast to the protonation of the disubstituted alkenes mentioned above, high ktranJkcis ratios of 9 x 108 and 3 x 103, respectively have been observed in the acid-catalyzed addition of methanol to the trans- and cis-isomers of cycloheptene (39a and 38a) and cyclooctene (37a and 36a) (114a). The rate constants reflect partially the release of strain in the transformation of the cyclic olefins to the appropriate cycloalkyl cations. Comparison of the relative activation energies for these addition reactions with the difference of strain release leads to the estimate that the response to strain effects is about 60%. In a more recent study of the acid-catalyzed hydration of cis- (36b) and trans- 1-methylcyclooctene (37c), it was concluded that two conformationally different 1-methyl-carbocationic intermediates are... 

The p-hydrido bridged cation [90] is equivalent to a resonance system such as exists in allyl cations (single energy minimum) and can be distinguished from a set of two degenerate cycloalkyl cations [88] undergoing a fast hydride shift (double energy minimum) as shown in (70) by comparison with suitable model cations. 

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