Cyclopentyl carbocation

Calculated structure of the cyclopentyl carbocation. (Reproduced from reference 229.)... 

Among 2° carbocations, only the isopropyl, sec-butyl, and cyclopentyl carbocations have been observed in solution. These three structures nevertheless illustrate the kinds of delocalization effects that may be expected in other 2° carbocations. The cyclic structure of the cyclopentyl cation prevents significant C-C-C bridging. The calculated geometry (Figure 5.43) shows a twisted C2 structure with the C -H bonds aligned parallel with the empty p orbital for maximum hyperconjugative overlap. ... 

Equilibration of protons due to rapid rearrangement in cyclopentyl carbocation (68). 

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 cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Another method by which a carbocation can be generated is by protonation of either an alkene or a carbonyl group. Thus, cyclobutylmethyl cations are formed by treatment of a ketone or an alkene with acid the cyclobutylmethyl cations subsequently rearrange to give cyclopentyl cations. 

A recent X-ray characterization of the cyclopentyl cation 33 generated with methyl carboranes has shown a perfect planar structure around the carbocation center [bond angles CH3-C+-CH2= 124.9 and 125.2°, CH2-C+-CH2= 109.9° bond distances CH3-C+ = 1.46 A, CH2-C+ = 1.45 A).123... 

Applying the additivity of chemical shift analysis55 to the 2-norbomyl cation also supports the bridged nature of the ion. A chemical shift difference of 168 ppm is observed between the ion (C7Hn+) and its parent hydrocarbon [i.e., norbornane (507)], whereas ordinary trivalent carbocations such as the cyclopentyl cation (33) reveal a chemical shift difference of =360 ppm.55... 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

However, not everyone was convinced by the existence of the non-classical carbocation. H. C. Brown 1977 pointed out that the norbornyl compounds are compared with cyclopentyl rather than with cyclohexyl analogues, 2.21 (eclipsing strain), and in such a comparison the endo-isomev is abnormally slow, the exo-isomer being only 14 times faster than cyclopentyl analogues. He also pointed out that the formation of racemic product is due to two rapidly equilibrating classical carbocation species (Scheme 2.17). The interconversion of enantiomeric classical carbocation species must be very rapid on the reaction timescale. 

Photoheterolysis of benzylic chlorides [204] yielded results signifying that simple benzyl cations, such as cumyl and 1-phenylethyl cations, can exist in the solution as free ions radicals arising from a competing photohomolysis are also observed frequently. Haloalkyl-carbocations are studied by heterolysis of the corresponding dihalides in super acid media [205]. NMR chemical shifts are interpreted as evidence for an interaction between the vacant orbital of cationic center of the haloalkyl carbocations with a lone electron pair of the halogen atom. 3-chloro-l-methylcyclopentyl cation 73, thermally eliminates hydrogen chloride and yields l-methyl-2-cyclopentyl cation 74, a similar behavior reported for y-chloroalkyl carbocations [206] (Scheme 5). 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

Within —50 to —130 °C there are 3 PMRsignals with an intensity ratio of 4 1 6. This points to the freezing of 3,2-hydride shifts (E = 10.8 0.6 kcal/mole A = 10 - s ). Judging from these data the 3,2-hydride shift rate in a stable 2-norbomyl cation is abnormally low compared with 1,2-hydride shifts in secondary carbocations. Thus the respective activation energies are 5 kcal/mole for the 1,2-hydride shift in the cyclopentyl cation and 10.8 kcal/mole for the 3,2-hydride shift for the 2-norbornyl cation. This corresponds to the rate ratio 10 at —150 °C and 10 at 25 °C. Olah has studied the models of both ions showing that torsional and non-... 

ESC A spectra-. Olah has shown that while in the spectrum of the 2-norbomyl cation the signal of the cation centre is separated from those of the other carbons by a barrier (AEb) of 1.5 eV, for the cyclopentyl cation it amounts to 4.3 eV this is due to far larger charge localization in classical carbocations. Kramer maintains that the cyclopentyl cation is an erroneous model to compare with the 2-norbomyl cation. However, in the spectrum of the 2-methylnorbornyl cation (which can obviously raise no objections as a model compound) the value of AE is 3.7 eV, i.e. again much higher than for the 2-norbomyl cation. 

Referring carbocations to classical or nonclassical on the basis of the value of AS ( C) it is also useful to compare these values for the secondary ion in question and for the corresponding methyl-substituted tertiary ion. The comparison of classical secondary and tertiary ions shows small differences (for isopropyl and tert-butyl the difference is —4 ppm, for cyclopentyl and methylcyclopentyl it is —10 ppm), for nonclassical ions these differences are far larger 2-norbornyl — 2-methylnorbomyl, — 129 2-bicyclo[2,l,l]hexyl — 2-methyl-2-bicyclo[2,l,l]hexyl, —97 ppm etc. 

One very interesting structure that might at first glance appear to be a 1° carbocation is the cyclopropylmethyl cation. Calculations indicated, however, that this species is best described by a pair of rapidly equilibrating nonclassical structures (62 and 63, Figure 5.46) that are approximately equal in energyAttempts to prepare the 1° cyclobutylmethyl cation were unsuccessful, however, with cyclopentyl cations being formed instead. ... 

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