Cyclopentyl cation 1,2-hydride shift

It has been argued that the energy required for the 3,2 shift is substantially higher than a 2-4 kcal mole estimate for 1,2-hydride shifts around the cyclopentyl cation, the 7-9 kcal mole difference being attributed to stabilization of the norbomyl system by the formation of the comer protonated non-classical stmcture (Olah et al., 1969). 

Treatment of cyclopentane with FSOsH-SbFj in SO2CIF below —10°C yields the cyclopentyl cation [49] (Olah and Lukas, 1968). The H-nmr spectrum showed only one singlet at 8 4.75 which was unchanged down to —130°C. Rapid 1,2-hydride shifts around the ring (42) were suggested to be... 

The absence of 1,2-hydride shifts in [331] is in sharp contrast with the behaviour of related systems, e.g. the cyclopentyl cation [49]. This may be explained in terms of both steric and electronic factors. Strain makes the five membered ring planar and lengthens the C(l)-C(2) bond. This prevents a favourable geometry for hydrogen migration to the empty p-orbital. Extensive charge delocalization into the aromatic rings, as shown by the C-nmr spectrum, must also decrease the rate of the 1,2-hydride shift. 

Furthermore, Yannoni and coworkers succeeded in freezing out the degenerate hydride shift in the 34 cyclopentyl cation in the solid state at -203°C. The observed chemical shifts at 5 C 320.0,71.0, and 28.0 indicate the regular trivalent nature of the ion and are in good agreement with the estimated shifts in solution based on the average shift data. 

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-... 

We might expect that the isopropyl cation would be immune to the 1,2 hydride shift exhibited by the cyclopentyl cation, since a simple hydride shift would convert... 

The tabulation shows that the t-butyl system is a reasonable model for some equilibrating ions. It fails badly, however, when applied to the norbornyl compounds. The isopropyl system is a poor model for sec-butyl and cyclopentyl ions and is a very poor model for the norbornyl cation. The failure of the models to provide reasonable estimates of the shifts in the tertiary norbornyl cations which are undergoing either VVagner-Meerwein shifts or hydride migration makes it clear that the experimentad shifts in the secondary system cannot be used as structural proofs. Rather they should be regarded as fascinating results to be rationadized in terms of the structure, whatever it may be. 

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