Non-classical ions

By contrast with the polymerization of propene, there seems little scope here for the formation of branched structures. Detailed elucidation of the polymer structure would provide evidence on the question whether a non-classical ion can act as chain-carrier. 

It was found that Cope rearrangement of the structurally rigid tetracyclic molecule 506 is remarkably accelerated by creating a remote (i.e. non-conjugated) carbenium ion center by an ionization of a ketal group (equation 191)249. The possibility of both classical and non-classical ion participation in this Cope rearrangement was revealed by using MNDO calculations. 

The non-classical ion may exist as edge or face protonated nortricyclene, represented by stmcture [9], as an alternative. This species is not a resonance stmcture of [7] or [8]. If it is a discrete intermediate, one might expect to be able to detect the displaced proton via a suitably designed exchange experiment. 

The extent of retention of optical purity differentiates Corey s results from those of Berson and Remanick. Corey found similar optical purity in the products of both epimers, from which it was concluded that a common ion was an intermediate in both reactions. Since optical purity could not be retained, if the symmetrical non-classical ion was an intermediate or transition state in the reaction, he concluded that deamination led to the classical cation from both the exo- and endo-reactants. 

For the non-classical ion an energy minimum was deduced with a bond length of 1-40 A between C-1 and C-2 and 1-837 A between C-1 and C-6. The energy was —268-04377 Hartrees or about 4-3 kcal mole higher than the classical structure. 

The C-ls x-ray electron spectroscopic spectra of the norbornyl ion has recently been reported (Olah et al., 1972). In principle, ESCA appears to provide an ideal method to decide upon the ion s structure, as one would expect a classical ion to exhibit two peaks with an area ratio of 6 1 and the non-classical ion to show two peaks with a 5 2 ratio. 

The norbomyl spectrum shows a much smaller shift, estimated as 1 7 eV, and this, together with the fact that the area ratio was reported as 5 2, suggested that the non-classical ion was being observed. 

This decision, however, appears to be based on a miscalculation. A closer inspection of the published spectra leads to the conclusion that the area ratio is much closer to 6 1 than 5 2 and therefore it cannot represent the non-classical ion. Thus, if the spectrum is that of a norbomyl cation it must be that of the classical ion and would therefore be consistent with the implications of the exchange study just described. 

While the area ratio apparently eliminates the non-classical ion, one must ask whether the small binding energy shift observed can be accounted for if the ion is classical. This might be possible because the shift is dependent on a large number of factors which differ... 

In Table 6, J( C—H) spin coupling constants are reported for the norbomyl ion. It has been noted (Olah et al., 1970) that the distinction between the classical and non-classical ion cannot be... 

These spectral changes are consistent with either (a) freezing out a stable non-classical ion, or (b) the presence of the classical ion in which the 6,2-hydride shift has been slowed down, but the Wagner-Meerwein rearrangement is still exceedingly rapid. If the ion is non-classical, the spectrum indicates that it must be a comer-protonated nortricyclene rather than the edge-protonated ion because the latter would have split into a 1 2 1 pattern. 

The proposal that the —154° spectra is consistent with the non-classical ion suffers in our view from results with the 1,2-dimethylnorbomyl cation (Olah et al., 1971). This cation was found... 

On the other hand, if the non-classical ion is a stable intermediate, the transition state for the 3,2 hydride shift requires a subst mtial reorganization, including the cleavage of the cyclopropyl ring, and, by analogy with unimolecular gas phase processes, a much higher pre-exponential factor might be expected. [In the cyclopropane-propylene reaction log A is 1ST 7 (Chambers and Kistiakowsky, 1954).] Contrary to expectation, the observed pre-exponential for the 3,2-shift is actually a little lower than for the 1,2,6-equilibration process. 

Inspection of strong acid solutions in which the ion may be stabilized by spectral and exchange probes fail, in our opinion, to establish the presence of a protonated cyclopropyl ring and therefore do not support the presence of the non-classical ion. The proton... 

Other noteworthy achievements of solid-state 13C CP/MAS NMR in the context of carbonium ions are that (1) the sec-butylcarbonium ion can be identified at low temperatures in a sec-butyl chloride/antimony pentafluoride matrix in the temperature range 80-190 K (2) the norbornyl carbonium ion has been characterized (356,396) at temperatures down to 5 K, there being a strong (but not yet incontrovertible) indication that the controversial non-classical ion (398) exists and (3) the homotropylium ion is best represented (399) by the completely delocalized (homoaromatic) seven-membered state (a below) rather than the incompletely delocalized state (b). 

H. C. Brown, The Non-Classical Ion Problem, with comments by P. v. R. Schleyer, Plenum,... 

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