Rapidly equilibrating classical

The differentiation of bridged nonclassical from rapidly equilibrating classical carbocations based on NMR spectroscopy was difficult because NMR is a relatively slow physical method. We addressed this question in our work using estimated NMR shifts of the two structurally differing ions in comparison with model systems. Later, this task... 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

Emster (1922) could also be explained by the alternative proposal that two rapidly equilibrating classical ions [4] and [5] were the actual reaction intermediates. 

For example, acetolysis of exo-2-norbornyl brosylate 254 produces exclusively exo-2-norbornyl acetate 255. The exo-brosylate 254 is more reactive than the endo-brosylate 256 by a factor of 350 and the acetolysis of optically active exo-brosyl ate gave completely racemic exo-acetate 255. Thus, the carbonium ion produced from exo-254 is more rapidly (thus more easily) formed than that from endo-256. These results were originally rationalized in term of a bridged (nonclassical) cation 257 (Winstein approach) (97) or as the rapidly equilibrating classical carbonium ions 258 and 259 (Brown approach (98, 99)). 

Fig. 7.9 The classical/nonclassical 2-norbornyl cation problem. Grey a pair of rapidly equilibrating classical cations with a nonclassical, bridged transition structure black the nonclassical cation as the minimum cation. The fully MP2/6-31G(d) optimised 2-norbornyl cations are depicted the nonclassical ion is 13.6 kcal mofi1 more stable at this and comparable levels of theory.

Based upon solvolysis work with the labelled />-nitrobenzoate [416] Coates and Kirkpatrick (1968) concluded that the carbocation intermediate was either a rapidly equilibrating classical cation [417] or the nonclassical [31]. 

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. 

Considerable support for Roberts suggestion has come from later studies, notably those involving NMR studies in superacid media. Thus, while the spectrum of the a,a-dimethylcyclopropylcarbinyl cation is best rationalized in terms of a classical structure that for the parent system is consistent with neither static nor rapidly equilibrating classical structures The parent system seems best to be described as a mixture of the equilibrating bicyclobutonium ions and, less than 4 kcal mol higher in energy another structure that might be the classical cyclopropylcarbinyl cation ... 

Indeed, many examples are now known of such rapidly equilibrating carbocations under stable ion conditions (see Table 13.1, Ref.11)). The question to be resolved is whether the behavior of the 2-norbornyl cation under solvolytic conditions is best interpreted in terms of such a pair of rapidly equilibrating classical carbocations or ionpairs, or as the stabilized a-bridged species. 

In order to differentiate between these two possibilities the Raman and 13C NMR spectra of the ion were studied10. As Raman spectroscopy is a fast physical method (assuming that vibrational transition rates are faster than any of the hydrogen or alkyl shifts) and the question of possible equilibration versus bridged ion is unimportant. The previously discussed technique of average 13C NMR shifts should be also applicable in this case to differentiate a static nonclassical bridged ion from rapidly equilibrating classical carbenium ions. 

The deuterium isotopic perturbation technique developed by Saunders et al. is capable of providing a convenient and valuable means to differentiate between rapidly equilibrating classical trivalent and nonclassical carbocations containing hypercarbons. 

The PMR spectra give no possibility to chose between the nonclassical ion 5 and the rapid equilibrium of a pair of enantiomeric classical ions 6. The ion structure was therefore studied by Raman laser spectra, because the rates of vibrational transitions are far higher than those of Wagner-Meerwein rearrangements. The spectrum of the norbomyl cation proved to be very similar to that of nortricyclejie and quite different from that of norbomane. For rapidly equilibrated classical ions 6, the RS spectrum would be similar to that of norbornyl derivatives. Besides, the rapid equilibration, according to Olah, would lead to the doublet splitting of the bands of some skeletal deformational vibrations which is not observed. 

Another variant to identify the classical or the nonclassical nature of carbocations by NMR data has been suggested by Olah, Schleyer et al. They calculate the difference between the sum of the chemical shifts of all the carbons of an ion and that of all the carbons of the corresponding hydrocarbon formed by adding a hydride-ion. For static and rapidly equilibrated classical carbocations the difference is usually 350 ppm. For nonclassical ions this value is by hundreds of ppm less thus for the 2-norbornyl ion A6 is +175 ppm, for the 7-norbomenyl one —1 ppm etc. 

Figure 2.15 Rapidly equilibrating classical carbocation model for the 2-norbornyl cation.

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

Rapidly equilibrating classical car-bocation (carbenium ion) model for 2-norbornyl cation. 

He concluded that all the available data were equally consistent with the intermediacy of rapidly equilibrating classical carbonium ions. The Wagner-Meerwein rearrangement that interconverts the two ions is presumed to be rapid relative to capture by nucleophile, and would lead to racemic product from an optically active norbornyl substrate ... 

The norbomyl cation was generated in SbF5-S02-S0F2, and the temperature dependence of the proton magnetic resonance spectmm was examined. Subsequently, the NMR spectrum was studied, and the proton spectrum was determined at higher field strength. These studies excluded rapidly equilibrating classical ions as a description of the norbomyl cation under stable ion conditions. 

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