Rapidly equilibrating classical carbocations

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... 

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. 

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. 

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.

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]. 

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. 

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. ... 

The question is - how can we decide between the formulation of the 2-norbornyl cation as a rapidly equilibrating pair of classical carbocations IS, and the various nonclassical formulations, 19-25 ... 

The racemization of optically active 2-norbornyl derivatives in the course of solvolysis can be accounted for, as Winstein himself appreciated8, either in terms of a rapidly equilibrating pair of classical carbocations (18) or in terms of the formation of a symmetrical nonclassical species (19-25). Consequently, the problem cannot be resolved solely on the basis of such racemization. 

The optically active exo-products from the exo-amine may result either from a chiral norbornyl cation, but, more likely, as emphasized by Collins (1975), by asymmetry in the diazonium-carboxylate ion pairs, which give carbocation-carboxylate ion pairs (see Scheme 7-17 in Sect. 7.3). The authors emphasized, however, that these product studies cannot distinguish a rapidly equilibrating and highly exo-selective classical norbornyl cation from the bridged species 7.107 and 7.108 as intermediates (Scheme 7-35). 

From the above data Brown assumed the intermrfiate carbocation not to be the nonclassical ion 19 but to have the structure of the classical tritgrclic carbocation 219 in the rapid equilibration with its epimer. As noted above, the solvolysis of tosylate 209 under weak-alkaline conditions yields only alcohol 210. 

A number of other carbocations similar to C5H5 have been studied, the most notable being Cr.Mel. It has been demonstrated that the structure of this cation is Csi,. 11.66. rather than a rapidly equilibrating series of classical structures, 11.67, where a bond-switching process permutes three two-centcr-two-electron... 

The experimental observations are consistent with the intermediacy of a non-classical carbocation called the norbornyl cation (see Figure 11.10), a structure primarily attributed to Winstein. The non-classical structure possesses an internal mirror plane of symmetry, and the endo face of the compound is protected by the bridging interaction. This, at the time, was a highly novel proposal, and it was not universally accepted. The experimental observations could alternatively be explained by invoking two classical carbocations rapidly equilibrating via carbon shifts (Eq. 11.38). The biggest proponent of this explanation was Brown. The distinction between these two possibilities is an important issue to discuss. 

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