2-Norbornyl cation controversy

Clearly there was no lack of devoted adversaries (perhaps a more proper term than enemies) on both sides of the norbornyl ion controversy. It is to their credit that we today probably know more about the structure of carbocations, such as the norbornyl cation, than about most other chemical species. Their efforts also resulted not only in rigorous studies but also in the development or improvement of many techniques. Although many believe that too much effort was expended... 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

The controversy regarding the classical versus nonclassical nature of the 2-norbornyl cation was one of the central topics of physical organic chemistry [47, 48]. It also has become amenable to computations but it is clear that this is all but easy because the solvolysis... 

See S. Weininger. What s in a name From designation to denunciation - the nonclassical cation controversy," Bulletin for the History of Chemistry 25 (2000) 123-131 and references therein C. Walling, "An innocent bystander looks at the 2-norbornyl cation, Accounts of Chemical Research 16 (1983) 448-454. 

This ion holds a unique position in the history of chemistry through the important role it has played in the extension of the bonding theory of carbon compounds. Winstein and coworkers and others pioneered this work (Bartlett, 1965) which has caused so much controversy. Thus the norbornyl cation is at... 

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). 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

This reaction has been the center of a controversy concerning the structure of norbornyl cations such as 74 and 75. Norbornyl cations have been described in terms of the resonance structures shown in 77, or as the nonclassical ion 78.33 This view was questioned, primarily by Brown,who did not believe that the evidence proved the existence of a bridged intermediate.34 Brown suggested that the facile rearrangements are explain-... 

The mechanistic controversies have not diminished the synthetically interesting transformations possible with controlled cation rearrangements." Mehta and Kapoor exploited the Wagner-Meerwein shift to synthesize himachalene via treatment of 79 with trifluoroacetic acid at 35°C, initially forming the norbornyl cation 80." 2 xbis cation gave the usual norbornyl cation rearrangement to 81, which was trapped by trifluoroacetate to give the final product... 

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 isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

The protracted and controversial discussions on the structure and dynamics of the 2-norbornyl cation have been recently summarised (Walling, 1983 Olah et al 1983 Brown, 1983 Grob, 1983). The question has been whether the 2-norbornyl cation has a symmetrically bridged hypercoordinated structure [99] with a pentacoordinated carbon atom or whether it is a rapidly equilibrating pair of trivalent cations [100], In the context of this review only the evidence from isotope effects on the nmr-spectra of the 2-norbomyl and related cations is discussed. 

The hydride shifts of norbornyl cation are well accepted it is the carbon shift that has generated so much controversy. The issue is whether Eq. 11.38 is very facile, or whether we have a single symmetrical structure (see the drawing below). A large fraction of physical or-... 

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

The corner-protonated cyclopropane configuration 45 is analogous to the non-classical norbornyl ion at the center of controversy for the past three decades. The non-classical cation concept had its origin with Wilson and coworkers in 1939 who depicted structure 58 as a possible intermediate in the camphene hydrochloride-isobornyl chloride... 

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