Nonclassical 2-norbornyl cation

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.

Table 7.7 Difference in free energies of solvation (kcal mol 1) for the classical and nonclassical 2-norbornyl cations by three very different computational approaches.

In further theoretical studies, Schleyer and coworkers found" that the nonclassical 2-norbornyl cation (126) is more stable by 13.6kcalmoE than the 127 classical structure, and 126 is the only stable form in the gas phase and in solution, and the classical form (127) is unlikely to be involved in solvolysis reactions [MP4(fc)SDQ/6-31G /MP2(full)/6-31G -p ZPVE level]. They also performed a rigorous ab initio calculation and defined the classical 127 structure as an artifact and found that it is a transition state in the rearrangement of 126 into the bridged 2-norpinyl cation. The computed MP2-GIAO 0 chemical shifts for 126 are close to the experimental values, whereas those calculated by Schindler (IGLO) for 127 deviate considerably. The comparison of the chemical shifts for the two structures with the experimental data proves once again the symmetrical nonclassical structure. 

It would be very interesting for the problem of the existence of the nonclassical 2-norbornyl cation to determine the crystal structure of one of its salts. Due to the generally low stability of such salts, it was not possible to analyze a salt of the unsubstituted 2-norbornyl cation. Laube (1987) was, however, successful in obtaining a crystal structure of the l,2,4,7-aAZ//-tetramethyl-2-norbornyl cation (as salt with Sb2Fn, 7.102). 

The IR spectrum published by Koch et al. (1990) was recorded using Vancik and Sunko s technique (1989) in which 2-chloronorbornane and SbF are deposited simultaneously from the gas phase on a Csl window at 77 K. During a slow temperature increase to 150 K, formation of the carbocation could be observed by the change in the IR spectrum. Koch et al. (1990) compared the frequencies and intensities of 21 absorption bands observed in the range 878-3110 cm with results that were calculated on the basis of a structure of the nonclassical 2-norbornyl cation obtained by Koch et al. (1989) using the 6-31G basis set and dynamic electron correlation on the MP2 level. The calculated IR bands correlated very well with the experimental frequencies and intensities. The calculated structure of this cation was. 

The description of the nonclassical norbornyl cation developed by Winstein implied that the bridged ion is stabilized relative to a secondary ion by C-C ct bond delocalization. H. C. Brown put forward an alternative interpretation,arguing that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical secondary 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 suggested that the rapid migration would lead to preferential approach of the nucleophile from the exo direction. 

The results of the oxidation of other unsubstituted cycloaliphatic compounds are summarized in Table 4, which indicates that cyclopentane and cyclohexane are oxidized in high yield and selectivity. The medium-sized rings, however, afford only low yields of the trifluoroacetates in a complex product mixture. This lack of selectivity is probably due to extensive rearrangments of the intermediates and easy follow-up oxidations of the products. In dichloromethane/acetic acid the same non-selectivity is encountered. The electrolysis of bicyclo[2.2.1]heptane (3) leads in dichloromethane/trifluoroacetic acid to exo-bicyclo[2.2.1]heptan-2-ol (4) in 83% yield as single product after hydrolysis of the trifluoroacetate (equation 6). The stereochemistry of 4 indicates that a nonclassical norbornyl cation is involved as intermediate. In dichloromethane/20% acetic acid/0.05 M TBABF4 a slightly lower yield (61%) of exn-2-norbornyI acetate was obtained. 

Another characteristic property associated with the nonclassical norbornyl cation, the high exofendo rate ratio, was also scrutinized. This was done by examining the rate ratio for related tertiary cations that have classical structures, such as the 2-phenylnorbornyl cation. It was found that the exo and endo 2-phenyl-norbornyl p-nitrobenzoates exhibited an exo j endo rate ratio of 140 1 in aqueous... 

Figure 9.2. Carbon Is photoelectron spectrum Is core-hole-state spectra for the 2-norbornyl cation of tert-butyl cation and Clark s simulated spectra for the classical and nonclassical ions.

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

SECTION 5.12. THE NORBORNYL CATION AND OTHER NONCLASSICAL CARBOCATIONS... 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

This approach has been applied to the norbornyl cation, where the NMR even at very low temperatures showed equivalent NMR signals for Cl and C2. The monodeuterated 26 was prepared, which showed a difference of only 2 ppm in the signals for Cl and C2. This small splitting is inconsistent with rapidly equilibrating ions but is expected for the symmetrical nonclassical cation. 

However, 2-aryl-2-norbornyl cations 510 show deviations from linearity in such chemical shift plots with electron-withdrawing substituents indicative of the onset of nonclassical o-delocalization fully supporting the nonclassical nature of the parent secondary cation (Figure 3.23). These conclusions were criticized by Brown et al.883 In a subsequent paper, Olah et al.67 have shown major flaws in such criticisms. 

Subsequently, Grunthaner reexamined the ESCA spectrum of the 2-norbornyl cation on a higher-resolution X-ray photoelectron spectrometer using highly efficient vacuum techniques.884 The spectrum closely matches the previously published spectra. Furthermore, the reported ESCA spectral results are consistent with the theoretical studies of Allen and co-workers885 on the classical and nonclassical norbomyl cation at the STO-3G and STO-4.31G levels. Using the parameters obtained by Allen and co-workers, Clark and co-workers were able to carry out a detailed... 

Theoretical quantum mechanical calculations903-908 have also been performed on the 2-norbornyl cation at various levels. These calculations reveal a significant preference for the o-delocalized nonclassical structure. An extensive calculation by Schaefer and co-workers906 using full geometry optimization for symmetrically and... 

The 7-Norbornyl Cation. 7-Norbomyl derivatives were found to be extremely unreactive in solvolysis studies and product formation was shown to occur with predominant retention of configuration.917 920 These observations led to the suggestion by Winstein et al.917 that the cationic intermediate is a nonclassical ion. Attempts to isolate the 7-norbomyl cation under stable ion conditions in superacid... 

Bicyclo[2.2.1]heptane (norbomane) and bicyclo[2.2.2]octane, when treated with nitronium tetrafluoroborate in nitrile-free nitroethane, unexpectedly gave no nitro products. Instead, only bicyclo[2.2.1]heptane-2-one and bicyclo[2.2.2]octan-l-ol were isolated, respectively.500 Observation of bicyclo[2.2.1]heptane-2-yl nitrite as an intermediate and additional information led to the suggestion of the mechanism depicted in Scheme 5.48. In the transformation of norbomane the first intermediates are the 2-norbornyl cation 126 formed by hydride abstraction and nonclassical cation 127 formed through insertion of N02+ into the secondary C—H bond. In the case of bicyclo [2.2.2]octane, the oxidation of bridgehead tertiary C—H bond takes place and no further transformation can occur under the reaction conditions. Again these electrophilic oxygenation reactions testify to the ambident character of the nitronium ion. 

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... 

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