The Norbornyl Cation

Theoretical analyses of such structures indicated that the C-H-C 3c-2e bonds in tricyclic systems are highly sensitive to molecular geometry. Ponec, R. Yuzhakov, G. Tantillo, D. J. /. Org. Chem. 2004, 69, 2992. 

Bartlett, P. D. Nonclassical Ions Reprints and Commentary W. A. Benjamin New York, 1965. Sargent, G. D. Q. Rev. Chem. Soc. 1966, 20, 301 suggested that the classical period in carbocation history lasted less than ten years, from the time of Whitmore s publication (reference 194) until the first report of a bridged ion by Nevell, T. P. de Salas, E. Wilson, C. L. /. Chem. Soc. 1939, 118 see also Saltzman, M. D. Wilson, C. L. /. Chem. Ednc. 1980,57, 289. 

This conclusion was based on the premise that tunneling of the carbon atoms was not important in the equilibration mechaiusm. For a discussion, see (a) Myhre, P. C. McLaren, K. L. Yannoni, C. S. /. Am. Chem. Soc. 1985, 107, 5294 (b) reference 261b. 

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

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy that is based on the difference between the total chemical shift of a carbocation and that of the corresponding alkane. Differences in total chemical shift of 350 ppm or more are associated with classical carbocations, while differences of less than 200 ppm are thought to indicate nonclassical, bridged carbonium ions. For example, the sum of the total NMR chemical shift of propane is 47 ppm, while the sum for the 2-propyl cation is 423 ppm. The difference, 376 ppm, indicates that the 2-propyl cation is a classical ion. For the 2-norbomyl system the total of the shifts is 408 ppm, while the total for norbomane is 233 ppm. The difference, 175 ppm, was taken as evidence for a nonclassical structure.  

The acetolyses of both ejco-2-norbornyl brosylate and endo-2-norbornyl brosylate produce exclusively ejco-2-norbornyl acetate. The exo-brosylate is more reactive than the endo-brosylate by a factor of 350, as measured by the solvolysis rate constants.Furthermore, acetolysis of optically active ejco-brosylate gave completely racemic exo-acetate, and e/ido-brosylate gave exo-acetate that was at least 93% racemic. 

Both acetolyses were considered to proceed by way of rate-determining formation of a carbonium ion. The rate of ionization of the endo-brosylate was considered normal, since its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene hydro- 

TTie Nonclassical Ion Problem, Plenum Press, New York, 1977 H. C. Brown, Tetrahedron 32, 179 (1976) P. D. Bartlett, Nonclassical Ions, W. A. Benjamin, New York, 1965 S. Winstein, in Carbonium Ions, Vol. Ill, G. A. Olah and P. v. R. Schleyer (eds), Wiley-Interscience, New York, 1972, Chap. 22, G. D. Sargent, ibid.. Chap. 24 C. A. Grob, Angew. Chem. Int. Ed. Engl. 21, 87 (1982). 

This intermediate serves well to explain the formation of racemic product, since it is achiral. The molecule has a plane of symmetry passing through C(4), C(5), C(6), and the midpoint of the C(l)-C(2) bond. The plane of symmetry is seen more easily in an alternative, but equivalent, representation  

Carbon 6, which bears two hydrogens, is pentacoordinate and serves as the bridging atom in the cation. 

This controversy has been instrumental in the development of important structural methods in physical organic chemistry with respect to critical evaluation of results as well as to concepts behind the methods. 

The methods that were developed in the early 1960s to generate and observe stable carbocations in low-nucleophilicity solutions18 were successfully applied to direct observation of the norbomyl cation (C7H114 ). Preparation of the ion by the c route from 2-norbornyl halides, by the n route from 4-(2-haloethyl)-cyclopentenes, and by the protonation of nortricyclene ( bent o route ) all led to the same 2-norbomyl cation. 

In a joint effort, Saunders, Schleyer, and Olah867 first investigated the 60-MHz 1H NMR spectrum of the 2-norbornyl cation in the early 1960s. Subsequently, Olah and co-workers38,39 carried out detailed 100-MHz 1H and 25-MHz 13C NMR spectroscopic studies in the early 1970s at successively lower temperatures. From the detailed H NMR investigations at various temperatures (RT to —154°C), the barrier for the 

The unsymmetrically bridged ions 506 would be undistinguishable from the symmetrically bridged system 504 in solution NMR experiments. (However, see subsequent discussion of solid-state low-temperature 13C NMR as well as ESCA studies.) It is important to recognize that equilibrating open classical cations 505 cannot explain the NMR data and thus cannot be involved as populated species. 

Applying the additivity of chemical shift analysis55 to the 2-norbomyl cation also supports the bridged nature of the ion. A chemical shift difference of 168 ppm is observed between the ion (C7Hn+) and its parent hydrocarbon [i.e., norbornane (507)], whereas ordinary trivalent carbocations such as the cyclopentyl cation (33) reveal a chemical shift difference of =360 ppm.55 

The 2-Norbomyl Cation The 2-norbornyl cation (bicyclo[2.2.1]hept- 

2-yl cation, C7H11 ) holds a unique position in the history of organic chemistry 

The norbornyl ion controversy centered on the question of whether the ion had the static bridged nonclassical structure (126) containing pentacoordinate hypercarbons or whether its structure was better depicted as a rapidly equilibrating pair of trivalent classical ions (127a and 127b). 

The NMR data support the bridged structure. Under stable ion conditions, the value of Av (Eq. 14.64) is 175 ppm. This is far below the usual range seen for conventional carbenium ions, although the deviation is not as dramatic as that seen for the Coates ion. The isotopic perturbation of equilibrium technique (Chapter 8) has also been applied. At very low temperatures, the Cl and C2 of norbornyl cation show a single, merged line in the C NMR spectrum. When a 2-deuterio precursor is used in an effort to break this degeneracy, a single. 

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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). 

D. Kinetic and Thermodynamic Control in the Reversible Carbonylation of the 2-Norbornyl Cation... 

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

The behavior of members of the bicyclo[2.2.1]heptene family is also different from that of other common 1,2-disubstituted alkenes.230 The parent bicy-clo[2.2.1]heptene gives bicyclo[2.2.1]heptane in only 3.5% yield when it is treated with Et3SiH/TFA. The major product is reported to be a 2-bicyclo[2.2.1]heptyl trifluoroacetate of unspecified configuration (Eq. 70).230 The carbocation intermediate is presumably the 2-norbornyl cation. Addition of small amounts of boron trifluoride etherate to the reaction mixture causes the yield of hydrocarbon product to rise to 22% after a reaction time of 24 hours at room temperature. Further... 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

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... 

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

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... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation... 

Figure 3.22. Solid-state 13C NMR spectra of the 2-norbornyl cation according to Yannoniand Myhre.870...

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... 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

Gas-phase mass spectrometric studies891-894 also indicate exceptional stability of the 2-norbomyl cation relative to other potentially related secondary cations. A study by Kebarle and co-workers895 also suggests that the 2-norbornyl cation is more stable than the tert-butyl cation in the gas phase (based on hydride transfer equilibria from their respective hydrocarbons). 

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... 

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... 

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

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. 

The complete Fourier transform C-nmr spectra (with all coupling constants and multiplicities) were obtained for the 2-norbornyl cation at —70°C and -150°C by Olah et al. (1973a). At -70°C in SbFs-SO ClF-SOjF the ion undergoes rapid equilibration via 6,2-hydride shifts and only one resonance was observed for cyclopropane-like carbons 101.8 (ref CSa, quintet, /isch = 53.3 Hz). This peak was split into two at —150°C. One peak... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

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