Stable ions 2-norbornyl cation

The stable 2-norbornyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et al. (1970) proved spectroscopically that this ion is a comer-protonated nortricyclene with a pentavalent carbon atom. The value for the carbonylation-decarbonyla-tion equilibrium constant K (= of the 2-norbomyl ion illustrates... 

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... 

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.

The norbornyl cation is at the heart of the nonclassical ion problem . The argument over the stable cation concerns whether it is a rapidly equilibrating pair of classical ions or rather a symmetrical ion (Brown, 1977). The isotopic perturbation studies by Saunders and Kates (1980) show that the postulated rapid Wagner-Meerwein rearrangement (4) is not consistent with the results but is a static structure like [4],... 

These results by Olah and coworkers that the norbornyl cation adopts a corner protonated nortricyclane structure [4] in superacid media have recently got strong support. Saunders isotopic perturbation technique (Section 4) has been applied to this ion (Saunders and Kates, 1980). They prepared the dideuterio derivative [210] of [4] under stable ion conditions and... 

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. ... 

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. 

The tertiary 1,2-dimethyl-2-norbornyl cation (732) is clearly unsymmetrical. Solvolyses of the p-nitrobenzoate (730)96) and of the chloride (7ii)518) proceed with partial retention of configuration, i.e., an excess of (734) over its enantiomer (734 ) is produced (57.5 42.5 with (731) in methanol-lutidine). Similar results were obtained with 1,2-dimethyl-5-norbornen-2-yl p-nitrobenzoate (42S)347 and the 5,6-benzo derivatives (733)519 in spite of eventual ir participation. The 1,2-dimethoxy-2-norbomyl cation (735) is unsymmetrical even under stable ion conditions520. The low temperature NMR shows distinct signals for the two methoxy groups, coalescence occurring at +7 C. 

The methods that were worked out in the early 1960s by Olah to generate and observe stable carbocations in low nucleophilicity solutions" were successfully applied to the direct observation of the 2-norbornyl cation (C7H1P). Preparation of the ion by the a route from 2-norbornyl halides, by the k route from cyclopentenylethyl halides, and by the half-bent a route via protonation of nortricyclene, all led to the same 2-norbomyl cation. 

Fig. 4.13. Contrasting energy profiles for stable and unstable bridged norbornyl cation. (A) Bridged ion is a transition structure for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged ion is the most stable structure.

Within —50 to —130 °C there are 3 PMRsignals with an intensity ratio of 4 1 6. This points to the freezing of 3,2-hydride shifts (E = 10.8 0.6 kcal/mole A = 10 - s ). Judging from these data the 3,2-hydride shift rate in a stable 2-norbomyl cation is abnormally low compared with 1,2-hydride shifts in secondary carbocations. Thus the respective activation energies are 5 kcal/mole for the 1,2-hydride shift in the cyclopentyl cation and 10.8 kcal/mole for the 3,2-hydride shift for the 2-norbornyl cation. This corresponds to the rate ratio 10 at —150 °C and 10 at 25 °C. Olah has studied the models of both ions showing that torsional and non-... 

The ab initio calculations taking into account electron correlations have led to the conclusion that the nonclassical 2-norbomyl ion is more stable than the classical one by 8-13 kcal/mole ( , cf. similar conclusions have been drawn by the authors of In the edge-protonated 2-norbornyl cation was shown to be comparable in stability with the comer-protonated one. 

In 2,3-dimethyl-2-norbornyl cations the rate of the exo-3,2-hydride shift is far higher than that of the endo-3,2-hydride shift (the difference in activation energies 5.5 kcal/mole Contrary to Olah s data of the two isomeric ions-2,3-endo-dimethyl-2-norbornyl 13S and 2,3-exo-dimethyl-2-norbornyl 139 the more stable one is the former As has been noted earlier, the vacant p-orbital at C interacts mostly with the exo substituent at C this hyperconjugative interaction with the C —H bond is more effective than with the C —CHj bond. 

Further Kramer has measured the peak area ratio in the ESCA spectrum of the 2-norbomyl cation (from the figure published by Olah) and found it is not 2 5 (as required by the nonclassical ion model) but 4.5 1 or 6 1, depending on the method of measurement. Olah, however, points out that in the ESCA spectrum the main thing is not the ratio of peak areas which is often distorted due to admixtures coming from the vacuum systems and so can somewhat vary from experiment to experiment, but that of the AEb value. Finally, Kramer assumes the reported spectrum to belong to a neutral compound or a mixture of substances rather than to a stable ion. But according to Olah, the NMR spectra of the solutions under study before and after the recording of the ESCA spectrum are perfectly identical and coincide with that of the 2-norbornyl cation. 

In 1980 Arnett determined the heat of isomerization of the secondary 4-methyl-2-norbornyl cation to the tertiary 2-methyl-2-norbomyl ion in SbFj— SOjFQ in such an experiment the stabilities of neutral molecules are of no significance. R rrangement of a 4-methyl-2-norbomyl cation into a 2-methyl-2-norbomyl releases 6.57 0.41 kcal/mole in contrast, rearrangement of sec-butyl to tert-butyl ion releases 14.20 + 0.60 kcal/mole. Thus, the secondary 2-norbomyl ion is more stable than the usual s ondary ions by 7.5 kcal/mole the 4-methyl group is assumed to exert an insignificant effect on the charge at C. Also the ionization heats (AHj) of 2-exo-norbomyl chloride and 4-methyl-2-exo-norbornyl chloride into the respective secondary ions are very close to each other —23.16 + 0.43 and —22.20 0.49 kcal/mole at —100 °C. All these data indicate a specific stabilizing effect in the secondary 2-norbomyl ions. 

The norbornyl cation was subjected to intense scrutiny by a variety of spectroscopic techniques under stable ion conditions by George Olah of Case Western Reserve University. These spectroscopic investigations constituted the next phase of the nonclassical ion story. Solvolysis studies of norbornyl and related systems continued, but in most cases added detail to the existing state of knowledge, rather than breaking new ground. 

Subsequently, the NMR, Raman, and ESCA spectra were measured for the norbornyl cation under stable ion conditions. The Raman spectrum exhibited similarities to the spectrum of tricyclene, and could be interpreted as indicating that the species under stable ion conditions is a protonated tricyclene. Such a corner-protonated tricyclene is simply an alternate description of the nonclassical ion. 

In order to go to even lower temperatures to try to freeze out the putative Wagner-Meerwein shift of Eq. 11.38, several studies in frozen stable ion media have been performed. The most impressive is the solid state NMR spectrum of 2-norbornyl cation taken at 5 K Under these conditions, the system still appears to be a single, symmetrical ion as shown in Figure 11.10. If there is a rapid equilibration of two structures over a finite barrier, that barrier must be 0.2 kcal/ mol. It is generally considered that, if anything, the solid state should artificially increase barriers due to steric hindrance to atom movement, so if the structure is not symmetrical, the barrier is very low. 

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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