Cations 2-norbornyl

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- 

Returning to the parent 2-norbomyl ion, several studies in stable ion media support the bridged non-classical structure. At low temperatures, the 3,2- and 6,2-hydride shifts are slow on the NMR timescale. However, at no temperature have the Cl and C2 carbons become inequivalent. This could either mean that, in fact, the stable structure is the symmetrical bridged ion, or that the barrier to the reaction given in Eq. 11.38 is so small that the reaction is fast even at the lowest temperatures that are feasible for solution NMR. 

In summary, the prevailing wisdom now finds that the 2-norbornyl cation is non-classical, and that this should not be all that surprising. Donation of filled orbitals to empty orbitals is always stabilizing, and when the filled orbitals are aligned with the empty p orbital of a carbocation, this donation can and will occur. 

Another one of the more extensively investigated carbocation systems derives from SnI reactions on homoallyl, cyclobutyl, or cyclopropylcarbinyl derivatives. Solvolyses of all three of these systems give very interesting product mixtures. Let s examine just a few 

The solvolyses of cyclopropylcarbinyl and cyclobutyl derivatives often give exactly the same products, in close to the same ratios, as observed by Roberts in 1951. The reaction of certain homoallyl derivatives will also give these products, but homoallyl structures are also very susceptible to Sn2 reactions and will therefore sometimes deviate in the product ratios. A comparison where all three derivatives give the same products in similar ratios is shown in Eq. 11.42. The data indicate that there is likely a common intermediate in all three of these reactions. 

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Figure 9.1. 395 MHz M NMR spectra of 2-iiorbornyl 50 MHz proton decoupled C NMR spectra cation in SbF5/S02CIF/S02F2 solution of 2-norbornyl cation ( C enriched) in SbF5/S02ClF/S02F2 solution.

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.

During my studies 1 realized that the formation of the cr-delocalized 2-norbornyl cation from 2-norbornyl precursors represented the equiv-... 

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

Draw a Lewis structure (or series of Lewis structures) foi 2-norbornyl cation which adequately describes its geometry, charge distribution and bond density surface, Relate this structure to your description of 3-methyl-1-butyl cation. 

D. Kinetic and Thermodynamic Control in the Reversible Carbonylation of the 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... 

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... 

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

Of particular importance are recent results on the energetics of the exo-2,3-hydride shift in the 2,3-dimethyl-2-norbornyl and the l,2,3,4-tetramethyl-2-norbornyl cations. Line broadening studies reveal a free energy of activation of 6-6 and 7-3 kcal mole for this process in the respective ions (Huang eta/., 1973 Jones et a/., 1974). 

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

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