2-Norbomyl cations

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

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

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbomyl cation has also been extensively studied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers prepared the 2-norbomyl cation in... 

Norbomyl cations, besides displaying the 1,2 shifts of a CH2 group previously illustrated for the isobomeol —> camphene conversion, are also prone to rapid hydride shifts from the 3 to the 2 position (known as 3,2 shifts). These 3,2 shifts usually take place from the exo side, that is, the 3-exo hydrogen migrates to the 2-exo position. This stereoselectivity is analogous to the behavior we have previously... 

Photolysis in MeOD indicates that all three of the above products arise via initial protonation of the double bond to form the 2-phenyl-2-norbomyl cation, which then undergoes either nucleophilic capture by solvent, hydride... 

Bicycloalkylidenes. a-Delocalization in bicyclic carbocations provides the opportunity to detect the protonation of bicycloalkylidenes. An obvious choice was the 2-norbomyl cation (66), for which symmetrical bridging (equivalence... 

The H-nmr parameters for the 2-methyl, 1-2-dimethyl and 1,2-dimethoxy-2-norbomyl cations are listed in Table 13. The 2-methyl derivative was found to form an ion in which the exo- and... 

No such stabilization is present in the 1,2-dimethyl-2-norbomyl cation and the proton spectrum does not permit a choice because the exo- and enrfo-6-protons exhibit a common peak. The methyl groups also afford a single non-resolvable b[Pg.219]

It was, therefore, not possible to observe the secondary 3-spirocyclopropyl-2-norbomyl cation 96101. 

The 3-spirocyclopropy 1-2-cyclopropyl-2-norbomyl cation 97, R = c-C,H5 is stable even up to -20 °C, whereas the phenyl and methyl analogues rearranged to the allylic cations 98 at -70 °C and -90 °C, respectively (equation 58). 

Comparison of Cl and C6 l3C chemical shifts showed that the ff-participation from the 2-norbomyl ring is significantly reduced in the 2-methyl analogue, whereas in the cyclopropyl and phenyl analogues it has essentially vanished. The STO-3G calculated structures show that the spirocyclopropyl participation is mainly from the exo-C—C bond. The l3C NMR studies of these cations adequately accounted for the vanishingly low values of solvolytic keJkenio rate constants, and show that 3-spirocyclopropyl groups effectively compete with the Cl—C6 ff-bond participation in the 2-norbomyl cation framework. 

More recently, Auner s and Schleyer s groups in a joint effort reported the experimental observation of an alkyl-substituted 6-sila-2-norbomyl cation110. The 6,6-Dimethyl-5-neopentyl-6-sila-2-norbomyl cation 41 was prepared by a hydride transfer reaction via the jr-route 111 from a suitable cyclopentenyl silane derivative at room temperature (equation 10). The toluene solution of the tetrakis(pentafluorotetraborate) salt of 41... 

Due to computational savings, the open and closed forms of the 6,6-dimethyl-6-sila-2-norbornyl cation, 42 and 43, the closed 5,6,6-trimethyl-6-sila-2-norbomyl cation (44) and their corresponding benzene complexes 45, 46 and 47, respectively, were optimized... 

The X-ray structure of a number of alkoxycarbenium ions has been determined.66 An interesting example is 2-methoxy-l,7,7,-trimethylbicyclo[2.2.1]hept-2-ylium tetrafluroborate 326.630 It is a substituted 2-norbomyl cation and, indeed, the C(2)-C(l)-C(6) bond angle (98.8°) and the C(l)-C(6) bond distance (1.603 A) indicate G-bond charge delocalization, that is, the contribution of the 326b resonance form. 

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. 

Figure 3.19. Degenerate shifts in the 2-norbomyl cation (one of the carbons is labeled for clarity).

Figure 3.20. 395-MHz1H NMR spectra of 2-norbomyl cation in SbF5-S02ClF-S02 solution.

Figure 3.21. 50-MHz 13 C NMR spectra of the 2-norbomyl cation in SbF5-S02ClF-S02F2 solution.

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... 

Yannoni, Macho, and Myhre869 obtained magic-angle spinning cross-polarization 13C NMR spectra of the 13C-enriched 2-norbomyl cation in SbF5 solid matrix down to — 196° C. The solid-state chemical shifts and measured barriers for the 6,1,2-hydrogen shift of 6.1 kcal mol-1 correlate well with the discussed solution data. Subsequently, they even obtained 13C NMR spectra in the solid state at —268°C (5K),870 a remarkable achievement indeed. 

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). 

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