4-Methyl-2-norbornyl cation

One of the crucial points in the debate about the norbornyl cation has been the extra stability gained from the presumed o-bridging in comprison with a classical carbocation. This stabilization was recently determined calorimetric-ally from the heats of isomerization of the 4-methyl-2-norbornyl cation [212] to the tertiary 2-methyl-2-norbornyl cation [213] in SbFj-SOgClF by Arnett et al. (1980), thereby avoiding the large initial state contributions that follow the use of different neutral precursors. As a classical reference system the... 

It is also instructful to discuss studies on 2-methyl- and 2-phenyl-2-norbomyl catons, 167 and 168, both of which have been investigated by proton10,210 and carbon-13 211 NMR spectroscopy. The 2-methyl-2-norbornyl cation can be characterized as a partially a-bond (i. e., C.1-C.6) delocalized carbenium ion, while the 2-phenyl-2-norbomyl cation exhibits the properties of a classical carbenium ion with no significant a delocalization (of course the positive charge is extensively delocali-... 

Further compelling evidence indicating additional stabilization of the 2-norbornyl cation comes from Arnett et al. s measured heats of isomerization of secondary cations into tertiary cations.The measured heat of isomerization (A// ) of 4-methyl-2-norbornyl cation (143) to the 2-methyl-2-norbornyl cation (142) is -6.6kcalmoF [Eq. (5.20)]. In contrast, the related isomerization of the 2-butyl cation (58) to the tert-butyl cation (50) involves a difference of -14.2kcalmoTi [Eq. (5.21)] ... 

Equilibrium isotope effects in 2-methyl-2-norbornyl cation have been investigated and it was inferred from the direction and the small size of temperature-dependent isotope effects on the carbon chemical shifts of 2-CDj-2-norbomyl cation (Lloyd, 1978) that the carbon spectrum is actually an averaged spectrum of three rapidly equilibrating isomeric cations. One tertiary cation [109] and two hypercoordinated cations [110] and [III] were estimated to have about the same energy and to be present in a ratio of about 70 25 5 at — 115°C. The equilibrium is nondegenerate and K is very large in favour of the tertiary cation [109] only small isotope effects are therefore observed. 

The deuterium isotope effect on the proton and deuterium chemical shift in methyl and/or ring-deuteriated 2-methyl-2-norbornyl cations have also... 

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. 

The spectra of other norbornyl cations have also been investigated at low temperatures. Spectra of the tertiary 2-methyl- and 2-ethylnorborny cations show less delocalization,153 and the 2-phenylnorbornyl cation (54) is essentially classical,154 as are the 2-methoxy-155 and 2-chloronorbornyl cations.156 We may recall (p. 170) that methoxy and halo groups also... 

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. 

The 3-spirocyclopropyl-2-cyclopropyl-2-norbornyl cation 97, R = C-C3H5 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). 

Irrespective of the final decision as to the nature of the secondary 2-norbornyl cation, it is now evident that tertiary 2-methyl- and 2-phenyI-2-norbornyl cation are... 

An ESCA study by Olah and coworkers " succeeded in observing the ESCA spectrum of the 2-norbornyl cation (126) and compared it with those of the related 2-methyl-2-norbomyl cation (142) and other trivalent carbenium ions such as the cyclopentyl (75), 1-methyl-1-cyclopentyl (141), and tert-butyl (50) cations. The Is electron spectrum of the 2-norbomyl cation shows no high binding energy carbenium center and a maximum separation of <1.5 eV is observed between the two equivalent cyclopropyl -type carbons... 

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. 

Of definite interest is the problem of the validity of such relationships in the case of more significant changes in the nearest environment. According to Sorensen s estimation the free activation energy for the 1,2-shift of the methyl group from to in the exo-3-methyl-2-norbornyl cation (53) whose C (C ) and 0(C atoms are directly bonded with the hydrogens rather than with the methyl groufs of the MCR amounts to 14.5 kcal/mole. The substitution of this value into Eq. (7) yields the value AG = 9.9 kcal/mole rather close to the experimental one (AG = 11.5 kcal/mole >). 

Berson, j. a., j. H. Hammons, A. W. McRowe, R. G. Bergman, A. Remanick, and D. Houston The Chemistry of Mcthylnorbomyl Cations. VI. The Stereochemistry of Vicinal Hydride Shift. Evidence for the Nonclassical Structure of 3-Methyl-2-norbornyl Cations. J. Amer. Chem. Soc. 89, 2590 (1967). 

Likewise in the case of the norbornyl cation (52), a methyl substituent is sufficient to displace the equilibrium in favor of the classical ion [see (54)]. 

Following the clarification of the structure of the norbornyl cation by spectroscopic methods, further study of the alkylnorbornyl cations has now been made. The detailed analyses of low-temperature n.m.r. results show that the 2-methyl-, 1,2-dimethyl-, and 1,2,3,3-tetramethyl-norbornyl cations are all classical species undergoing rapid Wagner-Meerwein rearrangement. A separate study of the hydrochlorination of (456) and subsequent methanolysis of the product (457) unequivocally demonstrates the importance... 

Brown, H. C., and K.-T. Liu Additions to Bicyclic Olefins. VII. Electrophilic Addition of Hydrogen Chloride and Deuterium Chloride to Norbornene, 2-Methyl-norbornene, and Related Bicyclic Olefins. Evidence for a Carbonium Ion Process and the Capture of Unsymmetrical (Classical) 2-Norbornyl Cations. J. Amer. Chem. Soc. 97, 600 (1975). 

Methyl-2-norbornyl (66) and 1,2-dimethyl-2-norbornyl (67) cations have partially bridged structures. 

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