Carbonium ions norbornyl cation

If the norbornyl cation is formed on solvolysis of < xo-norbornyl derivatives, C2 should become equivalent to C and C7 to C3. In a most elegant tracer experiment, Roberts and Lee synthesized w-2-norbornyl-[2,3-14C] brosylate (85), solvolyzed it in acetic acid, and degraded the product. Equation 6.39 shows the product distribution expected if the symmetrical carbonium ion (82) were formed. The label was found not only at Cx, C2, C3, and C7, but also at C5 and... 

For example, acetolysis of exo-2-norbornyl brosylate 254 produces exclusively exo-2-norbornyl acetate 255. The exo-brosylate 254 is more reactive than the endo-brosylate 256 by a factor of 350 and the acetolysis of optically active exo-brosyl ate gave completely racemic exo-acetate 255. Thus, the carbonium ion produced from exo-254 is more rapidly (thus more easily) formed than that from endo-256. These results were originally rationalized in term of a bridged (nonclassical) cation 257 (Winstein approach) (97) or as the rapidly equilibrating classical carbonium ions 258 and 259 (Brown approach (98, 99)). 

Protons and carbenium ions always add to C=C double bonds via carbenium ion intermediates simply because no energetically favorable onium ions are available from an attack by these electrophiles. An onium intermediate formed by the attack of a proton would contain a divalent, positively charged H atom. An onium intermediate produced by the attack of a carbenium ion would be a carbonium ion and would thus contain a pentavalent, positively charged C atom. Species of this type are at best detectable under exotic reaction conditions or in the norbornyl cation (Figure 2.25). 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

The synthesis of tosylate 19 was specifically carried out to test the kinetic importance of bridging in the parent norbornyl system. Fusion of the trimethylene fragment to a norbornyl skeleton works against ionization to a nonclassical car-bonium ion because bridging would lead to a large increase in strain energy. No increase in strain would be predicted in ionization to a classical carbonium ion. Compound 19 underwent acetolysis only 8 times faster than its cndn-isomer 20, and was 280 times less reactive than cjco-norbornyl tosylate under the same conditions. It was concluded that exo-norbornyl systems do exhibit a rate enhancement that is best understood as an electronic stabilization of the cationic intermediate. 

The question of the high exolendo rate ratio was examined with exo- and endo-2-phenyl-2-norbornanol. The tertiary and benzylic 2-phenylnorbornyl cation is classical, as evidenced by its NMR spectrum. Since the relative rates of hydrolysis of exo- and endo-2-phenylnorbornyl p-nitrobenzoate in 60% aqueous dioxane at 50°C are 140, and the reactions proceed by rate-determining formation of classical carbonium ions. Brown concluded that the exo-endo rate ratio of 340 for norbornyl brosylates does not require assisted ionization of exo-norbornyl brosylate. ... 

Pertinent reviews this year deal with a reappraisal of the structure of the norbornyl cation under stable ion conditions, theoretical approaches to the structure of carbocations, pyramidal mono- and di-cations, and dynamic n.m.r. studies of carbonium ion rearrangements. ... 

The evidence discussed to this point, both for and against the nonclassical structure, rests on indirect evidence derived from interpretation of kinetic results and stereochemical features of the substitution reactions. With the development of the techniques for directly observing carbonium ions, structural studies on the ion became possible. The norbornyl cation was subjected to intense scrutiny by George Olah at Case Western Reserve University. These spectroscopic investigations constituted a new approach to the problem. 

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). 

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