Norbornyl carbonium ion

Other noteworthy achievements of solid-state 13C CP/MAS NMR in the context of carbonium ions are that (1) the sec-butylcarbonium ion can be identified at low temperatures in a sec-butyl chloride/antimony pentafluoride matrix in the temperature range 80-190 K (2) the norbornyl carbonium ion has been characterized (356,396) at temperatures down to 5 K, there being a strong (but not yet incontrovertible) indication that the controversial non-classical ion (398) exists and (3) the homotropylium ion is best represented (399) by the completely delocalized (homoaromatic) seven-membered state (a below) rather than the incompletely delocalized state (b). 

G. M. Kramer and C. G. Scouten, The 2-Norbornyl Carbonium Ion Stabilizing Conditions An Assessment of Structural Probes, in Advances in Carbocation Chemistry (X. Creary, Ed.) 1989, 1, Jai Press, Greenwich, CT. 

Determination of the higher carbon Is binding energies by p.e. spectroscopy has been used to distinguish between classical and non-classical carbonium ions. The time-scale of n.m.r. spectroscopy makes a-distinction between a non-classical norbornyl carbonium ion and equilibration between two classical... 

Fig. 5. Free-enthalpy diagram ofthe interconverson of 2-en

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

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... 

Since evo-2-norbornyl arenesulfonates apparently do solvolyze with anchi-meric assistance, the less strained 2-bicyclo[2.2.2]octyl derivatives (92) are of interest. In this system no comparison of the. rates of exo and endo derivatives can be made, since a substituent exo to one bridge is endo to t fief other. Stereochemical studies are, however, possible. If the carbonium ion (93) were formed from chiral 92 by participation of the —C6 bond, it would be chiral. Scheme 5 shows that attack at C2 of 93 should give retained bicyclo[2.2.2.]octyl solvolysis products (92a), and attack at Cx should give chiral exo-2-bicyclo[3.2.1]octyl... 

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). 

A number of alkyl and aryl migrations, which may be tenuously regarded as similar to carbonium-ion rearrangements, have been summarized by P. Brown and Djerassi (1967). The close similarity of the mass spectra of norbornyl chlorides have been explained by rearrangement of carbonium ions (Bunton and Del Pesco, 1969). 

The low-temperature stability of bicyclo[2.2.1]heptane derivatives is well documented by the huge literature on liquid-phase norbornyl carbonium chemistry under such conditions, no ring expansion is observed, but instead, multiple Wagner-Meerwein rearrangements take place, preserving the bicyclo[2.2.1]heptane structure in the course of nucleophilic substitution or elimination reactions. This remarkable behavior is at the origin of the nonclassic carbonium ion concept (28). 

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). 

The solvolysis of exo-norbornyl brosylate in acetic acid gives 4% elimination products, 98% of which is nortricyclene and only 2% is norbornene. The larger yield of the cyclopropane than the olefin was quoted as support for a non-classical rather than a rapidly equilibrating pair of classical carbonium ions . ... 

With Robert s data one should consider the criticism of his results by Deno According to the latter the equalization of carbon and hydrogen atoms that precedes the reaction of the carbonium ion with the solvent results from intramolecular rearrangements so it should be regarded neither as a proof of the equivalence of carbon atoms in carbonium ion structures nor as a confirmation of resonance structures. Hence, Robert s evidence that on conversion of 2-norbornyl tosylate into norbomyl acetate all the atoms except C become nearly equivalent does not favour the structure of the nonclassical ion 5 because the experimental data on isotopic equalization indicate only relative rates of carbonium ion rearrangements. 

Although carbonylation of the 2-norbornyl ion at or below room temperature leads to exclusive formation of the 2-ea o-norhornyloxo-carbonium ion, reactions at higher temperatures have shown that the 2-cndo-norbornyloxocarbonium ion is just as stable as the cjco-isomer (Hogeveen and Roobeek, 1969). This means that at low temperatures the carbonylation is kinetically controlled, and at high temperatures thermodynamically controlled. The detailed free-enthalpy diagram in... 

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. 

He concluded that all the available data were equally consistent with the intermediacy of rapidly equilibrating classical carbonium ions. The Wagner-Meerwein rearrangement that interconverts the two ions is presumed to be rapid relative to capture by nucleophile, and would lead to racemic product from an optically active norbornyl substrate ... 

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. ... 

---