Norbomenyl cation

Mechanism a involves a comer-protonated cyclopropane (26) we have already seen examples of such ions in the 2-norbomyl and 7-norbomenyl cations (pp. 409, 414). Mechanism b involves an edge-protonated cyclopropane (27). Mechanism c consists of a one-step SE2-type attack by to give the classical cation 28, which... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

This is called the o route to a non-classical carbocation because of the participation of a o bond. If a n bond is involved then it is called a 71 route. Many chemists aruge that the structure written from 7-norbomenyl cation are not non-classical carbocations because they are not canonical forms but real structures and there is rapid EQUILIBRIUM between them. 

TABLE 8. Structures of 7-norbomenyl cations and related compounds"... 

Fig. 5.50 Visualization supports the view that the 7-methyl-7-norbomenyl cation is delocalized ...

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

However, two compounds which are potentially within this definition have been prepared and their spectra obtained (80 and 81, see Table 16). The 7-norbomadienyl cation, 80 220,227), definitely prefers not to avail itself of bicycloaromatic stabilization of the bridge carbonium ion by "both double bonds, as the expected C211 symmetry is ruled out by the n.m.r. spectrum which in fact resembles that of the 7-norbomenyl cation, 71. Thus, bishomoconjugation only is observed, and the barrier to bridge flipping is estimated to be >19.6 kcal/mole 228). 

The effect of increasing electron demand (increasing [Pg.184]

Participation of tt electrons from an adjacent double bond controls the stereochemistry of substitution in the case of cyclopent-3-enyl tosylates, even though no strong rate enhancement is observed. The stereochemistry has been demonstrated by solvolysis of a stereospecifically labeled analog. The product of formolysis is formed with complete retention of configuration, in contrast to the saturated system, which reacts with complete inversion under similar conditions. The retention of configuration is explained by a structure similar to that shown in the case of the anti-7-norbomenyl cation. 

Norbomenyl cation 7-Norbomenyl anion Fig. 7.10. The conjugation stabilization in 7-norbornenyl ions... 

After years of discussion, the nonclassical nature of the 2-norbomyl and 7-norbomenyl cations 9 and 15 seems to be beyond doubt and flieir structure and charge delocalization is discussed in almost every organic reaction mechanisms textbook (Figure 9.2). 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

Structure V would be ideally constituted (see Vb) for the construction of the second C-C bond required by structure II. One major roadblock, however, is found at this point. It is the formation of a very unusual bridgehead carbo-cation (Via) that violates Bredt s rule. Nevertheless, caibenium ions at bridgeheads of polycyclic structures are well accepted reaction intermediates." Furthermore, structure VI, being an anti norbomenyl cation, benefits from the additional potential stabilization of its nonclassical caibenium ion character. It would not sit around long before water converted it to the longifolene precursor... 

The norbomenyl cation. Shown in schematic form are the limiting structures. Also shown is a calculated geometry along with the HOMO and LUMO. 

Most of the work reported in this area is limited to carbocyclic systems. The recent developments with the boron analogues of the cyclobutenyl/homocyclopropenium and norbomenyl/norbornadienyl cations point to the potential importance of cyclopropyl homoconjugation and homoaromaticity in a much wider sphere of organic systems. This will likely be an area where there will be considerable further work. 

The cation 77 is stabilized by hyperconjugation and -)-I-effect of CHj groups. In nonclassical carbocations the MO is formed, at least partially, by the a-overlapping betwmi atomic orbitals. Such an overlapping may be relatively small, as in the unsymmetrical homoallylic ion 18 or much larger, as in the 7-norbomenyl ion 19 where the C —interaction is apurecr-rc-o lap, while C —and C —are practically P,o-overlaps. In the 2-norbornyl cation 5 (according to Streitwieser) there is a... 

Gassman and Fentiman proposed the so-called tool of increasing electron demand of the developing cation centre For their example of the solvolysis of 7-syn-aryl-7-anti-norbomenyl-p-nitroteizoates they confirmed the -participation by three criteria ... 

The major products of solvolysis of 2-exo- and 2-endo-norbomenyl or nortricyclyl halides or sulphonates are nortricyclyl derivatives 2-exo-norbomenyl derivatives are formed with a small yield. The predominant formation of nortricyclyl derivatives is associated with its higher stability compared to the norbomene one Even in the direct equilibrium between norbomene and nortricyclene the tricyclic isomer is predominant (3 1) at reflux temperature. This difference in the stability of products can be partially reflected on the activation energy for partitioning the cationic intermediates into bicyclic and tricyclic solvolysis products. 

The 2-exo-norbomenyl isomer can be assumed to yield first ion 171 while the nortricyclyl isomers are likely to yield directly the symmetrical cation 173. 

Later on Cristol repeated Lee s experiments on the solvolysis of brosylate 174, but the results were different reactions at temperatures from 24 to 65 °C yield equal amounts of 3-exo-deuterio-2-exo-norbomenyl acetate and of 7-syn-deuterio-2-exo-norbornenyl acetate. It does not seem possible, Cristol remarks, to explain the discrepancy between the results obtained in and According to Story a symmetrical homoallylic ion 173 is scarcely probable all the available data can be accounted for by equilibration of enantiomeric cations 171 and 171a such a rearrangement must proceed readily enough since the 1,4-interaction is negligible. 

Exo brosylate 176 forms the ion 181 which is attacked by a nucleophile on four sides. The less active endo epimer 175 also forms the ion 181, but with a parallel Sfj2-substitution which reduces the amount of nortricyclyl compounds and the fraction of rearranged norbomenyl derivatives. The authors maintain that the concept of equilibrated homoallylic cations is not necessary at all. 

Even the allegedly classical ions 219 which are in equilibrium have the structure of the nonclassical secondary a-cydopropylcarbinyl cations The solvolysis of tertiary 7-norbomenyl esters 225 and 226 yields only the products with the retained configuration the relatiwly low rate ratio a-GHjia-H (10 at 25 °C), the negative isotopic effect for the solvolysis of ester 225 as compared with that of 226 — all this testifies to an effective ir-participation of the double bond and to a relatively weak hyperconjugative stabilization of the cation centre by the 7-methyl group... 

As noted earlier, in exceptionally weakly nucleophilic media the NMR method is used to observe directly many nonclassical ions — intermediates postulated in explaining unusual rates, products and stereochemistry of the above solvolysis reactions. This enables research under stable-ion conditions may result in dis-coverii new, earlier unknown kinds of carbocation rearrangements illustrated by the 7-norbomenyl and 7-norbomadienyl cations. 

By comparing the NMR spectra of 7-aryl-7-norbomenyl with 7-aryl-7-nor-bomyl ions ° 01ah > has recently demonstrated (Fig. 11) the interaction between the cation centre of C and the 7t-bond, the increase in electron demand being... 

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