Stable ion media

While the gas phase data just described are very informative, we also have a significant amount of thermodynamic data on carbocations in solution. This is due to the collection of data in stable ion media for carbocations developed so brilliantly by Olah and co-workers, leading to the receipt of the Nobel Prize for Chemistry in 1994. In a stable ion medium the ions have no possibilities for reactions that quench the ionic charge. While carbocations are reactive, if we remove potential reaction partners we will generally not see dimerization or other reactions in which two carbocations react with each other. The most common reaction... 

For the most part the stable ion media developed by Olah are based on antimony penta-fluoride, SbFs. This is a very powerful Lewis acid, and exposure of an alkyl halide to it produces a carbocation with the SbaFioX counterion, as in Eq. 2.16. The counterion to is very... 

Whenever a carbocation is an intermediate in a mechanism, rearrangements are possible. Besides the study of carbocation structures and reactivity in stable ion media, the majority of the information chemists have on carbocation rearrangements comes from SnI solvolysis reactions. A hydrogen, alkyl, or aryl group on a carbon adjacent (p) to the cationic carbon can shift to form a different carbocation. This is called a Wagner-Meerwein shift. Eq. 11.30 shows a thermoneutral example. 

Returning to the parent 2-norbomyl ion, several studies in stable ion media support the bridged non-classical structure. At low temperatures, the 3,2- and 6,2-hydride shifts are slow on the NMR timescale. However, at no temperature have the Cl and C2 carbons become inequivalent. This could either mean that, in fact, the stable structure is the symmetrical bridged ion, or that the barrier to the reaction given in Eq. 11.38 is so small that the reaction is fast even at the lowest temperatures that are feasible for solution NMR. 

In order to go to even lower temperatures to try to freeze out the putative Wagner-Meerwein shift of Eq. 11.38, several studies in frozen stable ion media have been performed. The most impressive is the solid state NMR spectrum of 2-norbornyl cation taken at 5 K Under these conditions, the system still appears to be a single, symmetrical ion as shown in Figure 11.10. If there is a rapid equilibration of two structures over a finite barrier, that barrier must be 0.2 kcal/ mol. It is generally considered that, if anything, the solid state should artificially increase barriers due to steric hindrance to atom movement, so if the structure is not symmetrical, the barrier is very low. 

The unusual chemical shifts associated with carbonium ions carry over to H NMR as well. Eq. 14.65 shows the novel bridged structure obtained when cyclodecanol is exposed to the highly acidic environment associated with stable ion media. The remarkable upheld shift seen for one proton confirms its bridging nature. This system is a beautiful example of the types of transannular effects discussed in Section 2.3.2. 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Computed GIAO-NMR chemical shifts were compared with the experimental data when available, and the optimized geometries were compared with the X-ray data if known. The lowest energy annulenium cation derived from 76 and 77 (both cis) was formed by protonation at C-1, and this was in concert with experimental observation of 76H" in superacid media. " The resulting annulenium ions were less delocalized than those via 74 and 75. DFT and stable ion study were also in concert regarding the protonation of 78. The availability of X-ray structures and stable ion NMR data for the dieations and dianions of 78 and 79 enabled comparison with the DFT-optimized structures and GIAO-NMR shifts. 

Both 84 and 85 have been prepared as stable ions in super-acid media and their NMR properties studied198-201. The conclusions reached from the extensive amount of work done with the parent and a variety of substituted systems is that both ions can be considered to be bishomoaromatic. In the case of 85 it should be noted that the C(7) bridge was found to lean towards and interact with one of the double bonds. Cation 85 was found to undergo an inversion process in which there is an interchange of the participating double bond (Scheme 35). Winstein and coworkers were able to place a lower limit of 19.6 kcalmol 1 on... 

Other oxygen stabilized ethylene dications have been prepared and studied using stable ion conditions. For example, Olah and White were able to observe as early as 1967 diprotonated oxalic acid (74) in FSOsH-SbFs solutions.33 This species and related dications were also studied subsequently by theoretical methods. Diprotonated a-keto acids and esters have likewise been generated in superacidic media, such as 75-77 34... 

The rearrangement of [13] bears a close resemblance to the transannular reactions observed in medium sized rings that have been reviewed by Prelog and Traynham (1963) and Cope et al. (1966). Recently Sorensen and coworkers have studied medium sized cycloalkyl cations under stable ion conditions in non-nucleophilic media and demonstrated that their structures are ji.-hydrido-bridged. The bonding situation in these ions contrasts sharply with that in the ions described above and rather corresponds to that of transition states (or intermediates) for intramolecular hydride transfer in these ions. 

These results by Olah and coworkers that the norbornyl cation adopts a corner protonated nortricyclane structure [4] in superacid media have recently got strong support. Saunders isotopic perturbation technique (Section 4) has been applied to this ion (Saunders and Kates, 1980). They prepared the dideuterio derivative [210] of [4] under stable ion conditions and... 

The most comprehensive studies of the effect of acidity on potentials are those of Schilt 617, 618), where media up to 12 M in sulfuric acid were used. For M = Fe, Ru, or Os, the oxidation-reduction potential for M(bipy) +/M(bipy) + becomes less negative as the medium becomes more acid, while the converse is true for [M(bipy)2(CN)3]/[M(bipy)2(CN)2]. These results are interpreted as showing the formation of stable ion-pairs derived from the tris complexes and acid anions the CN groups in the mixed M(II) but not M(III) complexes may behave as bases yielding mono- and diprotonated species. 

The trishomocyclopropenium ion (CeH/, 199) was first proposed by Winstein and coworkers as an intermediate in the solvolysis of czs-bicyclo[3.1.0] hexyl tosylate and extensive efforts were directed toward its generation under stable ion conditions. The persistent cation 199 was first prepared by Masamune et al. by the ionization of czx-bicyclo[3.1.0]hexane in superacid media and it has since been generated from the corresponding alcohol [Eq. (5.29)]. The NMR spectra of structure 199 are consistent with an ion of Csv symmetry. The three equivalent C-H groups are found at high field in the C NMR spectrum (8 C 4.9,7c h = 195.4Hz) in accordance with their hypercoordinate environments. 

This is a remarkably stable ion, already observed in 1955. Its first NMR spectrum (in H2SO4 at room temperature ) was reported in 1962 and so were a little later, the spectra of a variety of cyclopropyl carbocations in superacid media. Its spectrum has been carefully analyzed. 

Indicate the structure of the final stable ion that would be formed from each of the following reactants in superacid media. 

Can the data on the structure of a stable 2-norbomyl ion be applied to the intermediates in the solvolysis of 2-substituted norbornanes Brown says no Indeed, in superacids the solvation of the cation, though feeble, is unspecific. Nucleophilic media exhibit specific interactions with the counter-ion and one or more solvent molecules, which may alter the structure, energies and properties of intermediates. Nevertheless, if we compare the same substrates in solvolytic media and in superacids, we obtain related information provided we really compare carbocations in different media, and not processes of S 2 type and stable ions. 

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. 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

C provides direct support for the dictum that tertiary carbonium ions are more stable than secondary, which are more stable than primary. Primary and secondary alcohols are protonated under these conditions, and the protonated alcohols are the species observed (entry 1), while tert-butyl alcohol yields ter/-butyl cation at rates too fast to measure. The rates of cleavage of protonated primary and secondary alcohols depend on their structure. Protonated sec-butanol cleaves with rearrangement to (CH3)3C and water slowly at -60°C, protonated isobutanol cleaves with rearrangement at —30°C, and protonated n-butanol at 0°C. It is typical of reactions in superacid media that the most stable ion of a particular class is observed because, under conditions in which the ions are long-lived, intramolecular hydride shifts and rearrangement processes occur that lead ultimately to the most thermodynamically... 

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