Superacids carbocation equilibria

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

The relative strengths of weakly basic solvents are evaluated from the extent of protonation of hexamethylbenzene by trifluoro-methanesulfonic acid (TFMSA) in those solvents or from the effect of added base on the same protonation in solution in trifluoroacetic acid (TFA), the weakest base investigated. The basicity TFA < di-fluoroacetic acid < dichloroacetic acid (DCA) < chloroacetic acid < acetic acid parallels the nucleophilicity. 2-Nitropropane appears to be a significantly stronger base than DC A by the first approach, although in the second type of measurement, the two have essentially equal basicity. The discrepancy is due to an interaction, possible for hydroxylic solvents such as DC A, with the anion of TFMSA. This anion stabilization is a determining factor of carbocationic reactivity in chemical reactions, including solvolysis. A distinction is made between carbocation stability, determined by structure, and persistence (existence at equilibrium, e.g., in superacids), determined by environment, that is, by anion stabilization. 

It is important to point out that thermodynamic equilibria of hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (traditional acid catalysts, longer contact time), the thermodynamic equilibrium mixture of hydrocarbons can be reached. In contrast, when a reaction mixture in contact with excess of strong (super) acid is quenched, a product distribution approaching the thermodynamic equilibrium of the corresponding carbocations may be obtained. The two equilibria can be very different. Since a large energy difference in the stability of primary < secondary < tertiary carbocations exists, in excess of superacid solution, generally the most stable tertiary cations predominate. This allows, for example, isomerization of n-butane to isobutane to proceed past the equilibrium concentrations of the neutral hydrocarbons, as the er -butyl cation is by far the most stable butyl cation. 

---