Superacids carbocation rearrangements

The reaction proceeds via a pentacoordinate hydroxycarbonium ion transition state, which cleaves to either fert-butyl alcohol or the tert-butyl cation. Since 1 mol of isobutane requires 2 mol of hydrogen peroxide to complete the reaction, one can conclude that the intermediate alcohol or carbocation reacts with excess hydrogen peroxide, giving fcrt-butyl hydroperoxide. The superacid-induced rearrangement and cleavage of the hydroperoxide results in very rapid formation of the dimethylmethyl-carboxonium ion, which, upon hydrolysis, gives acetone and methyl alcohol. 

As we have seen, carbocations rearrange even under the conditions of the SN1 reaction, in which their lifetimes are extremely short and rearrangemeni must compete with the fast reaction with a nucleophile. Therefore, in superacid solution, in which their lifetimes are much longer, it is not surprising that carbocations undergo extensive rearrangements. Usually, such rearrangements occur until a tertiary carbocation is formed. For example, consider the case in which 1-butanol is dissolved in superacid solution ... 

The rearrangements of both the 1-butyl and 2-butyl carbocations to the tert-butyl carbocation occur rapidly in superacid solution. Both of these rearrangements proceed through several steps and must involve an unfavorable secondary carbocation to primary carbocation rearrangement. Show the steps in the rearrangement of the 1-butyl carbocation to the terf-butyl carbocation. 

The Focus On box in Chapter 8 on page 298 showed that when carbocations are generated in superacid solution, they undergo extensive rearrangements, usually forming a relatively stable tertiary carbocation. As an example, when 1-butanol is dissolved in superacid at — 60°C, the protonated alcohol is formed. Water does not leave at this temperature because the carbocation that would be formed is primary. When the temperature is raised to 0°C, water leaves but the carbocation rearranges rapidly to the more stable tert-butyl carbocation ... 

At low temperature, in sufficiently polar, but nonnucleo-phihc solvents, some carbocations are stable. Even under these superacid conditions, rearrangements through hydride or alkyl shifts to the most stable possible carbocation are... 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

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). 

As mentioned above, persistent carbocation 9 underwent rearrangement into cation 10 which rearranged further into cation 11. To reveal general relations/factors governing cationic rearrangements in benzopentalene derivatives, the behavior of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,l-fl]indene (12) in superacids was studied (52). It had been expected that hydrocarbon 12 would transform into the long-lived 5,5,10,10-tetramethyl-4b,5,9b,10-tetrahydroindeno[2,l-a]inden-4b-yl cation (13). However, H and 13C NMR data showed that hydrocarbon 12 transformed firstly into isomeric ion 14 which transformed further into cation 15 (Scheme 11). 

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 secondary butyl and amyl cations can be observed only at very low temperatures, and they rearrange readily to the more stable tertiary ions. Generally, the most stable tertiary or secondary carbocations are observed from any of the isomeric alkyl fluorides in superacidic solvent systems. 

Because of the high stability of the tertiary ions, these are preferentially formed in the superacid systems from both tertiary and secondary, and even primary, precursors.353 If, however, the tertiary carbocation is not benzylic, rearrangement to a... 

The success of a carbocation preparation in superacid is frequently very technique dependent. Despite this fact, rather few detailed accounts have been published of the special techniques that have been developed. To be able directly to observe reactive (unstable) ions by nmr, for example, the ions not only have to be studied at low temperatures (where non-degenerate rearrangements are slow) they also have to be prepared at low temperatures for the very same reason. Furthermore, side reactions have to be suppressed. Common reactions of this type are dimerizations and polymerizations water and oxygen also have to be excluded. 

In an attempt to study the l-methylcyclopentyl-[70] to cyclohexyl-[71] cation interconversion, Olah et al. (1967) tried a number of cyclohexyl- and methylcyclopentyl-precursors under different superacidic conditions at —60°C. However, the only observed product was ion [70]. For the facile rearrangement of [71] to [70] Olah et al. favoured protonated cyclopropanes over primary carbocations as intermediates. 

When bicyclo[3.2.2]nonatrien-2-ol [429] reacted with superacid at —135°C and was observed at the same temperature by H-nmr spectroscopy, a sharp singlet at S 6.59 was obtained (275). A rapidly rearranging carbocation was concluded to be responsible for the observed singlet since there is no regular... 

---