Primary Superacids

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). 

Quantum chemical calculations predict the primary ethylcation, C H/" to have /(-hydrido-bridgcd structure 5 which is 6-8 kcal/mol more stable than the Kekule line-bond structure for the primary cation 4. The ethylcation is not stable enough to be observable directly in superacid media. The NMR chemical shifts were calculated for both isomers 4 and 5 using the GIAO-MP2 method for CCSD optimized structures.23... 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

H202 in superacids at —78°C converts simple straight-chain alkanes into primary alcohol (ethane), or secondary alcohols and ketones (propane, butane).1,62 89 9° Electrophilic hydroxylation of the secondary C—H bond by the incipient hydroxyl cation formed through the protolytic cleavage of hydroperoxo-nium ion accommodates these observations ... 

Up to a // value of — 10, all indicators are primary amines and are therefore suitable for the measurement of the Hammett H() function. For stronger acids, new indicators such as nitro compounds have to be used. Although the acidity function scale based upon nitro compounds as indicators may not be a satisfactory extension of the aniline indicator scale, Gillespie and Peel18 have shown that the most basic nitro compound indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest indicator in the aniline series, 2,4,6-trinitroaniline. Thus, the acidity measurements using the nitro compounds may be considered to give the best semiquantitative picture of the acidity of the various superacid 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 first direct NMR spectroscopic evidence for the existence of primary alkylox-onium ions (protonated alcohols) in superacid solutions was found in 1961 by MacLean and Mackor.50 The NMR spectrum of ethanol in HF-BF3 solution at —70°C gave a well-resolved triplet at about 81 H 9.90 for the protons on oxygen coupled to the methylene protons. In HS03F this fine structure is not observed, even at 95°C, due to the fast proton exchange.51... 

These results are in agreement with the alkane behavior in superacid media and indicate the ease of oxidation of tertiary alkanes. However, high acidity levels are necessary for the oxidation of alkanes possessing only primary C—H bonds. 

Under superacidic conditions, it is known that the deprotonation equilibria (Scheme 5.17, first reaction) lie too far to the left (K= 10 16 for isobutane127) to make this pathway plausible. On the other hand, among Q, isomers, 2MeP is by far the easiest to cleave by /3-scission. The 4-methylpent-2-yl ion 30 is the only species that does not give a primary cation by this process. For this reason, this ion is the key intermediate in the isomerization cracking reaction of Q, alkanes. 

Alkylation of methane, ethane, propane, and n-butane by the ethyl cation generated via protonation of ethylene in superacid media has been studied by Siskin,148 Sommer et al.,149 and Olah et al.150 The difficulty lies in generating in a controlled way a very energetic primary carbenium ion in the presence of excess methane and at the same time avoiding oligocondensation of ethylene itself. Siskin carried out the reaction of... 

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 ... 

The primary thrust of the material in this chapter will be to indicate to researchers not already familiar with the details of superacid chemistry that, for the synthesis of many inorganic fluorides and other compounds containing fluorine, a favourable reaction medium can frequently be found by careful selection of an appropriate superacid, by deliberate control of the acidity or basicity of that medium and by use of suitable redox and other reactants and precipitants. 

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. 

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