Carbocations skeletal rearrangements

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. 

Studies reveal an advantage to using boron trifluoride in dichloromethane at reduced temperatures instead of Brpnsted acids in the organosilicon hydride reductions of a number of dialkylbenzyl alcohols.126 129 The use of Brpnsted acids may be unsatisfactory under conditions in which the starting alcohol suffers rapid skeletal rearrangement and elimination upon contact with the acid, and also in which the alcohol does not yield a sufficient concentration of the intermediate carbocation when treated with protic acids.126... 

A mixture of EtsSiH/TFA in dichloromethane reduces 3-methyl-5-a-cholest-2-ene to give the pure equatorial methyl isomeric product, 3/3-methyl-5o -cholestane, in 66% yield (Eq. 79).126 On the other hand, attempts to reduce cholest-5-ene using the same technique yield neither 5a-cholestane nor 5/3-cholestane, but instead an isomeric mixture of rearranged olefins. This result is presumably because of the inability of hydride attack to compete with carbocation skeletal isomerization and elimination.126... 

A long-established feature of the carbocation intermediates of reactions, such as SnI solvolysis and electrophilic aromatic alkylation, is a skeletal rearrangement involving a 1,2-shift of a hydrogen atom, or an alkyl, or aryl group. The stable ion studies revealed just how facile these rearrangements were. Systems where a more stable cation could form by a simple 1,2-shift did indeed produce only that more stable ion even at very low temperatures (see, e.g., Eq. 3). 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

In contrast, 3,3-dimethylbutene readily undergoes skeletal rearrangement (over specially prepared alumina catalyst free of alkali ions possessing intrinsic acidic sites, at 350°C).101 The extent of isomerization strongly depends on reaction conditions. At low contact time, isomeric 2,3-dimethylbutenes are the main products (Scheme 4.8, a) in accordance with the involvement of tertiary carbocation 7. 

Dehydrations produce olehns from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rearrangement, as in file case of tertiary alcohols, mixtures of regioisomers are often produced during file loss of a proton from file carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method. 

The formation of sesquiterpenes by a carbocation mechanism means that there is considerable scope for rearrangements of the Wagner-Meerwein type. So far, only occasional hydride migrations have been invoked in rationalizing the examples considered. Obviously, fundamental skeletal rearrangements will broaden the range of natural sesquiterpenes even further. That such processes do occur has been proven beyond doubt by appropriate labelling experiments, and... 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

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