Rearrangements of Carbenium Ions

Wagner-Meerwein Rearrangements in the Context of Tandem and Cascade Rearrangements 

These isomerizations almost certainly involve [l,2]-shifts of H atoms as well as of alkyl groups. One cannot exclude that [l,3]-rearrangements may also play a role. The reaction product, adamantane, is formed under thermodynamic control under these conditions. It is the so-called stabilomer (the most stable isomer) of all the hydrocarbons having the formula C1QH16. 

This impressive cascade reaction begins with the formation of a small amount of the fert-butyl cation by reaction of A1C13 with tert-BuCl. The fert-butyl cation abstracts a hydride ion 

Meerwein Rearrangement Cascades ( Tandem Wagner-Meerwein Rearrangements ) in Biosynthesis 

The carbenium ion formed last is deprotonated in the last step of this biosynthesis, thus furnishing the central C=C double bond of the final product lanosterol (G). 

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Skeletal rearrangements of carbenium ion species 2, that involve nucleophilic 1,2-migrations of alkyl groups, are called Wagner-Meerwein rearrangements... 

Rearrangements of carbenium ions occur quantitatively only... 

Notwithstanding these cases, many [1,2]-rearrangements of carbenium ions occur reversibly because of the small differences in the free enthalpies. 

Rearrangement of carbenium ions occurs quantitatively only if ... 

Fluorination of cinchona alkaloids has also been investigated. For instance, fluorination of quinine acetate under similar superacidic conditions (HF—SbFs/CHCls) affords a mixture of difluorocompounds in the 10 position that are ephners in 3 (60% yield, 1 1 ratio). This reaction involves a mechanism similar to the one described earlier (protonation, isomerization of carbenium ions, and Cl— F exchange). Curiously, when the reaction is performed on quinine itself, fluorination does not occur and an unprecedented rearrangement takes place (Figure 4.51). ... 

Undoubtedly, such scrambling phenomena are possible only with the participation of carbenium ions as reaction intermediates or transition states. Molecular rearrangements that rationalize the scrambling of deuterium atoms are shown in Scheme 5. 

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... 

The olefin seems to disappear mainly in the second stage after the decomposition of 125. Probably, a nonclassical type of carbenium ion intermediate (128) is formed with the help of the olefin group located at an intramolecularly favorable position, and this intermediate obviously makes the polymer structure complicated by a possible cyclization-skeleton rearrangement prior to the nucleophilic attack of the monomer. 

In addition to covalent species and carbenium ions, the equilibria may involve onium ions, which are formed by reaction of carbenium ions with noncharged nucleophiles [Eq. (46a)]. This decreases the carbenium ions lifetime, and therefore the time available for isomerization to more stable and less reactive carbenium ions via hydride and alkyl anion shifts [Eq. (46b)]. Decreasing the probability of rearrangements by decreasing the carbenium ions lifetime is especially useful because such rearrangements can not be prevented by decreasing the polymerization temperature. 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

Therefore, intermediate IV may be safely postulated (see Scheme 9.1). When subjected to strong protic acid (HCl) the W-nitroso group of IV activates the C-N bond by providing an electron sink, which is a desirable component in the formation of carbenium ions. It is this carbocation that allows the Wag-ner-Meerwein rearrangement of the angular methyl that is illustrated in V. The more stable tertiary carbenium ion of VI is then ideally set up for the epoxide ring closure, following the departure of nitrous oxide from VI. 

Oligomerization of olefins and their homologation by methanol (or methanol -derived species) appear to be essential features of methanol conversion over ZSM-5 zeolite. A self-consistent-interpretation of the entire process is possible in terms of Rr nsted-acid ZSM-5 zeolite catalysing proton transfer and methylation reactions, and the formation of carbenium ions, and their various oligomerization, cracking, rearrangement and hydride-transfer reactions. 

A self-consistent explanation of all the chemistry can be developed on the basis of Rri6nsted acidity of the zeolite, proton transfer and electrophilic methylation reactions, and the well known rearrangement, oligomerization and cracking reactions of carbenium ions. 

Such an interaction is probable. However, according to the available data (see Sect. IV.2.Q, a change in the acid medium does not usually cause great changes in the rate constants of carbenium ion rearrangements (cf., however, the rearrangement of hydroxytenzenium ions in aqueous acids. Sect. IV.2.C). One can, therefore, assume the equilibrium between the isomeric ions not to be very sensitive to the nature of acid medium either. The equilibrium constants (K) for isomeric isopropyl-trimethylcyclopentenyl cations at 25 "C in different acids i - i > are as follows ... 

A characteristic property of carbenium ions is their aptitude to rearrangements due to the shift of the substituent R from a-sp -hybridized carbon atom to the positively charged sp -hybridized atom (1,2-shift of the R substituent) ... 

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