Carbocation from alkenes

Once it was discovered that Friedel-Crafts alkylations and acylations involve cationic intermediates, chemists realized that other combinations of reagents and catalysts could give the same intermediates. We study two of these reactions in this section the generation of carbocations from alkenes and from alcohols. 

In Section 11.4.1, ionization of the C-Br bond in tertiary halide 64 gives carbocation 66. Carbocations were discussed in Chapter 10, Section 10.2, in connection with the acid-base reaction of an alkene with acids such as H-X (HCl, HBr, etc.). To understand formation of a carbocation in a substitution reaction, remember that the stability of a carbocation is related to the number of substituents attached to the positive carbon. The formation of carbocations from alkenes was described in Chapter 10, Section 10.2, as was the relative stability of carbocations. 

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

Hydride transfer from alkenes was also proposed to occur during sulfuric acid-catalyzed alkylation modified with anthracene (77). Then the butene loses a hydride and forms a cyclic carbocation intermediate, yielding—on reaction with isobutene—trimethylpentyl cations. This conclusion was drawn from the observation of a sharp decrease in 2,2,3-TMP selectivity upon addition of anthracene to the acid. 

The major factor in determining which mechanism is followed is the stability of the carbocation intermediate. Alkenes that can give rise to a particularly stable carbocation are likely to react via the ion-pair mechanism. The ion-pair mechanism would not be expected to be stereospecific, because the carbocation intermediate permits loss of stereochemistry relative to the reactant alkene. It might be expected that the ion-pair mechanism would lead to a preference for syn addition, since at the instant of formation of the ion pair, the halide is on the same side of the alkene as the proton being added. Rapid collapse of the ion-pair intermediate leads to syn addition. If the lifetime of the ion pair is longer and the ion pair dissociates, a mixture of syn and anti addition products is formed. The termolecular mechanism is expected to give anti addition. Attack by the nucleophile occurs at the opposite side of the double bond from proton addition. 

Although the ketene iminium salt route has advantages over the ketene route, limitations of this method include the nonstereospecificity of these cycloadditions in some instances because of the nonconcerted nature of these reactions. Of greater concern is the fact that the major product from alkenes which can form a tertiary carbocation is often the Friedel-Crafts product. This method is therefore largely limited to mono- and 1,2-disubstituted alkenes. 

Subsequently it was found that the same cations could also be generated from alcohols in super acid-S02 at - 60°C 1 and from alkenes by the addition of a proton from super acid or HF-SbF5 in S02 or SO20IF at low temperatures.12 Even alkanes give carbocations in super acid by loss of H. For example,13 isobutane gives the r-butyl cation... 

The success of such addition reactions depends on formation of a stable carbocation from the alkene under conditions where the most reactive nucleophile present is the carboxylic acid. 

Surprisingly, the kinetic measurements now available for the nucleophilic trapping of carbocations with water are not always matched by measurements of rate constants for formation of the carbocation from the corresponding alcohol required to evaluate the equilibrium constant AR. Although carbocations are reactive intermediates in the acid-catalyzed dehydration of alcohols to form alkenes,85,86 the equilibrium in this reaction usually favors the alcohol and the carbocation forming step is not rate-determining. Rate constants may... 

Mayr has also commented on the need for compensation for Marcus curvature in an extended free energy relationship. In the context of a discussion of the reactions of carbocations with alkenes, he suggests the alternative possibility that this compensation might arise from a log A-dependent change in the relative energies of frontier orbitals on the carbocation and the nucleophile.30... 

Mayr s calculations are consistent with his experimental demonstrations that for hydride transfer the magnitudes of N and E are independent of each other. It seems likely that the same is true of reactions of carbocations with alkenes, which again yield a carbocation as immediate product of the reaction. In these reactions then, the lack of dependence of selectivity on reactivity can be interpreted in terms of the compensation between thermodynamic driving force and variable intrinsic barrier, as already discussed, which receives substantial reinforcement from Mayr s calculations. 

The Koch-Haaf reaction397 for the preparation of carboxylic acids from alkenes uses formic acid or carbon monoxide in strongly acidic solutions. The reaction between carbocations and carbon monoxide affording oxo-carbenium ions (acyl cations) is a... 

Regio- and Stereoselectivity of the Addition Reactions Like proton-induced HAT additions [66-68], additions of carbocations to alkenes proceed with strict regioselectivity, the orientation being determined by the stabilities of the intermediate carbocations (Markovnikov rule). In this respect, carbocation additions differ from other electrophilic additions, as sulfenylations or selenylations, where the orientation is controlled by the nucleophilic attack at the bridged cationic intermediate (Scheme 13) [67, p. 860]. 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

Methyl groups at the position of electrophilic attack exert exactly the same enthalpic and entropic effects as in the alkene series (Table 4), and one can summarize that the attack of carbocations at alkenes and at allylsi-lanes, allylgermanes, and allylstannanes follows the same mechanism. The differences between these classes of nucleophiles are encountered after the rate-determining step While ordinary carbocations (produced from alkenes) usually accept a chloride ion to give addition products, the /3-metal-substituted carbocations are generally demetalated to yield the Se2 products. It has been reported, however, that j8-silyl-substituted carbe-nium ions with bulky substituents at silicon may also act as chloride acceptors with the consequence that in these cases allylsilanes yield addition products in the same way as ordinary alkenes do [159],... 

An interesting but rather unusual reaction involves the direct carbonylation of carbocations to carboxylic acid derivatives. Carbenium ions can be generated from alkenes or alkanes in strong acidic media. Thus, tertiary carboxylic acids can be produced from C4 or higher alkenes Koch-Haaf reaction) [39] (e.g., eq. (8)). Interestingly, Koch carbonylations are known to be catalyzed by copper or silver cations [40]. 

The ion (19) plays an important role in the formatiotl of e-tehchene however, the reaction is more complex. In addition to the a- and e-fenchenes, small amounts of cyclofenchene (20), 3-fenchene (23), 7-fenchene (22) and 8-fenchene (21) are also formed. Cyclofenchene (20) represents the key to the formation of these substances (see Scheme 7). It may arise by the loss of a proton from the carb tion (24). Reprotonation of the cyclopropane ring may lead to a new carbocation from which the alkenes may be derived. 

As was discussed in detail in Sections 4.4.1 through 4.4.4, here are two mechanisms by which acids add to alkenes, the AdE2 and AdgS processes. The reaction is carried out in the dark to minimize free radical side reactions (Chapter 11). In the Ad 2, the proton adds by pathway Ag to produce the more stable carbocation intermediate (Markovnikov s rule) in a second step the cation is trapped by a nucleophile, path An-The Adg2 often gives a mixture of syn and anti addition since the nucleophile can approach the carbocation from either top or bottom face. Adg2 example ... 

This reaction and the corresponding intramolecular transfers [Eq. (8.124)] are responsible for the production of only low-molecular products from 1-alkenes. For example, reaction (8.133) produces a relatively stable tertiary carbocation from a more reactive propagating secondary carbocation. 

B. Formation of carbocation ion pair from alkene and electrophile... 

The generation of carbocations from these sources is well documented (see Section 4.4). The reaction of aromatics with alkenes in the presence of Lewis acid catalysts is the basis for the industrial production of many alkylated aromatic compounds. Styrene, for example, is prepared by dehydrogenation of ethylbenzene, which is made from benzene and ethylene. 

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