Carbocations as electrophiles

As we have seen in Section 8.1, reaction of an alkene with an electrophile produces a carbocation that is subsequently attacked by a nucleophile. However, the carbocation is itself an electrophilic species, and 

This carbocation now becomes the electrophile, second styrene molecule, the regiochemistry of attack 

Cationic polymerization is, of course, an inter-molecular electrophilic addition process. Intramolecular electrophilic addition involving two double bonds in the same molecule may be used to generate a cyclic system. Thus, the trienone shown is converted into a mixture of cyclic products when treated with sulfuric acid. 

This is easily rationalized by protonation of the terminal alkene, yielding the preferred tertiary carbocation. The carbocation is then attacked by rt electrons from the neighboring double bond, creating a new a bond and a ring system. Note that this results in a favourable tertiary carbocation and a favourable strain-free six-membered ring (see Section 3.3.2). 

The products are then formed by loss of a proton from this carbocation, with a choice of protons that may be lost, so that a mixture of products in varying proportions results. P-Ionone is the predominant product. This is the most substituted alkene, and has the added stability conferred by extending conjugation with the unsaturated ketone (see Section 2.8). 

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Indeed, we can also achieve alkylation of an aromatic ring by using any system that generates a carbocation. In effect, we are paralleling the concept of carbocations as electrophiles as in Section 8.3,... 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

In vanadium-dependent haloperoxidases, the metal center is coordinated to the imidazole system of a histidine residue, which is similarly responsible for creating hypochlorite or hypobromite as electrophilic halogenating species [274]. Remarkably, a representative of this enzyme class is capable of performing stereoselective incorporation of halides, as has been reported for the conversion of nerolidol to various snyderols. The overall reaction commences through a bromonium intermediate, which cyclizes in an intramolecular process the resulting carbocation can ultimately be trapped upon elimination to three snyderols (Scheme 9.37) [275]. 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 681), we invoke the Hanunond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes the carbocation by resonance ... 

A similar result (open cations) was obtained with carbocations Ar2CH as electrophiles Mayr, H. Pock, R. Chem. Ben, 1986, 119, 2473. 

Protonation of alkenes yields carbocations, as we have seen, and in the absence of other effective nucleophiles (e.g. HaO, p. 187) these ions can act as electrophiles towards as yet unprotonated alkene (c/. p. 108), e.g. with 2-methylpropene (41) ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

As a simple example, note that the major products obtained as a result of addition of HBr to the alkenes shown below are not always those initially expected. For the first alkene, protonation produces a particularly favourable carbocation that is both tertiary and benzylic (see Section 6.2.1) this then accepts the bromide nucleophile. In the second alkene, protonation produces a secondary alkene, but hydride migration then leads to a more favourable benzylic carbocation. As a result, the nucleophile becomes attached to a carbon that was not part of the original double bond. Further examples of carbocation rearrangements will be met under electrophilic aromatic substitution (see Section 8.4.1). 

Figure 7-9 illustrates a typical Friedel-Crcifts alkylation. Once formed, the carbocation is a very strong electrophile. A complication that may occur is the rearrangement of the carbocation to a more stable carbocation, as seen in Sfjl mechanisms of alkyl halides. These rearrangements may involve a hydride or other shift. 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

The first step (which involves electrophilic attack by bromine on the double bond) produces a bromide ion and a carbocation, as shown in Equation 10-1.1... 

The electronic description and hybridization of dihalocarbenes (3) are similar to those of carbocations. Not surprisingly, therefore, dihalocarbenes behave as electrophiles in their reactivity towards alkenic substrates and this is discussed in the following sections. 

For carbocations, an electrophilicity (Lewis acidity) scale can be based on ions other than the hydroxide ion as is shown in general for X- in Equation (6), for which the equilibrium constant can be denoted A1R. Scales based on chloride ion, for example, have been used in the gas phase2,17,36 and are also appropriate for nonaqueous solvents. 

A combination of electrophilic properties of H+ and relatively low nucleo-philicity of the counter anion in superacids makes these materials convenient media for the generation of different types of carbocations as a result of protonation of the carbon-element bond ... 

The O-alkylation of carboxylates is a useful alternative to the acid-catalyzed esterification of carboxylic acids with alcohols. Carboxylates are weak, hard nucleophiles which are alkylated quickly by carbocations and by highly reactive, carbocation-like electrophiles (e.g. trityl or some benzhydryl halides). Suitable procedures include treatment of carboxylic acids with alcohols under the conditions of the Mitsunobu reaction [122], or with diazoalkanes. With soft electrophiles, such as alkyl iodides, alkylation of carboxylic acid salts proceeds more slowly, but in polar aprotic solvents, such as DMF, or with non-coordinating cations acceptable rates can still be achieved. Alkylating agents with a high tendency to O-alkylate carboxylates include a-halo ketones [42], dimethyl sulfate [100,123], and benzyl halides (Scheme 6.31). 

Electron-deficient molecules are known as electrophiles (electron-loving) and react with nucleophiles. Positively charged ions can easily be identified as electrophiles (e.g., a carbocation), but neutral molecules can also act as electrophiles if they have certain types of functional groups (e.g., carbonyl groups or alkyl halides). 

The regiochemistry of the reaction is in accord with the rule that the electrophile— the boron, in this case—adds to the carbon that is bonded to more hydrogens. The reason given previously for this orientation was that the electrophile adds so as to produce the more stable carbocation. As shown in Figure 11.7, no carbocation is involved in the... 

Most addition reactions involve a second step in which a nucleophile attacks the carbocation (as in the second step of the SN1 reaction), forming a stable addition product. In the product, both the electrophile and the nucleophile are bonded to the carbon atoms that were connected by the double bond. This reaction is outlined in Key Mechanism 8-1, identifying the electrophile as E+ and the nucleophile as Nuc -. This type of reaction requires a strong electrophile to attract the electrons of the pi bond and generate a carbocation in the rate-limiting step. Most alkene reactions fall into this large class of electrophilic additions to alkenes. 

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