Reactions of Carbocations with Alkenes

Because reaction (14) generates a product (PA ), which may exhibit similar electrophilic properties as the reactant RA , it will depend on the relative 

Halogenated ethylenes react with carbocations at the nonhalogenated or least-halogenated position [70-77], not because the a-halogenated car- 

According to quantum chemical calculations, additions of carbocations to alkenes are expected to proceed via 7r-complexes [84, 85]. The high antistereoselectivity of carbocationic cyclizations had been explained by this hypothesis [86-88]. 

As discussed in Section III.B.l, these reactions are assumed to proceed via partially bridged cations. Attack of Ph2CH+ at (Z)- and ( )-2- 

In analogy to these observations, alkylations of cyclopentene and cyclohexene, which also represent (Z)-l, 2-dialkylated ethylenes, afforded complex product mixtures due to extensive rearrangements. 

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There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

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. 

If the propagation step of the sequence described in Scheme 9 could be suppressed, the gross reaction (14) would result, and this reaction would allow mechanistic details of the reactions of carbocations with alkenes to be investigated. 

We are not aware of any systematic investigations on the diastereoselectivities of the CC-bond-forming step in intermolecular reactions of carbocations with alkenes. Generally, we observed only low stereoselectivities in such cases, as illustrated for the Lewis acid catalyzed addition of 4-chloro-2-pentene to (Z)-2-butene (Scheme 19). The si,re transition state 6 is slightly favored (75 25) over the si,si (and re,re) transition state 7, and for the corresponding addition to ( )-2-butene, the advantage of si,re over si,si (or re,re) sinks to 57 43 [95]. 

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. 

Based on what we ve seen thus far, a possible mechanism for the reaction of bromine with alkenes might involve electrophilic addition of Br+ to the alkene, giving a carbocation that could undergo further reaction with Br- to yield the dibromo addition product. 

Reactions of carbocations with water as a base removing a [3-proton to form an alkene or aromatic product have been less studied than nucleophilic reactions with water. Nevertheless, the correlations included in Fig. 1 (p. 43) represent a considerable range of measurements and these can be further extended to include loss of a proton a to a carbonyl group.116 Indeed, if one places these reactions in the wider context of proton transfers, it can be claimed that they constitute the largest of all groups of reactions for which correlations of rate and equilibrium constants have been studied.116,244,245... 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

As the reactions of carbocations with alkynes have higher activation enthalpies and less negative activation entropies than analogous reactions with alkenes, the relative reactivities of styrene and phenylacetylene have been found to strongly depend on temperature (Table 8) [213],... 

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. 

The reactivity trends observed for both the nucleophilic addition and cycloaddition of alkene radical cations parallel trends observed for the reactions of carbocations with nucleophiles and alkenes. However, the observed variations in reactivity towards oxygen - and substituent effects on the competition between addition of methanol and the neutral monomer for diphenylethene radical cations indicate that variations in both spin and charge density are important in determining the overall reactivity patterns. It is clear that further experimental and theoretical studies are required to provide a detailed model for understanding and ultimately predicting the reactivity of radical cations. 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

A second difference between alkene addition and aromatic substitution occurs after the carbocation intermediate has formed. Instead of adding Br- to give an addition product, the carbocation intermediate loses H+ from the bromine-bearing carbon to give a substitution product. Note that this loss of H+ is similar to what occurs in the second step of an El reaction (Section 11.10). The net effect of reaction of Br2 with benzene is the substitution of H+ by Br+ by the overall mechanism shown in Figure 16.2. 

The first step in the reaction of HX with an alkene is protonation to yield the more stable cation. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, then we can see that the preferred carbocation will be produced if protonation occurs... 

If both cr-bonds form at the same time, the reaction is pericyclic, but carbocations are capable of reacting with alkenes, and so the first bond may form to give the cyclopentenyl cation 2.51 in one step, with the second bond formed in a separate step 2.51 (arrows). The reaction remains a cycloaddition whether both bonds are formed at the same time or not, but it is pericyclic only if they are both formed in the same step, as is probable in this case. Fig. 2.6 shows a small selection of other reactions that may similarly be pericyclic. >... 

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... 

Markovnikov s rule (Section 11.2) In addition reactions of HX to alkenes, the H bonds to the carbon with more hydrogens (fewer alkyl substituents) and the X bonds to the carbon with fewer hydrogens (more alkyl substituents). A more modern version of Markovnikov s rule, based on mechanistic reasoning, is that the electrophile adds so as to form the more stable carbocation. 

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