Carbocations addition

Note This reaction involves a polar acidic mechanism, not a free-radical mechanism It is a Friedel-Crafts alkylation, with the slight variation that the requisite carbocation is made by protonation of an alkene instead of ionization of an alkyl halide. Protonation of C4 gives a C3 carbocation. Addition to Cl and fragmentation gives the product. 

Now let s address the issue of why carbon 1 is the one that is initially protonated. According to the mechanistic rule, the electrophile—the proton—should add so as to produce the most stable carbocation. We have already seen that addition of the proton to carbon 1 produces a resonance stabilized carbocation. Addition to the other carbons produces the following carbocations ... 

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]. 

The few available data for carbocation additions to cycloalkenes (Scheme 41) show an analogous reactivity order Cyclopentenes are more reactive than the acyclic analogs, and the only cyclohexene derivative shown in Scheme 41 is less reactive. Because of the paucity of data, this analogy should not be overinterpreted. The location of norbornene between the compounds which give secondary and tertiary carbocations has already been mentioned (Scheme 36 in Section III.D.4.a.). 

Because hydrohalogenation begins with a planar double bond and forms a planar carbocation, addition of H and Cl occurs in two different ways. The elements of H and Cl can both be added from the same side of the double bond— that is, syn addition—or they can be added from opposite sides—that is, anti addition. Both modes of addition occur in this two-step reaction mechanism. 

The n bond attacks the H atom of HBr to form a new C-H bond, generating a vinyl carbocation. Addition foiiows Markovnikov s ruie H adds to the iess substituted carbon atom to form the more substituted, more stable carbocation. Nucleophiiic attack of Br then forms a vinyi bromide one moie of HBr has now been added. 

Since cationic polymerization involves carbocations, addition follows Markovnikov s rule to form the more stable, more substituted carbocation. Chain termination can occur by a variety of pathways, such as loss of a proton to form an alkene. Examples of alkene monomers that undergo cationic polymerization are shown in Figure 30.4. 

Since the product of the carbocation addition to an alkene via path Ag is also a carbocation that can rearrange and/or attack another alkene molecule (polymerization), unwanted product mixtures can result. 

The mechanism of this hydrolysis reaction has been studied in great detail. The mechanism is the reverse of that for acetal formation. Acetal protonation is followed by elimination of an alcohol molecule. The resulting intermediate is a stabilized carbocation. Addition of water and a second acid-catalyzed elimination lead to the product. 

Dienes which have been reacted with phenols include buta-1,3-diene, isoprene (2-methylbuta-1,3-diene) and cyclopentadiene, all of which are perhaps more familiar in their reactions towards dienophiles in Diels-Alder additions. All are available from the petrochemical industry and their use in alkylations is an aspect of electrophilic substitution through carbocation addition. Because of their reactive nature, alkenyiphenols during synthesis frequently undergo further transformations. Indeed this is an aspect of their reactions and reactivity which has been comparatively little studied although in previous chapters intermittent reference has been made in the case of certain specific structures. 

The disconnection for carbocation addition with strong acids HX is ... 

Solvolysis of 4-twistyl (145 both epimers) and 10-protoadamantyl (146) tosylates in 70% aqueous acetone, ethanol, or acetic acid affords a mixture of 4-exo-twistyl and the predominant 10-exo-protoadamantyl derivatives by way of the common bridged carbonium ion intermediate (147) kinetic data reveal that the reaction of exo-(145) is anchimerically assisted, and hence endo- l45) is converted into (147) by leakage from the initial classical carbocation. Additionally an energy difference of 4.4kcalmol between twistane and protoadamantane systems is estimated. ... 

Carbon—carbon double bonds are also nucleophiles and can act as neighboring groups in intramolecular displacement reactions. Among the examples you already know are the sequence of intramolecular carbocation additions leading to steroids (p. 560) and intramolecular carbene additions (p. 434). In each of these reactions, a carbon-carbon double bond acts as a nucleophile toward an empty carbon Ip orbital as Lewis acid (Fig. 21.29). 

All of the ether oxygens can be protonated, but only in one case can alcohol be lost with assistance from the neighboring group to give a resonance-stabilized carbocation. Addition of water to the cation followed by deprotonation leads to the tetraether hemiacetal shown in Rgure 22.41 (both anomers are formed). 

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