Resonance stabilization carbocations

An electrophilic aromatic substitution reaction takes place in two steps—initial reaction of an electrophile, E+, with the aromatic ring, followed by loss of H+ from the resonance-stabilized carbocation intermediate to regenerate the aromatic ring. 

PROBLEMS For each compound below, determine whether the LG leaving would form a resonance-stabilized carbocation. If you are not sure, try to draw resonance strnctnres of the carbocation you would get if the leaving group is expelled. 

The possibility of nucleophilic attack on different carbons in the resonance-stabilized carbocation facilitates another modification exploited by nature during terpenoid metabolism. This is a change in double-bond stereochemistry in the allylic system. The interconversions of geranyl diphosphate, linalyl diphosphate, and neryl diphosphate provide neat but satisfying examples of the chemistry of simple allylic carbocations. 

Initially, the reaction involves protonation of one of the oxygen atoms, followed by loss of this group as a neutral molecule and formation of a resonance-stabilized carbocation. If the oxygen protonated were that of the alkoxy group, then the product would merely be the protonated aldehyde, and the reaction becomes a reversal of hemiacetal formation. Only when the oxygen of the hydroxyl is protonated can the reaction lead to an acetal, and this requires nucleophilic attack of the second alcohol molecule on to the alternative resonance-stabilized carbocation. 

Treatment of 3-(2-furylmethylene)-5-aryl-2(3//)-furanones (38) with A1C13 in benzene led to alkyl-oxygen ring cleavage to give resonance stabilized carbocations 39, which upon electrophilic attack in the ortho position yield the corresponding benzofurancarboxylic acids (40) (Scheme 12) (77JPR689). 

This difference in behavior between furanones 41 and those having an exocyclic double bond at position 3 (which under the same conditions produce acids) was attributed to the fact that the latter can afford resonance stabilized carbocations because of the extended conjugation with the aryli-dene moiety in position 3. 

At this point, H+ transfer could occur directly to give the product [Cu2(R—XYL—0—)(0H)]2+ (12), or Y could undergo a Wagner-Meerwein rearrangement [170] (i.e., formal migration of Y in a N.I.H. shift) to produce another resonance stabilized carbocation intermediate. 

Protonation of the hydroxyl group and loss of water gives a resonance-stabilized carbocation. Nucleophilic attack of methanol on the carbocation followed by loss of a proton gives the acetal. 

Loss of methanol gives a resonance-stabilized carbocation ... 

Anodic oxidation of homo allyltrimethylsilylmethyl ethers 238 or homo allyl trimethyl-stannyl methyl ethers in the presence of tetrabutylammonium tetrafluoroborate results in the formation of fluorine- containing tetrahydropyrans 239249(equation 131). The process involves formation of a resonance stabilized carbocation and its intramolecular cycliza-tion by the participation of a neighboring vinyl group, followed by attack of fluoride ion. This process is a convenient way to form the C—F bond involving electrochemical steps. 

The carbocation is stabilized (tertiary or resonance stabilized carbocations are best secondary carbocations are acceptable if other factors are favorable primary carbocations are not formed). 

Protonation of the alcohol can be accomplished by using the halogen acids, HC1, HBr, and HI, which also provide the nucleophile for the reaction. These reaction conditions favor the SN1 mechanism, although primary alcohols still follow the SN2 path unless a resonance-stabilized carbocation can be formed. The acids HBr and HI work with most alcohols, but HC1, a weaker acid, requires the presence of ZnCl2 (a Lewis acid) as a catalyst when the alcohol is primary or secondary. Examples are shown in the following equations ... 

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 ... 

Then, reaction of the alcohol with triphenylphosphine under acidic conditions produces the phosphonium salt by an SNI reaclion. A resonance-stabilized carbocation is formed and reacts at the terminal position of the chain. 

In general, we should expect rearrangements in reactions involving carbocations whenever a hydride shift or an alkyl shift can form a more stable carbocation. Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations. 

In addition to losing water, alcohols commonly fragment next to the carbinol carbon atom to give a resonance-stabilized carbocation. This fragmentation is called an alpha cleavage because it breaks the bond next to the carbon bearing the hydroxyl group. 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

McLafferty Rearrangement of Ketones and Aldehydes The mass spectrum of butyraldehyde (Figure 18-4) shows the peaks we expect at m/z 72 (molecular ion), m/z 57 (loss of a methyl group), and m/z 29 (loss of a propyl group). The peak at m/z 57 is from cleavage between the /3 and y carbons to give a resonance-stabilized carbocation. This is also a common fragmentation with carbonyl compounds like the other odd-numbered peaks, it results from loss of a radical. 

Figure 5.6 Proposed mechanism for the cyclization of geranyl diphosphate to sabinene and sabinene hydrate under catalysis by monoterpene synthases the reaction begins with the hydrolysis of the diphosphate moiety to generate a resonance-stabilized carbocation (1) the carbocation then isomerizes to an intermediate capable of cyclization by return of the diphosphate (2) and rotation around a single bond (3) after a second diphosphate hydrolysis (4) the resulting carbocation undergoes a cyclization (5) a hydride shift (6) and a second cyclization (7) before the reaction terminates by deprotonation (8) or capture of the cation by water (9). Cyclizations, hydride shifts and a variety of other rearrangements of carbocationic intermediates are a characteristic of the mechanisms of terpene synthases. No known terpene synthase actually produces both sabinene and sabinene hydrate these are shown to indicate the possibilities for reaction termination. PP indicates a diphosphate moiety.

Markovnikov s rule applies to the addition of HX to vinyl halides because addition of H forms a resonance-stabilized carbocation. As a result, addition of each equivalent of HX to a triple bond forms the more stable carbocation, so that both H atoms bond to the less substituted C. 

Tautomerization of the enol to the keto form occurs by protonation of the double bond to form a carbocation. Loss of a proton from this resonance-stabilized carbocation generates the more stable keto form. 

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