Allylic carbocations resonance

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

The radical is much more stable if both stmctures exist. Quantum mechanical theory implies that the radical exists in both states separated by a small potential. Moreover, both molecular orbital theory and resonance theory show that the allyl carbocation is relatively stable. 

The ally carbocation is an example of an intermediate whose structure has been extensively investigated by MO methods. The hybridization/resonance approach discussed earlier readily rationalizes some of the most prominent features of the allyl carbocation. The resonance structures suggest a significant stabilization and imply that the molecule would be planar in order to maximize the overlap of the n system. 

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates (recall that allylic means "next to a double bond"). When 1,3-butadiene reacts with an electrophile such as H+, two carbocation intermediates are possible a primary nonal-lylic carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 11.5), it is more stable and forms faster than a nonallylic carbocation. 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

An example of a biological Friedel-Crafts reaction occurs during the biosynthesis of phylloquinone, or vitamin Kl( the human blood-clotting factor. Phylloquinone is formed by reaction of 1,4-dihydroxynaphthoic acid with phytyl diphosphate. Phytyl diphosphate first dissociates to a resonance-stabilized allylic carbocation, which then substitutes onto the aromatic ring in the typical way. Several further transformations lead to phylloquinone (Figure 16.10). 

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Geranyl diphosphate and farnesyl diphosphate are analogues of dimethylallyl diphosphate that contain two and three C5 subunits respectively they can undergo exactly the same SnI reactions as does dimethylallyl diphosphate. In all cases, a carbocation mechanism is favoured by the resonance stabilization of the allylic carbocation. Dimethylallyl diphosphate, geranyl diphosphate, and farnesyl diphosphate are precursors for natural terpenoids and steroids. 

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. 

At first glance, this appears to be a secondary carbocation, bnt on further exanfination one can see that it is also an allylic cation. Allylic carbocations are stabilized by resonance, reselling in dispersal of the positive charge (see Section 6.2.1). From these two resonance forms, we can predict that both carbons 2 and 4 will be electron deficient. Now this has particnlar consequences when we consider subseqnent attack of the nucleophile on to the carbocation. There are two possible centres that may be attacked, resnlting in two different prodncts. The prodncts are the result of either 1,2-addition or 1,4-addition. The addition across the fonr-carbon... 

Protonation on C-2 gives an unfavourable primary carbocation. On the other hand, protonation on C-1 gives a favourable resonance-stabilized allylic carbocation. The two products are then formed by captnre of chloride in a ratio that reflects the relative contribution of the limiting structures for the allylic carbocation one is tertiary and the other is primary. 

You may think that is the end of the problem but, since we have an unsymmetrical diene, it is also necessary to consider protonation of the other double bond. Protonation on C-4 also gives a favourable resonance-stabilized allylic carbocation, this time with primary and secondary limiting structures. Protonation on C-3 gives an unfavourable primary carbocation with no resonance stabilization. Since the products formed are related to initial protonation at C-1, it is apparent that, despite the stability associated with an allylic cation, a tertiary limiting structure is formed in preference to that with a secondary limiting structure. 

The electrophile (H ) adds to form an allylic carbocation with positive charge delocalized at C and C (resonance forms II and III). This cation adds the nucleophile at C to form the 1,2-addition product or at C" to form the 1,4-addition product. 

The transition state and the intermediate R formed by a-attack is a hybrid of three resonance structures which possess less energy the intermediate from -attack is less stable and has more energy because it is a hybrid of only two resonance structures. I and II are also more stable allylic carbocations V is not allylic. 

Double bonds favor allylic cleavage and give the resonance-stabilized allylic carbocation. This rule... 

SAMPLE SOLUTION (a) When writing resonance forms of allylic carbocations, electrons are moved in pairs from the double bond toward the positively charged carbon. 

The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-l gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Protonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. 

The seven resonance forms for tropylium cation (cycloheptatrienyl cation) may be generated by moving tt electrons in pairs toward the positive charge. The resonance forms are simply a succession of allylic carbocations. 

Conjugated dienes have alternating single and double bonds. They may undergo 1,2- or 1,4-addition. Allylic carbocations, which are stabilized by resonance, are intermediates in both the 1,2- and 1,4-additions (Sec. 3.15a). Conjugated dienes also undergo cycloaddition reactions with alkenes (Diels-Alder reaction), a useful synthesis of six-membered rings (Sec. 3.15b). 

Secondly, it explains why the alternative 1,2-addition product is not formed(Fig.K). The intermediate carbocation required for this 1,2-addition cannot be stabilised by resonance. Therefore, the reaction proceeds through the allylic carbocation instead. 

Double bonds favor allylic cleavage and give the resonance-stabilized allylic carbocation. This rule does not hold for simple alkenes because of the ready migration of the double bond, but it does hold for cycloalkenes. 

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 fragmentation of an alkene to produce an allylic carbocation is usually a major pathway because of resonance stabilization ... 

The benzyl carbocation, like the allyl carbocation, has substantial resonance stabilization. The base peak at mlz 91 in the spectrum of butylbenzene (see Figure 15.11) results from a fragmentation that produces a benzylic carbocation as illustrated in the following equation. (We will see in Chapter 16 that the benzylic carbocation rearranges to an isomeric carbocation, which is even more stable.)... 

A and D, which are resonance-stabilized, are formed in preference to B and C, which are not. The positive charge of allylic carbocation A is delocalized over two secondary carbons, while the positive charge of carbocation D is delocalized over one secondary and one primary carbon. We therefore predict that carbocation A is the major intermediate formed, and that 4-chloro-2-pentene predominates. Note that this product results from both 1,2 and 1,4 addition. 

Resonance effect further stabilizes the carbocations when present. By resonance the positive charge on the central carbon atom gets dispersed over other carbon atoms and this renders stability to the carbocation. The more the canonical (resonating) structures for a carbocation, the more stable it will be. For example, benzyl and allyl carbocations are very stable because of resonance. 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

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