Resonance-stabilized allyl carbocation

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). 

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. 

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

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. 

The electrostatic potential maps in Figure 16.2 compare the resonance-stabilized allyl carbocation with CH3CH2CH2, a localized 1° carbocation. The electron-deficient region—the site of the positive charge—is concentrated on a single carbon atom in the 1° carbocation CH3CH2CH2 . In the allyl carbocation, however, the electron-poor region is spread out on both terminal carbons. 

Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate. 

Dienes are conjugated systems of two pi bonds. Above, the simplest diene, 13-butadiene, adds the electrophile to the end to produce a resonance stabilized allylic carbocation. As with alkenes, the more substituted the diene is, the more reactive. 

DMAPP may act as an alkylating agent (isopropene unit) via an 8 2 nucleophilic displacement in which the diphosphate is the leaving group. In some cases, DMAPP may ionize first to the resonance-stabilized allylic carbocation, and thus an S l reaction occurs on the C-activated position (Figure 1.12). 

Kinetic versus thermodynamic control. A plot of Gibbs free energy versus reaction coordinate for Step 2 in the electrophilic addition of HBr to 1,3-butadiene. The resonance-stabilized allylic carbocation intermediate reacts with bromide ion by way of the transition state on the left to give the 1,2-addition product. It reacts with bromide ion by way of the alternative transition state on the right to give the 1,4-addition product. 

Why is the 1,2-addition product (the less stable product) formed more rapidly at lower temperatures First, we need to look at the resonance-stabilized allylic carbocation intermediate and determine which Lewis structure makes the greater contribution to the hybrid. We must consider the degree of substitution of both the positive carbon and the carbon-carbon double bond in each contributing structure. 

Is the 1,2-addition product also formed more rapidly at higher temperatures even though the 1,4-addition product predominates under these conditions The answer is yes. The factors affecting the structure of a resonance-stabilized allylic carbocation intermediate and the reaction of this intermediate with a nucleophile are not greatly affected by changes in temperature. 

The formation of these two products can be explained with a similar two-step mechanism protonation to form a carbocation followed by nucleophilic attack. In the first step, protonation creates the more stable, resonance-stabilized, allylic carbocation, rather than an unstabilized primary carbocation. 

But this compound is symmetrical, with Cl being equivalent to C4, and C2 being equivalent to C3. Therefore, there are only two distinct possibilities for the protonation step at Cl or at C2. The resonance-stabilized, allylic carbocation is only formed via protonation at Cl ... 

The pyrophosphate leaving group Is expelled to give a resonance-stabilized, allylic carbocation... 

FIGURE 12.45 Protonation (D" in this case) of 1,3-pentadiene gives a resonance-stabilized allylic carbocation in which both contributing resonance forms are secondary. Addition of chloride at the two positions must take place to give equal amounts of 1,2- and 1,4-addition. 

Carbocations, radicals, and carbanions can be stabilized by resonance. For example, if a carbon atom with a 71 bond is bonded to the trivalent carbon atom of the intermediate, the empty orbital of that carbon atom can interact with the 2p orbitals of the n bond. The result is a resonance-stabilized intermediate. The resonance forms of a resonance-stabilized allylic carbocation intermediate are shown below. 

First, draw the structure of the resonance-stabilized allylic carbocation that forms when the carbon-bromine bond breaks. Second, draw the structures of the resonance-stabilized allylic carbocation. Third, add a bromide ion to the carbocation to obtain the isomeric bromine compound whose ionization would give the same resonance-stabilized carbocation. 

The pyrophosphate leaving group is expelled to give a resonance-stabilized (allylic) carbocation. The k bond of isopentyl phosphate then functions as a nucleophile and attacks the carbocation. Finally, a basic amino acid residue of the enzyme removes a proton to give the product ... 

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