Electrophilic Additions to Conjugated Dienes

This section only refers to molecules in which the double bonds alternate and hence interact through the extended molecular orbitals that we saw in Chapter 3. When there are two double 

FIGURE 11.25 Addition of chlorine to nonconjugated double bonds. 

Let s think about what will happen if we protonate butadiene. We will prefer to make the allylic cation, 11.7, rather than the isolated primary cation 11.8. 11.7 has two resonance forms the secondary cation is the more important contributor to the overall structure. So in adding electrophiles to dienes, we might expect that most of the product would come from the secondary form of the allylic cation. 

Under conditions of thermodynamic control, 1,4-dimethylcyclohexadiene adds bromine to give a mixture of the products shown, with the 1,2-addition product predominating. Explain this observation. 

Our first step must be to react bromine with the diene to form the allyl cation (because the allyl cation is very well stabilized by resonance, the formation of bromonium ions is not favored). As usual, we must consider both resonance forms, and the tertiary allyl cation will contribute more to the overall structure. 

---

Electrophilic Additions to Conjugated Dienes Allylic Carbocations 48i... 

Figure 8.2 Energy profile 1,2 and 1,4 electrophilic addition to conjugated diene...

For a review of electrophilic addition to conjugated dienes, see Khrislov Angelov Petrov Russ. Chem. Rev. 1991, 60. 39-56. 

The composition of the product mixture formed upon electrophilic addition to conjugated dienes often changes with the temperature at which the reaction is conducted. For example, when HBr is added to 1,3-butadiene at — 80°C, the major product is formed by 1,2-addition. In contrast, when the reaction is run at 45°C, the major product is the one resulting from 1,4-addition ... 

Electrophilic additions to conjugated dienes usually involve allylic cations as intermediates. Unlike simple carbocations, an allylic cation can react with a nucleophile at either of its positive centers. Let s consider the addition of HBr to buta-1,3-diene, an electrophilic addition that produces a mixture of two constitutional isomers. One product, 3-bromobut-l-ene, results from Markovnikov addition across one of the double bonds. In the other product, 1 -bromobut-2-ene, the double bond shifts to the C2—C3 position. 

Thus the hybrid nature of the allyl cation governs both steps of electrophilic addition to conjugated dienes the hrst, through stabilization the second, by permitting attachment to either of two carbon atoms. 

The fact is that conjugated dienes are more reactive than simple alkenes. In the present case, then—and in most cases involving alkenes and free radicals, or alkenes and carbonium ions—the factors stabilizing the transition state are more important than the factors stabilizing the reactant. However, this. is not always true. (It does not seem to be true, for example, in electrophilic addition to conjugated dienes.)... 

As we look at more examples, notice that the first step in all electrophilic additions to conjugated dienes is addition of the electrophile to one of the sp carbons at the end of the conjugated system. This is the only way to obtain a carbocation that is stabilized by resonance (i.e., by electron delocalization). If the electrophile were to add to one of the internal sp carbons, the resulting carbocation would not be stabilized by resonance. 

As with alkenes, the regioselectivity of electrophilic addition to conjugated dienes is governed by the stability of the resulting carbocation. Protonation of a conjugated diene always occurs at the end of the diene unit because an allylic carbocation results. 

Electrophilic Addition to Conjugated Dienes UV-Visible Spectroscopy Pericyclic Reaction Theory... 

---