Conjugated dienes radical addition

Free radical addition to conjugated dienes involves addition of a radical at position 1 of the diene, generating a resonance-stabilized radical A which will attack other molecules, e.g. X-Y via its 2- or 4-position, yielding 1,2- and 1,4-addition products B and C, respectively (Scheme 2.45). 

Like other alkenes, conjugated dienes undergo addition not only by electrophilic reagents but also by free radicals. In free-radical addition, conjugated dienes show two special features they undergo 1,4-addition as well as 1,2-addition, and they are much more reactive than ordinary alkenes. We can account for both features—orientation and reactivity—by examining the structure of the intermediate free radical. 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

For a review of free-radical addition to conjugated dienes, see Afanas ev, I.B. Samokh-valov, G.I. Russ. Chem. Rev., 1969, 38, 318. 

A convenient route to triquinanes is based on a strategy of silyl radical addition to conjugated dienes to form allylic type radicals and their subsequent intramolecular addition to C=C double bonds. By exposure of 10 to (TMS)3SiH and AIBN at 80 °C (Reaction 7.16) the triquinane 11 is obtained with an unoptimized 51 %> yield [26]. 

Radical addition of HBr to an alkene depends upon the bromine atom adding in the first step so that the more stable radical is formed. If we extend this principle to a conjugated diene, e.g. buta-1,3-diene, we can see that the preferred secondary radical will be produced if halogenation occurs on the terminal carbon atom. However, this new radical is also an allylic radical, and an alternative resonance form may be written. 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Problem 8.36 Write initiation and propagation steps in radical-catalyzed addition of BrCCl, to 1,3-butadiene and show how the structure of the intermediate accounts for a) greater reactivity of conjugated dienes than alkenes, (f>) orientation in addition. 

Anionic polymerization of conjugated dienes and olefins retains its lithium on the chain ends as being active moities and capable of propagating additional monomer. This distinguishing feature has an advantage over other methods of polymerization such as radical, cationic and Ziegler polymerization. Many attempts have been made to prepare block copolymers by the above methods, but they were not successful in preparing the clear characterized block copolymer produced by anionic technique. 

The scope and utility of cation radical induced cyclobutanation1 is greatly enhanced by the option of cross additions, the first of which was the formation of a 3 2 mixture of diastereomeric cyclobutanes in the irradiation of an equimolar mixture of phenyl vinyl ether and 1,1-dimethylindene in the presence of tetraphenylpyrylium tetrafluoroboratc in acetonitrile.2 The scope of PET cyclobutanations was further extended in a synthetic sense by the observation of cross additions of electron-rich alkenes to conjugated dienes.3 4 Examples of such reactions are shown below in the formation of compounds 1, 2, 3 and 4. 

In the reduction of conjugated dienes cis 1,4-addition of trimethyl-chlorosilane to give c -l,4-bis(trimethylsilyl)-2-butene is favored with sodium in THF, lithium naphthalide in THF, and with lithium in diethylether. It appears that the anion radical, which in nonionizing solvents should exist in a cis configuration, leads to the cis 1,4-addition of the silyl groups, whereas the dianions produced by further reduction lead to trans products (140). 

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