1,3-Dienes radical attack

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

In conclusion, the regiochemistry of radical attack at dienes appears to be rather predictable by considering steric and electronic effects. Attack is almost always preferred at the terminal carbon atoms of the diene. a-Substituents retard the addition significantly in all known cases while the steric effects of -substituents depend on the nature of the attacking radical. 

As far as the mechanisms of branching and crosslinking are concerned, there appear to us to be certain weaknesses in those commonly accepted. With ethylenic monomers, there can be little doubt that if branching were to occur at all, it will arise from radical attack upon the polymer already formed. It would be immaterial whether this transfer takes place on backbone carbon atoms or via side chains, as is almost certainly true for, say, vinyl acetate. When dienes are present, it has been generally accepted that the residual double bonds are the main seat of reaction, thereby creating the immediate possibility of crosslinking. However, the internal residual double bonds—that is, those... 

Encounter complexes between photoexcited 1-cyanonaphthalene and norbor-nadiene lead to products of several structural types. In polar solvents containing methanol, adducts are formed from exo-attack by methanol on the norboma-diene radical cation. In less polar solvents 1-cyanonaphthalene and norboma-diene form [2+2]adducts, exclusively on the norbomadiene exo-face. In non-polar solvents an endo-encounter complex between 1-cyanonaphthalene and norbomadiene is attacked by methanol from the exo-face to yield 1 1 1 adducts. SET to 1,4-DCB from readily accessible tertiary aminocyclopropanes yields cation radicals which undergo facile ring opening. A subsequent 1,5-hydrogen shift and hydrolytic workup provides ring-opened ketones for example E- or Z-(211) yields (212) in 85% yield. ... 

Perfluorobuta-1,3-diene becomes incorporated in the terpolymer via both 1,2- and 1,4-addition, and similar reactions involving the backbone unsaturation (CF=CF) sites are also possible analysis of vulcanizates by i.r. spectroscopy reveals that radical attack at the less hindered pendant olefinic sites is preferred. ... 

Also developed by Hill is a photochemical system (equations 41 to 48) based on a polyoxoacid, H3PW12O40 (P)< The excited state of the acid probably oxidizes the alkane in the first step. The radical can dien either attack the solvent to give an iminium radical, which leads to ketone on hydrolysis, or it can be oxidized to the carbonium ion, in which case attack on the solvent leads instead to the iV-alkyl-acetamide. If the substrate has two adjacent tertiary C—bonds, then alkenes tend to be fonned. The Barton reaction, normally known as an intramolecular C—activation, can give some intermolecular reaction in some examples. Thus, when /i-octyl nitrite is photolyzed in heptane, some nitrosoheptane is observed. ... 

Several groups have reported studies of free-radical additions to fluorinated acetylenes. The y-ray-initiated addition of ethanol or isopropanol to per-fluorobut-2-yne, which yields allylic alcohols capable of dehydration to l,l,l-trifluoro-3-trifluoromethylpenta-2,4-dienes, has been claimed in a patent." Also cited was addition of ethanol to 3,3,3-trifluoropropyne the hydroxyalkyl radical attacks the terminal carbon atom in this acetylene ... 

The extreme readiness with which tetrafluoroallene homopolymerises has prevented a study of the orientation of free-radical attack on the allenic system. Perfluoropenta-1,2-diene, however, does not polymerise readily, and so its photochemical chlorination and hydrobromination have been studied. ... 

With 1,3-diene based polymers, greater scope for structural variation is introduced because there arc two double bonds to attack and the propagating species is a delocalized radical with several modes of addi tion possible (see 4.3.2). 

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. ... 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

Most of the dienes investigated experimentally show high regioselectivity in radical addition reactions. The preferred position of attack is shown in Scheme 2. 

The preferred site for radical addition to a cumulated Jt-system of a 1,2-diene depends on (i) the degree of substitution of the allene, (ii) the nature of the attacking radical and (iii) reaction parameters such as temperature and initial reactant concentrations. 

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