Vinyl radical attack

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

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... 

Free-radical cyclization reactions nicely complement the Pd(0)-catalyzed intramolecular Heck reaction, which also provides cyclic products from unsaturated halides. Free radicals can be generated easily at saturated carbons from saturated alkyl bromides, and the products are reduced relative to the reactants. In contrast, intramolecular Heck reactions work best for vinyl and aryl bromides (in fact they do not work for alkyl halides), and the products are at the same oxidation level as the reactants. Moreover, free radicals attack the double bond at the internal position, whereas palladium insertion causes cyclization to occur at the external carbon. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

Vinyl radicals also add to carbon-carbon double bonds intramolecularly to give 2,6-cw-disubstituted cyclic ethers (Equation (5)).41 In the tin hydride-mediated cyclization of the substrates including alkynes, alkyl radicals attack to carbon-carbon triple bonds leading to uco-alkylidene allylic alcohols (Equation (6)).42 The coupling reaction between alkyl radicals may afford cyclization products. Thus, the reduction of 1,3-diiodopropane derivatives with a tin hydride provides substituted cyclopropanes.4... 

The co-condensation reaction of SiF2 with vinyl chloride produced, in addition to the addition products, a small quantity of insertion product. However, here the results mainly reflect the very large difference in the preference of initial radical attack on the two sides of the double bond (63). 

Random copolymers will be formed, or course, if each radical attacks either monomer with equal facility (kn =k 2, kn = 21, 1 = 2 = I). Free-radical copolymerization of ethylene and vinyl acetate is an example of such a system, but this is not a common case. Random monomer distributions are obtained more generally if k /k 2 is approximately equal tok2i/k22- That is to say, r I jr2- This means that k /k22 and A 2i / 22 will be simultaneously either greater or less than unity or in other words, that both radicals prefer to react with the same monomer. 

In this protocol the pyridine- and the pyrrolidine-ring of 1 are built up in a one-pot radical domino reaction." Photolysis of iodopyridone 11 in the presence of hexamethylditin provides radical 56, which attacks the reactive isonitrile 15. The resulting radical 57 reacts with the alkyne group in a 5-exo-dig cyclization (see Chapter 11). Next, the newly formed vinyl radical 58 cyclizes onto the aryl ring generating speeies 59. Final oxidation via a so far unknown mechanism yields 1 with 31 % yield. For the generation of radical 56 by photolysis two ways (A and B) are possible. 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

The proposed mechanism involves formation of a vinyl radical that attacks the M(l) reagent, leading to a transient M(II) species that reacts with a second equivalent of radical, affording the M(I) vinyldene and isobutene. 

Homolytic processes Evidence also has been presented for the radical-induced decomposition of thiolsulfinates (6,7). Homolytic cleavage is facilitated by the weak S-S bond ( 40kcal). The availability of sulfidic sulfur for radical attack is indicated by the observation that thiolsulfinates strongly retard the free radical polymerization of vinyl monomers (8). 

Finally, a novel three-component radical cascade reaction involving isonitriles has just been published [6]. In this paper, aromatic disulfides, alkynes, and isonitriles have been reported to react under photolytic conditions to afford -arylthio-substituted acrylamides 49 or acrylonitriles 50 in fair yields as mixtures of the E and Z geometric isomers (Scheme 21). The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical on the isonitrile. A fast reaction, for example, scavenging by a nitro-derivative (route a) or f-fragmentation (route b), is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The reaction provides very easy access to potentially useful poly-functionalized alkenes through a very selective tandem addition sequence. 

BusSnH-mediated intramolecular arylations of various heteroarenes such as substituted pyrroles, indoles, pyridones and imidazoles have also been reported [51]. In addition, aryl bromides, chlorides and iodides have been used as substrates in electrochemically induced radical biaryl synthesis [52]. Curran introduced [4-1-1] annulations incorporating aromatic substitution reactions with vinyl radicals for the synthesis of the core structure of various camptothecin derivatives [53]. The vinyl radicals have been generated from alkynes by radical addition reactions [53, 54]. For example, aryl radical 27, generated from the corresponding iodide or bromide, was allowed to react with phenyl isonitrile to afford imidoyl radical 28, which further reacts in a 5-exo-dig process to vinyl radical 29 (Scheme 8) [53a,b]. The vinyl radical 29 then reacts in a 1,6-cyclization followed by oxidation to the tetracycle 30. There is some evidence [55] that the homolytic aromatic substitution can also occur via initial ipso attack to afford spiro radical 31, followed by opening of this cyclo-... 

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