Intramolecular reactions radical propagation

These equations are similar to those assumed for the reactivity ratio determination. In contrast to what has been observed for conventional styrene-MMA copolymers, however, these equations indicate that a substantial proportion of the (SMM+MMS)-type resonance appears to occur in the C-area. The proportion of methoxy resonance observed in the C-area, in fact, exceeds P(SMS) by a substantial amount for many of the copolymers. This can be due to the assumption of an inadequate model for the copolymerization reaction, to the use of incorrect reactivity ratios and cyclization constants for the calculations or to an inadequate understanding of the methoxy proton resonance patterns of S/MMA copolymers. It is possible that intramolecular reactions between propagating radicals and uncyclized methacrylic anhydride units present on propagating chains result in the formation of macrocycles. Failure to account for the formation of macrocycles would result in overestimation of rc and rc and in underestimation of the proportions of MMA units in SMS triads in the derived S./MMA copolymers. This might account for the results obtained. An alternate possibility is that a high proportion (>50%) of the M-M placements in the copolymers studied in this work can be expected to have meso placements (], J2), whereas only a small proportion of such placements ( 20%) are meso in conventional S/MMA copolymers. Studies with molecular models (20) have indicated that the methoxy protons on MMA units centered in structures such as the following can experience appreciable shielding by next nearest styrene units. 

When steric and electronic features of the radical intermediates do not allow the desired radical propagation steps to be sustained, the product yield may be low. In such cases, more favorable - but often undesirable - processes, such as H-abstrac-tion from the solvent, may become dominant [503]. Especially intriguing are scaffolds on which radical and ionic cyclizations lead to different products. Thus, Ber-teina et al. described the formation of l-alkylidene-5H-dihydrobenzofuran and l-alkyl-5-dihydrobenzofuran from o-iodobenzyl vinyl ethers. The former product was formed by an intramolecular Heck reaction, while the latter was formed in high yields in a radical cyclization [504],... 

However, the units without double bonds such as cyclobutane and other unknown large cyclic units generated by the intramolecular reaction of double bonds with propagating radical species were also formed in the main chain (5). As a result, vinylcyclopropanone cyclic acetals showed small volume shrinkage. 

Although there may be some minor contribution of intermolecular chain transfer, these systematic studies have provided a clearer perspective of the mechanism of the intramolecular chain transfer reaction of propagating acrylate radicals. Nevertheless further investigation was required to provide decisive proof of the mechanism. 

The mechanisms of intramolecular free-radical cyclization reactions are no different from their intermolecular counterparts. The propagation part of the mechanism usually involves (1) abstraction of -Br, T, or -SeR by Bu3Sn- to give an alkyl radical, (2) one or more additions of an alkyl radical to a tt bond, and... 

The primary cydization occurs by propagation of a radical with pendant double bonds on the same chain. It is an intramolecular reaction that strongly depends on chain configuration. The instantaneous secondary cydization occurs between the chains that are connected by instantaneous cross-linking. The additional secondary cydization occurs between the chains that are conneded by additional aoss-linking. As the first approximation... 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

The peroxyl radical of a hydrocarbon can attack the C—H bond of another hydrocarbon. In addition to this bimolecular abstraction, the reaction of intramolecular hydrogen atom abstraction is known when peroxyl radical attacks its own C—H bond to form as final product dihydroperoxide. This effect of intramolecular chain propagation was first observed by Rust in the 2,4-dimethylpentane oxidation experiments [130] ... 

It was proposed that an initially generated silyl radical 3, by reaction of i-BuO radical and polysilane 2, attacks another silicon atom in the same backbone to give a cyclic polysilane that contains an acyclic chain and another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize or abstract a hydrogen atom from another macromolecule, thus propagating the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular homolytic substitution (ShO, a class of reactions discussed in Chapter 6. 

The degree of polymerization depends on the duration of the process. After 7 min, the molecular mass is equal to 9400 (the polydispersity index is 5.30). When the reaction is carried out for 15 min, the molecular mass of the polymer increases to 37,000 and the polydispersity index reaches 7.31 (Bauld et al. 1996). Depending on whether cation-radical centers arise at the expense of intramolecular electron transfer or in a stepwise intermolecular lengthening, polymerization can occur, respectively, through a chain or a step-growth process (Bauld and Roh 2002). In the reaction depicted in Scheme 7.17, both chain and step-growth propagations are involved. 

The very high value of Cm for vinyl chloride is attributed to a reaction sequence involving the propagating center XVIII formed by head-to-head addition [Hjertberg and Sorvik, 1983 Llauro-Darricades et al., 1989 Starnes, 1985 Starnes et al., 1983 Tornell, 1988]. Intramolecular migration of a chlorine atom (Eq. 3-114) yields the secondary radical XIX that subsequently transfers the chlorine atom to monomer (Eq. 3-115) to yield poly(vinyl chloride)... 

The importance of intramolecular cyclization was emphasized when Butler and coworkers found that the radical polymerization of N, N, N, /V-diallyldimethylammonium chloride (DADMAC) gave soluble, uncrosslinked polymers with little or no unsaturation (Eq. 6-101) [Butler and Angelo, 1957 Butler and Ingley, 1951 Wandrey et al., 1999]. There is a very low tendency for radical IV to propagate intermolecularly and undergo crosslinking. The predominant reaction is intramolecular cyclization, and the product is a linear product with cyclic structures in the backbone. The reaction is referred to as alternating intra/intermolecular polymerization or cyclopolymerization. 

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

The reason these so-called cyclopolymerization reactions occur is not known with certainty. Perhaps the best explanation that seems to fit most of the experimental results is that the two alkenic groups are associated in the ground state. Intramolecular cyclopolymerization is thus favored over intermolecular polymerization even before attack by a radical, and the overall cyclization process, from attack by the radical to formation of the propagating cyclic radical, is concerted. 

The free-radical construction of C—C bonds either inter- or intramolecularly using a hydride as mediator is of great importance in chemical synthesis. The propagation steps for the intermolecular version are shown in Scheme 2. For a successful outcome, it is important (i) that the R sSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene and (ii) that the alkyl radical reacts faster with alkene (to form the adduct radical) than with the silane. In other words, for a synthetically useful radical chain reaction, the intermediates must be disciplined. Therefore, in a synthetic plan one is faced with the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and, often, the concentration of silane provides the variable by which the products distribution can be influenced. 

The photostimulated reaction of 1,8-diiodonaphthalene with p-methyl-benzenethio-late ions in DMSO yields the substituted cyclized product 10-methyl-7-thia-benzo[de] anthracene (31) in moderate yield (Scheme 10.58) [54], The mechanism proposed to explain product 31 involves an intramolecular radical cyclization after monosubstitution in the propagation cycle of the SRN1 process. 

Exploring different methods for the intramolecular radical cyclization of 78 (Scheme 15)95, Usui and Paquette observed that (TMS SiH under normal conditions affords the expected functionalized diquinane 79 in 80% yield and in a a fi ratio of 82 18. MM2 calculations suggest it is the result of a kinetic controlled process. It is worth mentioning that the endothermic reaction 42 is expected to be one of the propagation steps in this chain process (vide infra). By replacing the silane with tin hydride under similar experimental conditions, the unexpected product 80 was obtained in a 77% yield. 

The fragmentation of the radical anion (RNu)- along the chain propagation cycle of the process has been taken as evidence of the proposed mechanism. For example, in the case of dihalobenzenes YArX, the radical anion formed upon the first substitution YArNu- may transfer the extra electron to the C—Y bond (intramolecular ET) or to YArX (inter-molecular ET). The ratio between monosubstituted and disubstituted products formed will depend on the relative rate constants for both types of competing ET reactions. 

These results indicate that when the intermediate 3-halo- 1-adamantyl radical couples with Ph2P" ions, it forms the radical anion (38)" which reacts via two competitive reactions. One is the intramolecular ET to the C—X <7 bond, which fragments to give the radical 39 that enters the propagation cycle to give ultimately 36 or is being reduced to 37. The other possibility is the intermolecular ET to the substrate to give the monosubstitution product 38 (equation 46). 

In the photostimulated reaction of 2-naphthoxide ion (198) with an o-dihalobenzene, an aromatic a radical may be formed very close to the oxygen functionality along the chain propagation cycle of the S l mechanism. This spatial proximity and the fact that the intramolecular coupling between the two moieties will form a relatively stable radical anion will favour the reaction between both reactive centres. Thus in the photostimulated reaction of o-dihalobenzenes with 198 in liquid ammonia, the formation of the monosubstitution 351 and of the cyclization product 352 were reported in yields that depend on the substrate and on the reaction conditions (equation 205)346. 

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