Homolytic Fragmentation

Carbon-centered radicals may undergo homolytic P-fragmentation reactions, whereby an olefin and a new radical is formed. This reaction is, in fact, the reverse of the polymerization reaction. With neighboring C-C bonds, these P-frag-mentation reactions are usually slow, and only observable, at least on the pulse radiolysis time-scale with negatively-charged polymeric radicals whose lifetime is prolonged by electrostatic repulsion. Then, even the situation of equilibrium polymerization maybe approached (Ulanski et al. 2000 Chap. 9.4). 

In the nucleobases, this type of reaction is not possible due to the lack of adequate structural elements. 

When the binding energy of the bond to be broken is considerably lower than that of a C-C single bond such as a C-SR bond, the rate of 3-fragmentation can go up dramatically. This is the reason why the addition of thiyl radicals to the double bonds of the nucleobases is so strongly reversible (Chaps 7.4 and 10.5). 

Water Elimination, Heterolytic ((-Fragmentation and Formation of Radical Cations 

In DNA, the carbohydrate part has no free OH groups. Yet, salient aspects have been obtained by studying the free-radical chemistry of carbohydrates (von Sonntag 1984). A general feature in carbohydrate free-radical chemistry is the water elimination of 1,2-dihydroxyalkyl radicals [reactions (31)-(35)]. 

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The rates of homolytic fragmentation of peroxyesters 25 are also enhanced when the size of the side chains R1 —R3 = alkyl is increased. This is shown for several examples in Table 4. The rate enhancing effect is smaller than for the azoalkane thermolyses... 

The mechanism of successive homolytic fragmentation of tetroxide and formed radicals seems to be the very probable [167,193,194] ... 

SCHEME 13. Main homolytic fragmentation path of acetyl henzoyl peroxide undergoing CID with 107 Ag+ ions... 

A large number of homolytic fragmentation reactions of carbon radicals with p-leaving groups are known from studies in the gas and condensed phase. 

Some homolytic fragmentation reactions are driven by formation of small, stable molecules. Alkyl acyloxyl radicals (RCOp decarboxylate rapidly (fe > 1 x 10 s ) to give alkyl radicals, and even aryl acyloxyl radicals (ArCOp decarboxylate to aryl radicals with rate constants in the 10 s range." Azo radicals produced in the homolysis of azo initiators eliminate nitrogen rapidly. Elimination of carbon monoxide from acyl radicals occurs but is slow enough (fe 10" -10 such that the acyl radical can be trapped in a bimolecular process,... 

Homolytic fragmentation of a photo-excited molecule, giving a radical pair (intramolecular process). 

The two-step deoxygenation of benzoyl cyanide to phenylcyanocarbene, which added quickly to alkenes via photolysis of the appropriate 2,2-dihydro-1,3,2,-dioxaphos-pholanes, has been performed70. A homolytic fragmentation mechanism was assumed. When the 1,3,2-dioxaphospholane 36 in trans-butene was irradiated, the range of observed substances included 37 and 38 as the major products, in addition to fluorenone and bifluorenylidene (equation 16)70. 

The idea of this synthesis was suggested by a phenomenon observed during the mechanochemical destruction of polyamides. In addition to the main reaction of homolytic fragmentation of macromolecular backbones, a polycondensation occurred, involving the interaction of the amino and carboxylic end groups. These were condensed until they disappeared from the reaction medium ... 

Such is the case for halogen derivatives. Cleavage of the C-X bond may occur as a homolytic process, usually with low quantum yield for aryl chlorides (27) and more efficiently for bromides and iodides (Sch. 10) (28), or as a heterolytic process with a high quantum yield for some (hetero) arylfluorides, notably fluoroquinolones (Sch. 11) (29). Likewise, sulfa drugs are usually susceptible to homolytic fragmentation of the C-S02 bond (Sch. 12) (30). 

The mechanism of these transformations seems to be substrate-dependent and only the cycloisomerization of aryl and primary iodides was thought to proceed as shown in Scheme 31. The stereoselectivity of the isomerization of 110 to 111 is better accommodated with the intermediacy of l-methyl-5-hexenyl radical59. Later, it was proposed that the isomerization of 6 to 109 also proceeds via a radical-mediated atom transfer process initiated by homolytic fragmentation of an ate-complex intermediate 112 (Scheme 32)60. 

Homolytic fragmentation (with polar effect) (P. D. Bartlett 1958)... 

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