Carbon radicals homolytic fragmentation

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,... 

UV spectroscopic measurements revealed that the polymerization reaction initiated with CD-complexed photo initiator is faster and, thus, ends up with higher yields than the polymerization reaction initiated with the same molar concentration of uncomplexed photo initiator. Radical photo polymerization is achieved by the homolytic fragmentation of carbon-carbon bonds of a photo-excited molecule as illustrated in Fig. 21. 

The mechanism of the key fragmentation involves initial transfer of an electron to hydroperoxide 30 (Eq. 1.2) from Fe, forming intermediate 30a, which then cleaves homolytically to the carbon radical 30b. Oxidative coupling with Cu(OAc)2 then forms 30c in which the ester moiety is in the stable Z-configuration, stabilized by internal coordination via a psuedo six-membered ring. From this intermediate, only one hydrogen atom (Ha) is available for syn elimination and, accordingly, only the (E)-olefin is produced. 

The initially formed parent ion (1) is a radical cation, so it usually fragments via a homolytic process (see the green and blue single-headed arrows) of the carbon-carbon bond. In this case, homolytic fragmentation may lead to two new ions—2 and 3—but not at the same time. Either ion 2 will form and be detected or ion 3 will form and be detected. Ion 2 has an mtz of 43 and methyl ion 3 has an m z of 15. If 1 is the parent ion, ions 2 and 3 are referred to as daughter ions because they must result from fragmentation of the parent or molecular ion. If all three ions are detected, the result is the mass spectrum of acetone shown in Figure 14.8. 

In many metal-u-organo complexes (represented by M—R where M can be a complex with other ligands) it seems probable that the first step in their decomposition is the dissociation of the M-carbon bond. This may dissociate homolytically or heterolytically, forming either a carbon radical or carbon ion species. In both cases, the carbon fragments would normally be very reactive and readily form stable products, for example by dimerization, or polymerization (see Figure 51). It is the formation of these more... 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

The first step in cracking is the thermal decomposition of hydrocarbon molecules to two free radical fragments. This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain. The following represents the initiation reaction ... 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

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). 

Crich and Yao have exploited a homolytic substitution at sulfur to trigger a radical cascade that includes a loss of carbon monoxide and a radical fragmentation of a 4,6-0-benzylidene moiety to give esters such as 113 after a final diastereoselective reduction (Scheme 32) [107]. This impressive outcome opens a direct avenue to 6-D-rhamnopyranosides and other 6-deoxy sugars. 

This kind of scission is typical of polyethylene (PE). The backbone of the polymer is broken randomly as all C-C bonds are of the same strength (Figure 27.5). Hence, the hydrocarbon chain breaks randomly and the resulting products are of the form of alkanes, alkenes and alkadienes of smaller size. This is a free radical mechanism. The covalent bond between two carbon atoms is cleaved homolytically to form fragments carrying one... 

The subsequent cycMsation of the alkoxy-radical depends upon its ability to attack a suitably placed C-H bond on the -carbon atom. The alternative to cyclisation under homolytic conditions is fragmentation of the alkoxy radical into a carbonyl compound (ii) and an alkyl radical (10), which affords a mixture of stable products by further transformations. Heusler [44] reached similar conclusions from a study of steroid reactions, and has demonstrated a close similarity between thermally and photolytically-induced homolytic reactions with lead tetraacetate in hydrocarbon solvents. 

More recently, a radical-mediated variation of this addition-fragmentation has been explored. The reaction, summarized in Scheme 77 for a one-carbon expansion, involves the generation of a radical at the terminus of a chain by homolytic cleavage of a carbon-heteroatom bond. Addition of the radical to the carbonyl produces a bicyclic intermediate, which on cleavage of the alternate bond regenerates the ketone carbonyl group with formation of a new radical. The sequence is terminated by the reduction of the radical with the tributyltin hydride reagent. The near neutral conditions of the reaction avoid the reclo-... 

Homolytic additions. Xanthates in which the 5-bearing carbon site can support a free radical readily undergo homolysis by heating with t-Bu202. The two radical fragments can be taken up by an unsaturated compound. Group transfer to a remote double bond concurrent with cyclization of certain unsaturated xanthates is an intramolecular version. An oxocane precursor of lauthisan can be acquired (35%) in this manner. ... 

The ESI product ion spectrum of florfenicol (Scheme 4,1) does not show a molecular ion peak at m/z 358 (Fig. 10.4). The significant peak corresponding to the loss of water is formed by heterolytic fragmentation as shown in Scheme 4 (II, m/z 340). The II decomposes to III (m/z 320) by a neutral loss of HF, and this fragmentation is promoted by the formation of a substituted tropylium ion (Scheme 4). The product ion spectrum of florfenicol is characterized by the unusual feature of a most abundant peak occurring at odd mass, namely at m/z 241. The IV, a radical ion, is formed by homolytic cleavage of the sulfur—carbon bond in III, and loss of the methanesulfinic radical (Scheme 4). 

If the mechanism is different (suggested by formation of a different product), then some chemical process must occur before HBr can react with the alkene. Logically, this event involves the new additive, the peroxide, which is known to undergo homolytic bond fragmentation to produce radicals. If radicals are involved, does the bromine go in first or second If a secondary radical is assumed to be more stable than a primary radical (as with carbocations, which are electron deficient), the most reasonable mechanism generates a bromine radical (Br ), which reacts with the C=C unit of the alkene. The bromine must add to the C=C unit before the H in order to generate a secondary radical, and this will place bromine on the less substituted carbon. 

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