Alkoxy radicals cleavage

Consequently some of the predicted compounds could not be detected while others were formed which were not expected from direct alkoxy radical cleavage. Furthermore, the amounts of volatiles present varied markedly with heating time. It is not surprising, therefore, that at high temperatures, and particularly with polyunsaturated systems, it has been extremely difficult to establish the nature and extent of preferential hydroperoxide scission. 

Diperoxyketals. Some commercially available di(/ f2 -alkylperoxy)ketals and their corresponding 10-h half-life temperatures (deterrnined in dodecane) are hsted in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually foUowed by P-scission of the resulting alkoxy radicals (40). For acychc diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

Diall l Peroxydicarbonates. Some commercially available diaLkyl peroxydicarbonates and their corresponding 10-h half-life temperatures (deterrnined in trichloroethylene solutions) are Hsted in Table 7 (45). These peroxides are active at low temperatures and initially undergo homolytic cleavage to produce alkoxycarbonyloxy radical pairs that may subsequendy decarboxylate to produce alkoxy radicals ... 

Another common fragmentation reaction is the cleavage of an alkoxy radical to an alkyl radical and a carbonyl compound ... 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

Ceric ions react rapidly with 1,2-diols. There is evidence for chelation of cerium and these complexes are likely intermediates in radical generation10 106 The overall chemistry may be understood in terms of an intermediate alkoxy radical which undergoes p-scission to give a carbonyl compound and a hydroxyalkyl radical (Scheme 3.59). However, it is also possible that there is concerted electron transfer and bond-cleavage. There is little direct data on the chemical nature of the radical in termediates. 

Ring cleavage of the vinyloxaspirohexane 208 to the alkoxy radical 211 followed by radical recyclization leads to ring expansion to the cyclic ketones 209 and 210 [114]. (Scheme 82)... 

The Mn + complexed ions appear to be the most selective and efficient initiator system for grafting to polysaccharides so far described. The mechanism of the initiation reaction has been studied in our laboratories by model experiments and ESR spectroscopy. There are two possible reactions indicated. Bond cleavage of a vicinal diol according to reaction (22) is one possibility. Another and faster reaction with Mn3+ giving radicals appears to be oxidation of aldehyde groups (24) to alkoxy radicals ... 

A third possible fate of the alkoxy radical is shown in Scheme 18.3 (lower reaction) and was postulated by Grossetete et al. [11], In this reaction, the hydroxy radical in the cage from hydroperoxide cleavage yields an anhydride by extraction of the a-hydrogen to the radical. These workers cite IR evidence in support of this reaction which seems quite reasonable. In addition, the report by Valk et al. [21] of glycolic acid would seem to confirm this path as well. 

The present review article is concerned with the production and utilization of alkoxy radicals generated by photolytic cleavage of the type (X = NO, halogen, etc.)... 

Hypohalites IRO-Hal) are similar to nitrates (see p. 155 in their photochemical behaviour. Ultraviolet irradiation gives an (n,Ji > excited state that cleaves to form an alkoxy radical and a halogen atom. The radical may undergo alpha-cleavage before recombination with the halogen atom occurs, and this accounts for the formation of 5-iodopentanal (5.69) from the hypoiodite of cydopentanol such hypoiodites are generated in situ from the alcohol. Iodine and merturyfll oxide. In open-chain systems the alkoxy radical can... 

Regardless of which mechanism occurs, it is necessary that either C2FsO or C2F5CFXO or both spontaneously decompose by C—C bond cleavage at room temperature. This is in marked contrast to the corresponding hydrocarbon alkoxy radicals. Further evidence for the instability of perfluoroalkoxy radicals has been discussed by Barr, Francis, and Haszeldine.19... 

As illustrated in Scheme 3.26, the role of the Pd(II) porphyrin complex is the photochemically sensitized generation of singlet oxygen Oj), which forms the allylic hydroperoxide 14. Subsequent Fe3+ porphyrin-catalyzed hydrogen abstraction from 14 yields the peroxy radical 16, while the catalytic Fe3+ species is recycled by homolytic 0—0 bond cleavage of 14 to give the alkoxy radical 17. Both radicals 16 and 17 are involved in the dark reaction where the remaining cyclohexene is consumed. 

Photolysis of sugar nitrates in the presence of hydrogen donors represents an efficient technique for their cleavage with the intermediate formation of alkoxy radicals. 

This alkoxy radical can explain the observed inversion of configuration at the C-3 position of 1,2-5,6 di-CMsopropylidene 3-0-nitro-oc-D-a//ofuranose 100 during the deprotection. The cleavage of the C2, C-3 bond is a possible explanation for the epi-merization [111] into 101 (Scheme 53). 

Singlet oxygen must attack the 2-position of a 3-hexylthienyl unit in order to abstract a p-hydrogen and form the hydroperoxide. Photochemical cleavage of the 0-0 bond yields the corresponding alkoxy radical and one hydroxyl radical. Chain scission then proceeds as a result of p-cleavage of the polymeric alkoxy radical. 

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