Oxygen-centered radical, reactions

This cleavage will therefore be discussed first. Several substrates have proven useful for the generation of the pivotal oxiranylcarbinyl radicals. Sharpless epoxides are valuable starting materials for radical precursors [18]. After formation of the oxygen-centered radical, reactions typical for radicals are observed. With a suitably positioned double bond, cyclization occurs as shown in Scheme 10 [19]. It is interesting that the cw-disubstituted carbon-centered radical can react further to yield a bicyclo[2.2.1] system. 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

There is an excellent, if non critical, compilation of absolute and relative rate data for reactions of oxygen-centered radicals covering the literature through 1982389 and for 1982-1992.39 1 Selected data from these and other sources are summarized in Table 3.7 and Table 3.8. The reactions of oxygen-centered radicals and their use in organic synthesis has been recently reviewed by Hartung el uIS )]... 

The pathways whereby oxygen-centered radicals interact with monomers show marked dependence on the structure of the radical (Table 3.8). For example, with MMA the proportion of tail addition varies from 66% for f-butoxy to 99% for isopropoxycarbonyloxy radical. The reactions of oxygen-centered radicals are discussed in detail in the following sections. 

Table 3.8 Specificity Observed in the Reactions of Oxygen-Centered Radicals with Various Monomers at 60 °C...

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Watanabe et al,25-5 52s applied AMS dimer (116) as a radical trap to examine the reactions of oxygen-centered radicals (e.g. r-butoxy, cumyloxy, benzoyloxy). AMS dimer (116) is an addition fragmentation chain transfer agent (see 6.2.3.4) and reacts as shown in Scheme 3,96. The reaction products are macromonomers and may potentially react further. The reactivity of oxygen centered radicals towards 116 appears to be similar to that of S.2 1 Cumyl radicals are formed as a byproduct of trapping and are said to decay mainly by combination and disproportionation. 

Correa and Waters50 also proposed a mechanistic scheme where the key step of the overall reaction involves the recombination of sulfonyl radicals to form an intermediate with an O—S bond, the decomposition of which yields a sulfinyl radical (ArSO ) and an oxygen-centered radical ArS020 Later this suggestion was further strengthened on the basis of ESR studies51 and thanks to the elucidation of the electronic structure of sulfonyl radicals. However, it seems to the author that the available literature data point to an overall mechanism for equation 17 more complex that the one suggested50 (cf., for... 

Nitrogen- and oxygen-centered radicals offer some specific features. The rate constant of the reaction of dialkylaminyl radicals with (TMSlsSiFl is close to the analogous reaction of secondary alkyl radicals. In the reactions of... 

For this reason we have concentrated on the study of ketone photolysis in the presence of oxygen and the interaction of the oxygen-centered radicals arising in this reaction with certain tetramethylpiperidine derivatives. ... 

Formation of a Charge Transfer Complex between the nitroxide and one of the oxygen-centered radicals c or d. In the next step the complex is assumed to react with the remaining peroxyradical to form the products actually found, with release of the additive (reactions (13)/(14) or (13 )/(14 )) ... 

The observed formation of isobutyrate (Figs. 5 and 6) would appear to be one of the possible reasons for the slow decrease in the nitroxide concentration. The formation of isobutyrate can be seen as a reaction competing with the capture of the acyl radicals by oxygen. The absence of isopropyl ether in the reaction mixture is explained by its immediate cleavage - following its formation analogous to isobutyrate - to nitroxide by oxygen-centered radicals (mainly acyl peroxy radicals). 

In the case of DiPK, we were likewise able to show in additional experiments that in all probability it is the isobutyryl peroxy radical c and not the isopropyl peroxy radical d that is responsible for oxidation of the amine II to the nitroxide I. When namely the two oxygen-centered radicals are produced independently of one another in accordance with reactions (19) and (20) only in the case of the acylperoxy radical the formation of the nitroxide can be observed ... 

It can be seen that the steric effect is profound in radical reactions of Ar2OH with peroxyl and methyl radicals. It will be shown later that the steric effect exists in other free radical reactions of Ar2OH. The AES values of the reactions of alkyl radicals with Ar2OH are considerably higher than those for phenols reacting with oxygen-centered radicals. The steric effect can also manifest itself in the inverse reactions of sterically hindered phenoxyl radicals Ar20 with various molecules (see later). 

One side reaction commonly encountered in the reaction of alkyl-substituted polyenes with oxygen-centered radicals is hydrogen abstraction from the alkyl group in positions adjacent to the polyene jr-system. For reactions of the ferf-butyloxy radical, this reaction becomes so dominant that it can be used to form polyenyl radicals by hydrogen abstraction... 

As with silicon hydrides, the reaction of atoms or radicals with germanium hydrides is the key step for the majority of reactions forming germyl radicals. However, kinetic data for the reactions of organic radicals with germanium hydrides in solution are limited to carbon- and oxygen-centered radicals. 

Concerning the general reaction Scheme 1, attention is restricted to two special areas A), cases where X is a carbon-centered radical and Y is an oxygen atom joined by a double bond to some center Z (Eq. 4), and B), cases where X is a hetero atom, in most cases oxygen centered radical and Y is a carbon (Eq. 5) [11]. One is then dealing with formation and heterolysis of a bond between a carbon- and a hetero-atom. Of the two, the hetero-atom is of course always more electron-affinic and therefore in the heterolysis the electron pair joining the two will go to the hetero-atom. 

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