Alkoxy radical addition

A unique alkoxymethylation reaction can be acconplished by treatment of a-alkoxycarboxylic acid chlorides with ketones in the presence of Sml2 (equation 28). llie reaction is postulated to proceed by a reductive decarbonylation process, leading to a relatively stable a-alkoxy radical. Addition ctf this radical to the Sm -activated carbonyl and further reduction and hydrolysis provides the observed product. An... 

Alkoxy radical addition followed by rapid 3-scission (process a) leads to overall oxidation Alternatively, a rapid 3 scission reaction (path h) of 5 yields the product of substitution. Addition of X (path c) followed by 3 scission is a third overall reaction, a free radical Arbuzov (reaction 2) (20). 

Mn (IT) is readily oxidized to Mn (ITT) by just bubbling air through a solution in, eg, nonanoic acid at 95°C, even in the absence of added peroxide (186). Apparently traces of peroxide in the solvent produce some initial Mn (ITT) and alkoxy radicals. Alkoxy radicals can abstract hydrogen to produce R radicals and Mn (ITT) can react with acid to produce radicals. The R radicals can produce additional alkylperoxy radicals and hydroperoxides (reactions 2 and 3) which can produce more Mn (ITT). If the oxygen feed is replaced by nitrogen, the Mn (ITT) is rapidly reduced to Mn (IT). 

Dialkyl peroxydicarbonates have been reported as low temperature sources of alkoxy radicals (Scheme 3.30)lfMJfb and these radicals may be formed in relatively inert media. However, it is established, for primary and secondary peroxydicarbonates, that the rate of loss of carbon dioxide is slow compared to the rate of addition to most monomers or reaction with other substrates.186,187 Thus, in polymerizations carried out with diisopropyl peroxydicarbonate (47), chains will be initiated by isopropoxycarbonyloxy (48) rather than isopropoxy radicals (49) (see 3.4.2.2).188... 

Various /-alkoxy radicals may be formed by processes analogous to those described for /-butoxy radicals. The data available suggest that their propensities for addition vs abstraction are similar.72 However, rate constants for [3-scission of /-alkoxy radicals show marked dependence on the nature of substituents a to oxygen (Figure 3.6).210 420,421 Polar, steric and thermodynamic factors are all thought to play a part in favoring this trend.393... 

The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other... 

Epoxides can also be reductively opened to form a radical. An example of an intramolecular cyclization of such a radical has recently been reported <06TL7755>. Treatment of 40 with Cp2TiCl generates an intermediate alkoxy radical, which then adds to the carbonyl of the formate ester. The product, 41, is formed as a 2 1 mixture of isomers at the anomeric carbon. This reaction is one of the first examples of a radical addition to an ester. The major byproduct of this reaction is the exo-methylene compound, 42, arising from a P-hydrogen elimination. 

The literature presents a large number of examples concerning the use of known oxazolidinones as chiral auxiliaries in many kinds of reactions. Rare is the use of A-amino derivatives of oxazolidinones, which were used to synthesise new A-acylhydrazones 207. Radical addition reactions occurred with high diastereoselectivity <00JA8329>. The use of glycolate oxazolidinones 210 proved to be efficient for the enantioselective preparation of a-alkoxy carboxylic acid derivatives . Photochemical reaction of vinyl... 

The alkoxy radical of Scheme 18.3 (upper reaction) could scission to produce the same carboxyl radical as seen in the Norrish type 1 path (Scheme 18.1, path A) discussed above. As such, it is an additional source of CO2 but not taken into account in the report by Day and Wiles [25], Not reported but still obvious, the other fragment of this scission is an aliphatic aldehyde that could also have been one of the aldehyde carbonyl IR signals reported [11, 25], Hydrolysis of this chain end would yield the reported glyoxal [21],... 

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. 

It should additionally be noted that a number of the paths of the schemes above have received some confirmation in a number of literature reports dealing with the photolysis and photo-oxidation of other polyesters [32-35], Because these reports investigated poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) and poly(butylene naphthalate), however, they may not have direct application to understanding of the processes involved in PET and PECT and so have not been discussed in this present chapter. All do contain support for the formation of radicals leading to CO and C02 evolution, as well as the hydrogen abstraction at glycolic carbons to form hydroperoxides which then decompose to form alkoxy radicals and the hydroxyl radical. These species then were postulated to undergo further reaction consistent with what we have proposed above. 

It should be noted that some alkoxy radicals derived from oxygenated compounds may have some unique chemistry in addition to these reactions. For example, Tuazon et al. (1998b) have shown that alkoxy radicals derived from OH + acetate reactions, i.e., R C(0)0CH(0 )R2> rearrange and decompose to RCO and the acid RC(0)0H. 

Also formed are a variety of hydroxynitrates, nitro-oxyhydroperoxides, and hydroxycarbonyls anticipated from the reactions of alkylperoxy and alkoxy radicals formed after the initial addition of N03 to the double bond (e.g., Kwok et al., 1996c). The yield of 4-nitroxy-3-... 

It should be noted that, although it is not important in this particular case, there is an additional path available for some alkoxy radicals formed in the oxidation of certain halogenated organics, that is, the intramolecular elimination of HC1 from a-monochloro-alkoxy radicals. For example, the alkoxy radical CH2C1CHC10 both reacts with 02 and eliminates HC1,... 

As Schaffer has found 2.4.6-triphenyl-X -phosphorin 22 and other 2.4.6-tri-substituted X -phosphorins react smoothly with aryl diazonium salts in benzene. Nitrogen develops and the aryl residue bonds with the phosphorus. In presence of alcohols as nucleophiles, l-alkoxy-l-aryl-2.4.6-triphenyl-X -phosphorins 100 can be isolated. The aryl diazonium-tetrafluoroborate without any nucleophile in DMOE yields l-aiyl-l-fluoro-2.4.6-triphenyl-X -phosphorin 70i. As with other oxidants like halogen or mercury-Il-acetate, we suppose that in the first step triphenyl-X -phosphorin radical cation is formed. This could be shown by ESR spectroscopy. The next step may be a radical-radical addition to the X -phosphorin cation or a nucleophileradical addition respectively ... 

Two H-addition radicals (18, 19) were also detected but complete hyperfine tensors not reported. The only deoxyribose moiety radical detected in 5 -dCMP at 10 K in this study was the well known 03 alkoxy radical, first identified by Box et al.15 It was observed at 10 K and at 65 K. 

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