Radical Alkoxyl radical

Hydrogen peroxide can be replaced in these reactions by a fatty acid hydroperoxide (ROOH). Then, instead of hydroxyl radicals, alkoxyl radicals (RO ) are formed. 

It is usually postulated that the final product in the accepted mechanism, the alkoxyl radical (4), cleaves (eqs. 14 and 15) before or after hydrogen abstraction, and that this accounts for the drop in molecular weight of the... 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

The excited carbonyl groups have radical character at both carbon and oxygen, and the oxygen is rather similar in its reactivity to alkoxyl radicals. The MO diagrams for the n-n and 7t-7t states can be readily depicted ... 

Oxaziranes derived from isobutyraldehyde react with ferrous salts to give only substituted formamides fEq. (23)], The chain propagating radical 30 thus suffers fission with elimination of the isopropyl group. An H-transfer would lead to substituted butyramides, which are not found. Here is seen a parallel to the fragmentation of alkoxyl radicals, where the elimination of an alkyl group is also favored over hydrogen. The formulation of the oxazirane fission by a radical mechanism is thus supported. 

PD—S) to yield phosphates and alcohols, see Scheme 5 reaction a. Sterically hindered aryl phosphites (e.g., AO 14) have an additional chain breaking activity, i.e. they react with peroxyl and alkoxyl radicals during their function as antioxidants (reactions 5b and 5c) [18]. 

Cyclic ethers were also obtained by cyclization of alkoxyl radicals, generated in a radical chain reaction by reacting the thione 42 with (TMSfsSiH under photochemical conditions at 20 °C (Reaction 46). Regioselectivities of cyclization have been investigated and a progressive increase of the 6-endo-trig selectivity along the series R2 = H[Pg.140]

Whereas bromine radicals (133) and succinimidyl radicals (134) react by the Sh2 mechanism at the tin center in tetraalkyltins, but not in alkyltin halides, alkoxyl radicals (135) and ketone triplets (136) react with alkyltin halides, but not tetraalkyltins this may reflect the conflicting, electronic demands of the radical reagents which, as electrophilic species, should be more reactive towards tetraalkyltins than alkyltin halides, but which would also tend to make use of a 5d orbital... 

Scheme 27 Trimethyl phosphite as alkoxyl radical scavenger to characterize C-0 vs C-C fragmentation for oxyranylcarbinyl radicals. Reprinted with permission from [64]. Copyright 2003 American Chemical Society...

This type of fission has been observed in a detailed examination of the oxidation of tertiary alcohols by Co(ril). The kinetics are similar to those reported for cyclohexanol vide supra) although the rate is about 40 times less. The possibility of alkoxyl radical formation seems attractive, for Co(III) is known to oxidise... 

Product studies show (i) that the ease of elimination of Rp from R1R2R3COH depends on the relative stabilities of Rp, R2 and R3- and (//) that Rp can be eliminated from RjRjHCOH, i.e. that secondary alcohols can undergo a measure of C-C fission via an alkoxyl radical. The extent of C-C fission increases when RjRjDCOH is employed, but the h/ d values for several alcohols do not accord with the observed products . This paradox has been discussed at length . 

Fig. 16.4 Interaction between quercetin (Quer) and iron and the balance between pro-oxidative and antioxidative effects. Quercetin may reduce Fe(H20) to yield Fe(H20) active in the Fenton region forming hydroxyl radicals ( OFI) or alkoxyl radicals ( OR), in effect being pro-oxidative. In contrast, quercetin may form a complex with iron(II), inactive in reducing FI2O2 to OFI, but rather oxidised in the quercetin ligand, in effect being antioxidative. Quer (-H) is the phenoxyl radical.

Injury to cells and tissues may enhance the toxicity of the active oxygen species by releasing intracellular transition metal ions (such as iron) into the surrounding tissue from storage sites, decompartmentalized haem proteins, or metalloproteins by interaction with delocalized proteases or oxidants. Such delocalized iron and haem proteins have the capacity to decompose peroxide to peroxyl and alkoxyl radicals, exacerbating the initial lesion. 

Thus the alkoxyl radical formed is susceptible to interaction with polyunsaturated fatty acid chains, effettively reinitiating further damage, or interaction with a... 

Lipid peroxides Lipid peroxy radicals Lipid alkoxyl radicals Aldehydes... 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

The fragmentation of alkoxyl radicals is especially favorable because the formation of a carbonyl bond makes such reactions exothermic. Rearrangements of radicals frequently occur by a series of addition-fragmentation steps. The following two reactions involve radical rearrangements that proceed through addition-elimination sequences. 

Among the most useful radical fragmentation reactions from a synthetic point of view are decarboxylations and fragmentations of alkoxyl radicals. The use of (V-hydroxy-2-thiopyridine esters for decarboxylation is quite general. Several procedures and reagents are available for preparation of the esters,353 and the reaction conditions are compatible with many functional groups.354 f-Butyl mercaptan and thiophenol can serve as hydrogen atom donors. 

In this context Fernandez-Mateos and his group reported efficient cyclizations with aldehydes and ketones as radical traps [115]. The authors propose a reduction of the intermediate alkoxyl radicals by a second equivalent of... 

Reaction 6 representing /1-scission of alkoxyl radicals leading to the reduction of molar mass competes with transfer of a free radical centre to surrounding groups with consequent formation of alcoholic groups (reaction 7), which subsequently loose water and C = C unsaturation appears randomly along the polymer chain. 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

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