Alkoxy radicals reviews

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

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.)... 

Small alkylperoxy and alkoxy radicals can decompose uni-molecularly, though their rate constants are often in the second-order region. They abstract hydrogen atoms from alkanes, aldehydes, esters, and acids, add to olefins, and may react with 02. Furthermore, interactions with other radicals can lead to disproportionation or combination. These reactions are reviewed, and particular attention is given to CH 02 and CH30 a number of rate constants are estimated. 

More than 140 different alkenes have been identified in the atmosphere [27]. The sources of alkenes are similar to those for the alkanes with combustion of fossil fuel being a major source. The presence of unsaturated bonds makes these compounds much more reactive than the alkanes. The most persistent member of this class of compounds (ethene) has an atmospheric lifetime of the order of a day, while more typically the lifetimes for alkenes are measured in hours. As a result of their short lifetimes the atmospheric concentrations of alkenes are highly variable and decrease dramatically away from their source locations. The mechanisms of atmospheric oxidation of alkenes have recently been reviewed [55]. As with the alkanes the reaction of OH radicals is an important loss mechanism. This reaction proceeds mainly via addition to the unsaturated bond as illustrated for ethene in Fig. 4. In one atmosphere of air at 298 K the dominant atmospheric fate of the alkoxy radical HOCH2CH2O is decomposition via C - C bond scission, while reaction with O2 makes a 20% contribution [56]. The fate of alkoxy radicals resulting from addition of OH to alkenes is generally decomposition via C - C bond scission [8]. Thus, the OH radical initiated oxidation of propene gives acetaldehyde and HCHO, oxida-... 

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (conunonly r-butyl peracetate or r-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hyttogen atom abstractitm and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

Other applications of nitrite photolysis include the functionalisation of the i4a-methyl group of the lanosterol skeleton by irradiation of the nitrite of a, y a-alcohol [20], attack on C(i9> by irradiation of 5a-halo-6ji mtrito derivatives [2X,22] or nitrites of 2/ alcohols [23], intramolecular addition of a 20 alkoxy radical to a Ai -double bond [24], and some reactions involving radical-induced fragmentation of adjacent C-C bonds [23]. A comprehensive review by Nussbaum and Robinson [26] surveys the wide variety of reaction paths open to alkoxy radicals generated by photolysis of nitrites. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

Alkoxy radicals that arise from the reaction of NO with alkylperoxy radicals also may enter into several competing processes. Four such reactions must be considered thermal decomposition, isomerization, reaction with oxygen, and addition to either NO or N02. Falls and Seinfeld (1978) have presented a brief review of the various possibilities. Table 6-13 summarizes current information on rate coefficients and projected rates in the atmosphere for several small alkoxy radicals. 

Table 6-13. Left Rate Coefficient Estimates for Reactions of Alkoxy Radicals in the Atmosphere, Taken Partly from the Review of Atkinson and Lloyd (1984) Right Rates Calculated from the Rate Coefficients Assuming Number Densities n(02) = 5 x 10 8, n(N02) = 2.5 x 10 ° molecules / cmib... 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

The S—C bond fission path has been exploited as a method for photodeprotection of alcohols. The earliest examples of this were reported by Pete and his group226-228, who demonstrated that the free alcohol could be obtained in reasonable yields on irradiation of tosyloxy derivatives. Scheme 19 shows the proposed mechanism for the process where irradiation brings about S—C bond fission affording a biradical. Loss of S02 affords the alkoxy radical and ultimately the alcohol. Many examples of this deprotection path have been reported over the years. This area of study has been reviewed by Binkley in a number of articles229-231 with particular reference to deprotection of carbohydrate derivatives. Thus the compounds shown in Scheme 20 are converted into the free alcohols on irradiation in yields up to 87%232,233. Even higher yields can be obtained as with the irradiation of 261 which affords 100% detosylation affording 262234. 

Both competitive methods give fairly reliable relative rate coefficients in most cases. However, discrepancies between them have been found when t-butyl hypochlorite was used as the source of alkoxy radicals and when aralkanes (e.g. toluene) were the substrates because of the incursion of a chlorine atom chain, and relative reactivities to Cl- rather than to t-BuO- were determined. Absolute rate coefficients reported in this review do not include this suspect-data. 

Williams, 1959 Gray et al., 1967 Heicklen, 1968 Howard, 1972), but no general update seems to have t peaied. However, Batt (1979) has reviewed the thermochemistry and thermal-decomposition kinetics for aikoxyl radicals and alkyl nitrites. It is the purpose of the present review to report all work through June, 1986 (and some later) on the reactions of alkoxy radicals. In addition, since aikoxyl radicals often are produced in laboratory experiments from alkyl nitrites, the photochemical and thermal decomposition of these species will also be reviewed. 

Perhaps most pertinent to this review, model compounds play an integral role in the development of detailed chemical kinetic mechanisms. For instance, w-propylperoxy," 1-butoxy radical," and n-pentyl" radical are, respectively, the simplest alkylperoxy radical, alkoxy radical, and alkyl radical capable of undergoing facile 1,5-H atom transfers (each via a favorable six-membered-ring transition state, as illustrated in Figure 2). Thus, these smaller systems are commonly used as models to investigate the comparable isomerizations possible in the larger radical, oxy radical, and peroxy radical species that are involved in FlC fuel combustion. 

The chemistry of peroxyesters (38) also commonly called pcrcsters has been reviewed by Sawaki,199 Bouillion el al.m and Singer.194 The peroxyesters are sources of alkoxy and acyloxy radicals (Scheme 3.32). Most commonly encountered peroxyesters are derivatives of /-alkyl hydroperoxides (< .g. /-butyl peroxybenzoate, BPB). 

The chemistry of alkyl hydroperoxides (40) has been reviewed by Porter,217 Sheldon204 and Hiatt.2lS Alkyl hydroperoxides are high temperature sources of alkoxy and hydroxy radicals.219 They are often encountered as components of redox systems. 

Hendry, D.G., Mill, T., Piszkiewicz, L., Howard, J.A., Eigenman, H.K. (1974) A critical review of H-atom transfer in the liquid phase chlorine atom, alkyl, trichloromethyl, alkoxy and alkylperoxy radicals. J. Phys. Chem. Ref. Data 3, 944-978. 

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