Alkoxy radicals fragmentation reactions

When the reaction of cyclic alcohols 578 is conducted under an oxygen atmosphere, the initially produced C-radical traps a molecule of O2, yielding peroxide 579 according to Scheme 3.229 [622]. This and similar sequential alkoxy radical fragmentations can be applied to the preparation of various synthetically interesting medium-sized ketones and lactones. 

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

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 number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

Because of the absence of a suitably positioned C—H bond, the alkoxy radical derived from 546 cannot undergo heterocyclization. -Fragmentation therefore ensues to give 547 along with a small amount of parent ketone. An empirical predictive rule has been developed to account for the stereoelectronic control observed in such reactions... 

Photolysis studies of alkyl nitrites in the vapor phase has been limited to the verification of the presence of a nitroso dimer moiety among the products by ultraviolet spectrophotometry. The principal product of photolysis of butyl nitrite in the vapor phase is the trans-isomer of the dimer of nitrosomethane43 86 87 281 286 the reaction has been explained by a mechanism involving fragmentation of alkoxy-radicals. 

Table 9.2 presents the results of experiments with the 9-decalyl radical, generated as indicated in Scheme 2 from two isomeric precursors.27 In this reaction, the alkyl radicals, which form by fragmentation of the alkoxy radical,... 

Lipid alkoxy radicals (LO ) decompose in chain scission reaction to a great variety of reactive aldehydes such as malonaldehyde, hydroxyalkenals, 2-alkenals, 2,4-alkadienals and alkanals. Lipid alkoxy radicals also cause the degradation of the apolipoprotein B (apoB) to smaller peptide fragments. 

The subsequent cycMsation of the alkoxy-radical depends upon its ability to attack a suitably placed C-H bond on the -carbon atom. The alternative to cyclisation under homolytic conditions is fragmentation of the alkoxy radical into a carbonyl compound (ii) and an alkyl radical (10), which affords a mixture of stable products by further transformations. Heusler [44] reached similar conclusions from a study of steroid reactions, and has demonstrated a close similarity between thermally and photolytically-induced homolytic reactions with lead tetraacetate in hydrocarbon solvents. 

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. 

There is no evidence in any of the gas phase systems for initial multiple bond rupture (i.e., fragmentation reactions). Because of the low reaction temperatures, the alkoxy radical intermediates of the bond fission reactions (or radicals resulting from alkoxy radicals) are mainly involved in radical-radical termination processes ( 0) rather than participating in hydrogen abstraction from the parent peroxide E oi 6-8). Thus it has been commonly believed that the peroxide decompositions were classic examples of free radical non-chain processes. Identification of the rate coefficients and the overall decomposition Arrhenius parameters with the initial peroxide bond fission kinetics were therefore made. However, recent studies indicate that free radical sensitized decompositions of some peroxides do occur, and that the low Arrhenius parameters obtained in many of the early studies (rates measured by simple manometric techniques) were undoubtedly a result of competitive chain processes. The possible importance of free radical reactions in peroxide decompositions is illustrated below with specific regard to the dimethyl peroxide decomposition. 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

Homolytic cleavage has been found to result from excitation of the para-nitrobenzenesulphenates (434). The reaction is proposed as a convenient means of generating alkyl radicals since these are produced along with a ketone by fragmentation of the tert -alkoxy radical initially formed, ... 

Small molecule-polymer "moiety" reactions. Groups formed on the backbone of polymer chains (such as ROO peroxy, RCO keto, RO alkoxy, etc.) do react with "small molecule" species, including oxygen and some radical fragments. The effect of the polymer medium apparently is to reduce the bimolecular collision (reaction) rates by a factor of about 10 2 relative to fluid solution rates, due to the reduced mobility of the chains. 

Radicals also undergo fragmentation reactions. Most of these are p>-scission reactions, such as illustrated by decarboxylation and fragmentation of alkoxy radicals, but decar-bonylation, an a-cleavage, is also facile. 

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