Alkoxy radicals, fragmentation

Alkoxy radical fragmentation is also involved in ring expansion of 3- and 4-haloalkyl cyclohexanones. The radical formed by halogen atom abstraction adds to the carbonyl group, after which fragmentation to the carboethoxy-stabilized radical... 

Tertiary alcohol 293, when reacted with iodobenzene diacetate and iodine, underwent a formal alkoxy radical fragmentation and provided the nine-membered diketone 294 in 80% yield as a separable 1.2 1 mixture of epimers (Equation 40) <1999JOC4576>. 

Other options have also been established for promoting alkoxy radical fragmentations. Two different methods were proposed by Suarez and collab-... 

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. 

Saicic, R.N., Synthesis of bridged cyclooctane derivatives via alkoxy radical fragmentation. Tetrahedron Lett., 38, 295,1997. 

For example, photolysis of steroidal hemiacetals with (diacetoxyiodo)benzene and iodine in the absence of oxygen affords medium-sized lactones as a result of alkoxy radical fragmentation (Equation 110.4). ... 

Boto, A., Betancor, C., Hernandez, R., Rodriguez, M.S., and Suarez, E., Sequential alkoxy radical fragmentation. A one-step method for breaking two 1,3-positioned carbon-carbon bonds. Tetrahedron Lett., 34, 4865,1993. 

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

In cyclic systems, the fragmentation of alkoxy radicals can be a reversible process. The 10-decaIyloxy radical can undergo fragmentation of either the C(l)—C(9) or the C(9)-C(10) bond ... 

Fragmentation of alkoxy radicals finds use in construction of medium-size rings.364 One useful reagent combination is phenyliodonium diacetate and iodine.365 The radical formed by fragmentation is normally oxidized to the corresponding carbo-cation and trapped by iodide or another nucleophile. 

Since alkoxy radicals are known precursors to chain scission in autoxidation (24), the "hot" alkoxy radicals formed as shown should undergo facile chain scission or fragmentation. The chain scission is illustrated for the alkoxy radical derived from either the ethylene or propylene monomer unit in EPM ... 

We next examine the possible fates of the alkoxy radical produced as a result of hydroperoxide fragmentation. It should be noted that the other fragment produced in this process, an hydroxy radical (not shown), would be an extremely reactive species. Since it is not attached to a polymer chain end, it is also capable of more readily diffusing through the polymer matrix than most of the radicals discussed to this point. This also makes the photo-oxidation of the glycol potentially more destructive. 

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,... 

These fragments are produced in a solvent cage if the two alkoxy radicals Scheme 12... 

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