Hydrogen abstraction by alkoxy radicals

Radical trapping by a nitroxide was used to determine the ratio between (3-fragmentation and hydrogen abstraction by alkoxy radicals generated by thermal decomposition of peroxy pivalate esters.The ratio of fragmentation hydrogen abstraction increased sharply with radical substitution and stability. 

Intramolecular hydrogen abstraction by alkoxy radicals generated from hypochlorites has also been observed ... 

Hydrogen abstraction by alkoxy radicals at any of the allylic sites jdelds a site for styrene polymerization, resulting in a graft (282,283). Model studies based on styrene-butadiene block copolymers show a strong influence of graft structure on particle morphology (81,82). The double bonds in polybutadiene are much less... 

Since hydrogen atom abstraction by an excited carbonyl closely resembles hydrogen atom abstraction by alkoxy radicals, it is mainly compounds with n,n excited states, namely ketones and aldehydes, that... 

There are also useful intramolecular functionalization methods that involve hydrogen atom abstraction by oxygen radicals. The conditions that were originally developed involved thermal or photochemical dissociation of alkoxy derivative of Pb(IV) generated by exchange with Pb(OAc)4.374 These decompose, giving alkoxy... 

From the above it is clear that DMPO can undergo the addition-oxidation mechanism with water as the nucleophile, provided a suitable oxidant is present. With a primary alcohol competing, the O-connected alkoxy spin adduct is formed in addition to HO-DMPO". On the other hand, with a hydroxyl radical source a competing alcohol will undergo hydrogen abstraction by HO" and form an a-hydroxyalkyl radical which forms a C-connected spin adduct. This criterion clearly can distinguish between the two mechanisms at least in model systems (for recent examples, see Reszka and Chignell, 1995 Janzen et al., 1995 Thomas et al., 1996). 

Another source of the hydroperoxyl radical is the abstraction of a hydrogen atom from alkoxy radicals by molecular oxygen ... 

The initial OH chemistry of HCFC-123 (CHC12CF3) produces a fully halogenated alkoxy radical where further hydrogen abstraction by 02 is not an option ... 

However, in the presence of a suitable substrate as illustrated by RH in reaction 4, an intermolecular hydrogen abstraction by the intermediate alkoxy radical favorably competes with the decomposition reaction (2). The resulting intermediate alkyl radical, R, normally combines with... 

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

Similar internal cyclizations occur when 2- (l-adamantyl)-ethanol is treated with lead tetraacetate to give the cyclic ether 83 217 >283 or with HOC1 to give 84 (Eq. (70)) 2831. These reactions apparently involve hydrogen abstractions by the initially formed alkoxy radical. 

The 20/1 y/5 ratio, when both carbons are unactivated methylenes, is also displayed by alkoxy radicals 75>. The slower 1,6-hydrogen transfer presumably reflects some of the strain in a 7-membered ring. It must also involve a more negative entropy. Rotations about three C—C bonds are frozen in the transition state, whereas only two C—C rotations need be frozen for y-hydrogen abstraction. 

The intramolecular hydrogen abstraction reactions promoted by alkoxy radicals in carbohydrates are particularly useful for the stereoselective synthesis of various polycyclic oxygen-containing ring systems [643-647], This reaction can be illustrated by the intramolecular 1,8-hydrogen abstraction between glucopy-ranose units in disaccharide 613 promoted by alkoxy radicals and leading to the 1,3,5-trioxocane derivative 614 (Scheme 3.240) [644]. 

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