Radical-trapping reagents

The formed acyl radicals are reactive towards efficient radical trapping reagents such as 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO), diphenyl diselenide and diphenyl disulphide, and A-f-butyl-a-phenylnitrone giving the respective adducts. ... 

Adding radical-trapping reagents should have no peculiar effect on the relative proportions of propylene and propane formed. However, the yield of hydrocarbons would be expected to drop as isopropyl adducts of the reagent appear in their place (Reaction 8). 

In order to show the intermediacy of a cyclopenta-l,3-diyl diradical, benzophenone sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene was studied in the presence of radical trapping reagents. 

Anodic Decomposition of Toxic Compounds (Anodic Mineralization), Fig. 4 ESR signals of electrolyzed carbonate solution containing 20 mM phenol. The number is elapsed times (min) after the addition of DMPO (radical trapping reagent) to the electrolyte extracted from the cell... 

Radical production from [2+2] cycloaddltlon reactions also has been clearly demonstrated by a number of workers, using both experimental and theoretical techniques (9a). However, the Interception of the Intermediate 1,4-dlradicals by added radical trapping reagents has been reported In only a few cases (5,10-16). 

The formation of the quinoidal p-xylylene intermediate can be monitored by the appearance of a peak in the UV spectrum around 310 nm. This has been used to optimize reaction conditions for polymerizations involving unreactive sulfonium salts [48]. There has been some controversy over the precise nature of the polymer coupling reaction. The initial assumption was that the polymerization was a radical-promoted process [46]. The presence of radicals was very hard to prove, and the pendulum swung for a while toward an anionic mechanism [51]. However, careful work by Lahti and coworkers [53] showed that radical trapping reagents did indeed suppress the polymerization. As an example, the addition of TEMPO to the reaction mixture not only dramatically lowered the yields and molecular weights but also caused the disappearance of the spin label. The mechanism of radical initiation is unknown it may involve spontaneous coupling of two quinoidal p-xylylene intermediates to form a biradical. 

The mechanism of these reactions has been the subject of much debate. The intermediacy of the quinodimethane 9 has been established by spectroscopic means, but there was much discussion as to whether the polymerizations proceeded via a radical or an anionic mechanism. Experiments showing that radical trapping reagents suppressed the polymerizations demonstrated that the Wessling and Vanderzande ... 

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