Traps for radicals

The outstanding chemoselectivity of Cp2TiCT was amply demonstrated by Merlic [50,51] and by Dotz [52] who employed a, /3-unsaluraled tungsten and chromium carbenes as radical traps for C - C bond formation. In the latter contribution the very acid sensitive glycal epoxides were used with good success. An example is shown in Scheme 8. 

The anionic methylruthenium(II) species was autoxidized to a Ru(III) compound, RuMe(OEP). The methyl group of this compound was accidentally transformed into a coordinated carbon monoxide molecule by an excess of 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) [158] on an attempt to use TEMPO as a radical trap for the measurement of the Ru-C bond energy in solution. This was the first transformation of a methyl group to carbon monoxide to be observed in the proximity of a metal. 

Quantitative nitroxide-trapping experiments should be carried out under thoroughly de-oxygenated conditions since oxygen will act as a competitive radical trap for carbon-centred radicals. Consequently, several freeze-thaw degassing cycles using pressures <10 3 mm Hg are usually required. 

On addition of the triplet quenchers sdlbene or piperylene to solutions of pinacolone, all polarizations strongly decrease. Low piperylene concentrations, in particular, suppress the isobutane polarizations completely while they affect those of acetaldehyde and isobutylene to a lesser extent. Piperylene may also act as radical trap. For 7 M solutions in piperylene, very weak polarizations of isobutylene and acetaldehyde of the types given in table 1 can still be observed. Apparently the polarizations may arise from primary triplet pairs in this case, too s). 

There is some evidence that surface processes and complex formation " may obscure the trapping reactions. Hydrogen sulphide may also be classed with these types of radical traps . Aldehydes have also been used as radical traps . For acetaldehyde, the major reaction should be abstraction of the alde-hydic hydrogen atom, viz. so that the CO at least, should provide a material balance for RH. 

The major part of the reactions of a-arylation which have been reported were performed on substrates containing active methylene groups, such as p-diketones, p-ketoesters, P-ketonitriles and malonic acid derivatives. Less activated compounds, such as P-ketosulfides have also been efficiently arylated on the a-carbon. (Table 5.3) The yield of the arylated product can be increased by addition of 1,1-diphenylethylene, acting as a free radical trap. For example, in the reaction of phenylation of ethyl cyclohexanonecarboxylate, addition of 1,1-diphenylethylene reduced the radical chain decomposition in such a way that an 80% yield was obtained instead of 55% in absence of 1,1-diphenylethylene. ... 

The search for a nitrogen-centered radical trap for the radicals produced from... 

Methyl oxalyl chloride was also employed as a radical trap for the same purpose (Scheme 18) [40c], Radical reaction of an alkyl iodide with methyl oxalyl chloride in the presence of hexabutylditin under photochemically initiated conditions affords an acid chloride along with a small amount of the corresponding methyl ester. Sequential radical reaction involving cyclization and carboxylation can be performed using methyl oxalyl chloride. 

Morie, G.P. Evaluation of free-radical traps for the removal of nitrogen oxides from cigarette smoke Tob. Sci. 18 (1974) 80-81. 

The second category is that of free radicals traps. For example, hindered amines are excellent light-stabilizing compounds their oxidation generates stable free radicals called nitroxide that are able to efficiently trap reactive free radicals. Very recently, such nitroxide radicals were used to prevent a... 

Likewise, photochemical reactions of N-oxides are considered in the following, not the use of N-oxides in photochemistry. Therefore, the use of nitrones, in particular of cyclic polysubstituted cyclic nitrones such as 5,5-dimethylpyrroline N-oxide (DMPO, 8, see Equation 99.4), as radical traps for oxygen-centered radicals, in particular the hydroxyl radical or the superoxide anion,is not discussed here. In fact, these nitrones are used for determining the mechanism of photoinduced oxidation reactions, for example, in lipid photooxidation " or in oxidative DNA damage but are not undergoing a photochemical reaction under such circumstances. 

---