Phenyl radicals, trapping

Figure 1 is an ESR spectrum acquired after 10 minutes of sonicating an Ar - saturated solution of 2-PCB and PBN at 20 kHz. Samples that were irradiated at different power intensities for the same time period (10 minutes) are compared. Peaks consist of a triplet of doublets with the parameters of och = 4.2, ocn =16.0, which are characteristic a values for phenyl radical trapped by PBN. These peaks appeared in all ESR spectra obtained from sonicated solutions of 2-PCB (n = 20) and 4-PCB (n = 19) with irradiation times longer than 10 minutes (n = number of experiments). 

In this section indirect methods will be explored. One was given in Section 3.6. There, data were presented for two members of a large family of reactions of the phenyl radical with organic halides. Pairs of halides were used, and the ratio of two competing products was determined. Expanding the general scheme of Eq. (5-1) further, to use a pair of competing traps, we have... 

Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

The trapping of alkyl, alkoxyl and alkylthiyl radicals by trivalent phosphorus compounds, followed by either a-scission or p-scission of the ensuing phosphoranyl radical, is a powerful tool for preparation of new trivalent or pen-tavalent phosphorus compounds [59]. However, the products of these reactions strongly depend on the BDE of the bonds, which are either formed or cleaved. For example, the addition of phenyl radicals on a three-coordinate phosphorus molecule occurs irreversibly, while that of dimethylaminyl (Me2N ) or methyl radicals is reversible, the amount of subsequent P-scission (formation of compound C) depending on the nature of Z and R (Scheme 25). For tertiary alkyl radicals and stabilized alkyl radicals no addition is observed (Scheme 25) [63]. 

Cleavage of the radical-anion to give a phenyl o-radical also occurs in aprotic solvents and can be demonstrated using substrates where the phenyl radical is trapped in an intramolecular addition process. Substi ate 12 gives products analogiis to those from reduction of the 2-haiogeno-N-methylbenzanilides (p. 129) at a mercury cathode in aprotic. solvents [79], Similarly 13 undergoes a related cyclization of the o-radical intermediate [80],... 

Reduction of phenyldiazonium chloride in acetonitrile containing a high concentration of an aromatic substrate, which can act as a free-radical trap, leads to phenylation of the substrate in 14 - 33% yields together with 50 - 50% of benzene formed by phenyl radical attack on the acetonitrile [132], Intramolecular capture of the phenyl radical, in an electrochemical equivalent of the Pschorr reaction, is much more successful and phenanthrene derivatives can be prepared in 90 - 96% yield [133],... 

The thermolysis of benzoyl peroxide in thiophene gives a complex mixture, with the phenylation products accounting for only 3%. Considerable amounts of 2,2 -bithienyl and products containing the benzoyloxy group are formed. The suggested mechanism is shown in Scheme 59, according to which the benzoyloxy radicals are efficiently trapped before fragmentation to C02 and phenyl radicals. 

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

Irradiation of phenyliodonium salts lead to the formation of phenyl radicals. In the presence of C60 these radicals are efficiently trapped under formation of pheny-lated C6o derivatives, mainly the monoadduct. In reaction mixtures of C6o, phenyliodonium salts and spin traps like nitroso-tert-butane ( BuNO) or nitroso-durene (ND) no phenyl adducts with the spin traps could be observed after irradiation. This suggests that C6o is a more efficient scavenger for phenyl radicals than the spin traps [177], Other investigations yielded similar results, e.g., the photolysis of organomercury compounds in the presence of fullerenes leads to fullerene-derived radical adducts. These radical adducts can combine to form dimers that are thermally stable and accumulate in the samples [Eq. (7)] [178],... 

Anion 213 is trapped by Mel to give 211. Phenyl radicals can react with PhS ions to give finally the product Ph2S. This step has been proved to be irreversible under these experimental conditions278, although the equilibrium constant (ca 3 x 105) has been measured 279. 

The mechanism of the ligand coupling or reductive elimination step is not well understood and rather speculative. However, the possible intermediacy of radical species, either free or in-cage, has been considered unlikely, since the use of free-radical traps did not alter the outcome of the reactions. The use of an external trap (1,1-dipheny-lethylene)31,40 (Equation (13)) or the use of the classical internal intramolecular free-radical probe [(o-allyloxy)phenyl radical]41 or its diphenyl analog ]( -2,2-diphenylallyloxy)phenyl radical]42 (Equation (14)) did not interfere with the outcome of the arylation reaction, thus excluding the possibility of the intermediacy of free radicals. 

The reaction of oxyhaemoglobin with phenylhydrazine was studied in whole blood samples (Maples et al., 1988). Hydrazine-based drugs induce destruction of red blood cells with resulting haemolytic anaemia. Using DMPO as a trap, nitroxide radicals were detected, but these had solid-state or immobilized spectra with broad parallel and perpendicular features. Whilst no firm identification is possibly based on ESR spectra, except that the trapped radical must be a high polymer, various lines of evidence lead to the conclusion that the adduct was formed from a sulphydryl radical on oxyhaemoglobin. Chloroform extracts gave the phenyl radical adduct in accord with in vitro studies. 

The triphenyl methyl or trityl radical behaves as a radical trap and favors the polymerization-termination which is thermoreversible and thus allows the insertion of a new polymeric sequence. In 1982, Otsu et al. [49,213,214] proposed an interesting example involving phenylazotriphenylmethane as Initer (initiator-terminator) able to initiate a free radical polymerization from the phenyl radical. Alternatively, the trityl end-capped polymer can be utilized as an original macroiniter for the polymerization of a second monomer and yields block copolymers as follows ... 

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