Fragmentation of radicals

An intramolecular mechanism of the rearrangement has been shown in the special ESR study (refs. 21, 22), conducted on the model radicals, generated by abstraction of a bromine atom from T2 " the rate constant K2, equal to (5.0 + 0.3) X 104 sec- at 22°C, has been also determined. In addition, fragmentation of radical... 

Fragmentation is the reverse of radical addition. Fragmentation of radicals is often observed to be fast when the overall transformation is exothermic. 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

Scheme 6 Observation of a CIDNP effect on fragmentation of radical 5...

Radical fragmentation of radicals containing the carbonyl group (RCO) ... 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

In carbohydrates, P-fragmentation of radicals adjacent to the glycosidic linkage [e.g reactions (14) and (15)] will also lead to chain scission. 

One possibility to avoid the fragmentation of radical anions lies in the lowering of their antibonding n MO. For instance, the photostimulated reactions of polycyclic or heterocyclic halides, such as 1-chloronaphthalene, 2-chloroquinoline, 4-chlorobiphenyl and 9-bromophenanthrene with PhSe" ions give good yields of substitution products ArSePh (50-72%)306. In this case a stable k radical anion is formed. 

Table 1. Rate constants for fragmentation of radical ions...

The fragmentation of radical anions and the reverse reaction, the addition of anions to radicals, are the critical steps of SRN1 reactions [110] which constitute perhaps the largest class of fragmentation reactions initiated by photoinduced electron transfer. These reactions are chain processes and photoinduced ET is involved only in the initiation step, which is usually poorly defined. The reactions may also be initiated by other means, not involving absorption of a photon. The SRN1 reactions and related redox-activation processes have been recently extensively reviewed [72a, 110,127] and will not be discussed here. 

Even though fragmentation of radical anions represents a key step in SrnI reactions (Scheme 76) and in aliphatic nucleophilic substitution reactions (Sn2) proceeding via single electron transfer (Scheme 77), such processes and their mechanistic implications will not be discussed in this section (several reviews are available [271-277]). 

Radicals also can be trapped by addition to the nitroso group to form nitroxide radicals. In the following equation, diacetyl peroxide is a source of methyl radicals, which are trapped by the nitroso compound. (For further discussion of the fragmentation of radicals, see Section 7.)... 

Can distinguish isomeric Leu/lle via secondary fragmentation of radical z ions. 

Radicals can also be generated via the fragmentation of radical anions. Single electron reduction of various alkyl or aryl halides will lead to bond cleavage, as shown in Eqs. 10.44 and 10.45. The reductions commonly occur from dissolving metals such as Na and K, sodium naphthalenide (Na Ar ), sodium benzophenone ketyl, or from pulse radiolysis (e ). 

The cyclopropanone fragments of radicals appear to be unstable and [91], and elimination of CO leads to the following conversion ... 

The fragmentation of radical cations formed in a mass spectrometer gives lower molecular weight fragments, often characteristic of a functional group (13.3). 

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