Radical 1,2-group migrations

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

The anthraquinone group of the UAQ sensitizer is intercalated on the 3 -side of its linkage site [15]. Use of UAQ permits assessment of the directionality of long-range radical cation migration. Both AQ and UAQ enable the selective and efficient introduction of a radical cation in duplex DNA, whose lifetime is controlled by its relatively slow bimolecular reaction primarily with H20. 

Fig. 4 Schematic representation of long-distance radical cation migration in DNA. In AQ-DNA(l), irradiation of the anthraquinone group linked at the 5 -terminus leads to reaction at GG steps that are 27 A and 44 A from the site of charge injection. The amount of reaction observed at each guanine is represented approximately by the length of the solid arrow. In UAQ-DNA(2), irradiation of the anthraquinone leads to reaction at each of the eight GG steps. However, replacement of a G by 7,8-dihydro-8-oxoguanine (8-OxoG) introduces a deep trap that inhibits reaction at guanines on the same side of the DNA as the trap...

Barton has reported a wide variety of elegant studies in which various silenes or silylenes have been created, usually thermally, and their subsequent rearrangements investigated in terms of the observed products of trapping (51,53,65,145). It has been clearly established that interconversion between silenes and silylenes, especially where H atoms or Me3Si groups migrate, are facile processes. In some cases, radicals can be the precursors to silenes (65). 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

It is believed that the reaction starts with homolytic cleavage of the cobalt-carbon bond (at a cost of perhaps 100 kJ mol-1)8 to yield a Co(ll) atom and a 5 -decxy-adenosyl radical. This radical then abstracts a hydrogen atom (in Eq. 19.35 from the methyl group). Migration of the —GO)SR group takes place, followed by return of the hydrogen atom from 5 -deoxyadenosine to the substrate. This regenerates the 5 -deoxyadenosyl radical, which can recombine with the Co( I) atom to form the coenzyme. 

The biochemistry of coenzyme B12 generally revolves around either mutase enzyme activity, involving functional group migration, notably by stereospecific 1,2-shifts (Scheme 2.8), or methylation by methionine synthetase. The general mechanism for the mutase activity is a radical-based one and has been established by EPR spectroscopy to be of the general form shown in Scheme 2.9. 

In close analogy to the 2-hydroxyethyl radical 21 (R = H) the 2-aminoethyl radical 24 faces a substantial barrier for the (most likely stepwise) 1,2-migration process. A transition state for the concerted migration pathway such as 25 could up to now not be located. What differentiates the amino 1,2-migration from the corresponding hydroxy group migration is that the former appears to be less affected... 

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