Radical exchange

In another version of this method, the radical generated by radical exchange from the aryl telluride carbohydrate 83 and the N-acetoxy-2-thiopyridone affords, after intramolecular cyclization and desulfanylation, the polyhydroxylated and phosphorylated pseudo sugar 84 [54] (Scheme 23). 

The Rate Constants of Peroxyl Radical Exchange Reaction with Hydroperoxides... 

Figure 8.9 Radical exchange reaction of alkoxyamine derivatives.

The radical exchange reaction of alkoxyamines has some advantages for the design of dynamic polymers. The reaction is controlled by temperature and affords the exchanged alkoxyamine units without by-products. The reaction can be applicable to polymers having various functional groups... 

Higaki, Y Otsuka, H. Takahara, A. A thermodynamic polymer cross-linking system based on radically exchangeable covalent bonds. Macromolecules 2006, 39, 2121-2125. 

It was discovered that the acetyl derivalive of iV-hydroxy-2-thiopyridone is especially suitable as a source of methyl radicals, being photolysed by irradiation with a simple tungsten lamp. On this basis, and because of the high radicophilicity of tellurides (especially anisyl tellurides), a radical exchange occurs when the reagent is irradiated in the presence of an... 

Valence Bond State Correlation Diagrams for Radical Exchange Reactions... 

As an application, let us consider a typical class of radical exchange reactions, the hydrogen abstractions from alkanes. Equation 6.14 describes the identity process of hydrogen abstraction by an alkyl radical ... 

Exercise 6.4 Based on the analysis of the barrier for radical exchange reactions, we can use the relationship G = 2DHx for the promotion gap. If we assume only linear transition states, we can use the relationship B = 0.5DHx given in Equation 6.19 (16). Combining the two expressions and the value f= 1/3, we get the following expression for the barrier (16) ... 

C02Me, etc.) were formed in good yield. A short and high-yielding synthesis of showdomycin 133 (Scheme 34) illustrated the utility of the method.88 D-Ribose was converted to the known derivative 134 which, on mesylation and displacement with anisyltelluride anion, gave 135. Methyl radical exchange on 135 in the presence of maleimide gave 136 which, on oxidation to sulfoxide and elimination, afforded the derivative 137. Standard deprotection procedures readily transformed 137 into the antibiotic showdomycin 133. The overall yield was about 30%. 

Ugelstad et al. proposed a model in which the possibility of radical exchange is respected [20], Under stationary conditions it must hold that... 

The PET-sensitization experiment uses a very high concentration of A to make the exchange of X for A" fast on the CIDNP timescale, that is, realizes the second of the two above-mentioned limiting cases. It is displayed as trace (d). The polarizations of the donor olefin are seen to be a mirror images of those in trace (a), which is simply due to the fact that the polarizations now arise from the pair D + A", for which Ag is negative. The phase inversion is thus a spectacular demonstration of the radical exchange in the three-component system. 

One simple solution to this longstanding problem in radical chemistry is based on the degenerative radical exchange of dithiocarbonyl derivatives [3-6]. The principal, unwanted competing pathway is eliminated and, as a consequence, the intermediate radicals acquire a sufficient lifetime to allow them to add to un-activated olefins or, more generally, to undergo slow inter-or intra-molecular processes. 

Akhtar, M., Barton, D. H. R., Sammes, P. G. Some radical exchange reactions during nitrite ester photolysis. J. Am. Chem. Soc. 1965, 87,... 

Vitvitskii, AI. Activation energy of some free-radical exchange reactions. Theoretical and Experimental Chemistry 1969 5(3) 276-278. 

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