Radical reactions fragmentation method

McNeill and Rincon studied thermal degradation of PC by means of TGA, TVA, DSC, FT-IR, mass spectrometry (MS) and gas chromatogra-phy-MS (GC-MS) method (see Fig. 2.5). PC is stable up to 300°C. Above that temperature, small quantities of phenol and p-cresol were detected, at 375-400°C CO2 appeared and then at T > 455°C CO and CH4 were formed the peak on the DTG curve was at F = 462°C. At 500°C the main products are q chc dimer and bisphenol A, with small quantities of CO2, p-cresol, p-ethyl phenol, phenol, p-vinyl phenol, p-isopropyl phenol, CO and CH4. In the absence of air and moisture, degradation of PC proceeds by hemolytic decomposition of the polymer chain, radical reactions, fragmentations and molecular rearrangements. 

Relatively few kinetic data are available for the carbon-carbon bond forming reactions of alkene radical cations. Nevertheless, rate constants for the cyclization illustrated in Scheme 9, with generation of the alkene radical cation by the fragmentation method, have been measured. These cyclization rate constants are significantly faster than those of the corresponding neutral radicals [89]. 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

As radical reactions are increasingly applied in synthesis, a convenient and informative method to notate these reactions in retrosynthetic analysis becomes desirable. Currently, two fragments united by a radical reaction are sometimes represented simply by dots Seebach has introduced the radical syn-thon, notated as r (Scheme 18).74 Both of these notations have two related limitations first, they imply that C—C bonds are formed by radical-radical coupling, and second, they do not indicate which site provided the radical and which site accepted it. I introduce here a notation for radicals in retrosynthetic analysis that strives to be in harmony with current ionic notations without artificially imposing75 on radical reactions the features needed for the planning of ionic reactions. 

The radical chain mechanism involving allytin belongs to the fragmentation method family and can be schematically presented as in equation 38, which involves five steps (i) initiation step, (ii) reaction with the substrate, forming the carbon-centred radical, (iii) a possible evolution of this radical, (iv) addition of the newly formed radical to the allylic double bond and (v) /3-scission, regenerating the chain-carrying tin radical. 

Abstract This review provides an overview of some of the more recent work directed to exploit radical-based chemistry for the modification of some of Natures most important biomolecules, such as amino acids, peptides, and carbohydrates. Radical reactions are particularly advantageous for carrying out a variety of structural modifications on biomolecules as the reaction conditions are typically compatible with a wide variety of functional groups and solvents. An array of effective synthetic transformations will he discussed including selective side chain and backbone modifications of amino acids and peptides, along with methods for the transformation of carbohydrate substituents, as well as fragmentation and cyclizations reactions for the preparation of either structurally modified carbohydrates or chiral building blocks. 

This reaction demonstrates that fluorinated alkenes are especially reactive towards nucleophilic radicals. Throughout the text, examples of free—radical reactions initiated by -rays are interspersed with peroxide—initiated processes and it is worth emphasising that the o -ray technique is an extremely useful experimental probe because it is, in effect, a universal initiator, i.e. the method can be applied over a wide range of temperature. Furthermore, it leaves essentially no fragments in the product, as do chemical initiators, and the technique may be used with metal apparatus. 

Among several radical techniques, the free-radical addition-fragmentation chain transfer reaction appears to be an unrivaled method for the synthe-... 

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