Eliminations from Radical Intermediates

Radicals undergo various elimination reactions, sometimes as side reactions during the transformations we describe in this and the next chapter. These reactions are not part of standard radical mechanisms, so we simply group a few of them together here as examples of reactions you should know. 

Just as deprotonation adjacent to a carbenium ion can form an olefin, similarly removal of H adjacent to a free radical will form an olefin (Eq. 10.95). As we noted in Section 10.10.4, the process of Eq. 10.95 is referred to as radical disproportionation when the radicals are the same. Unimolecular elimination from a radical is the simple reverse of the addition of a radical to an alkene (Eq. 10.96). Since the addition is typically exothermic, it takes heat to reverse the addition. One example is the depolymerization of polystyrene, which will occur at temperatures of 300 °C (Eq. 10.97 see Chapter 13 for a discussion of the polymerization reactions). Strain in an adjacent ring will favor elimination, as shown in Eqs. 10.98 and 10.99. These two examples convert one radical to another, and such reactions will be discussed in more detail in Section 11.11. 

Combining Addition and Elimination Reactions (Substitutions at sp Centers) 

Now that we have covered both additions and eliminations, we can combine them. Many functional group transformations involve such a combination. The result is a substitution reaction that occurs at an sp hybridized carbon. They are best described as addition-elimina- 

For the next several sections we will focus upon addition-elimination reactions at carbonyl centers. All these reactions are easily understood using our paradigm of reactivity. In every case we will show how a nucleophile with a full or partial negative charge attacks the partially positive carbonyl carbon. It is the subtle details that make the examples interesting. 

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The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

Primary alkyl phenyl tellurides undergo elimination to form terminal olefins in high yields on treatment with an excess of iV-chloro-iV-sodio-4-methyl-benzenesulphonamide (chloramine-T) in refluxing THF/ ufc-Dinitro compounds and /3-nitrosulphones are converted into olefins via free-radical elimination processes on treatment with tributyltin hydride in the presence of catalytic quantities of azobis(isobutyronitrile) (AIBN). Elimination from the dinitro compounds shows no stereocontrol by contrast, elimination from /3-nitrosulphones is highly stereoselective, e.g. (26)- (27), presumably because elimination from the intermediate radical is faster than rotation about the central carbon-carbon bond. 

Nugent and RajanBabu described that with Cp2TiCl , that had been isolated and purified prior to use, an (E) to (Z) ratio of 3-4 1 of 5-decenes was observed from either cis- or trans-5-decene oxide [28,29]. Therefore, it seems clear that a common long-lived /f-lilanoxy radical intermediate was formed from both epoxides. After further reduction and elimination the formation of the mixture of olefin diastereoisomers was observed. 

Mechanistically quite different from the process described in (19) is the process of methyl elimination from the cation radicals of 93 and its metal para isomers 9b40 . In the case of 93 methyl loss is initiated by a hydrogen transfer from the benzylic site to the ester function, 93- 94, thus forming a reactive intermediate 94 from... 

Propoxur (313) is another herbicide which can be eliminated from the environment by means of photochemical treatments. In fact, it has been shown that direct irradiation of aerated aqueous solutions of propoxur leads to formation of PFR products, disappearing almost completely the starting material. By laser flash photolysis, it has been demonstrated that the key intermediate in this reaction is the 2-isopropoxyphenoxy radical [297]. 

Experimental evidence indicates that the alkyl radical intermediates from the anodic oxidation of carboxylates are generated in free solution and have no memory of the configuration of the carboxyl group that was eliminated. Where the carboxylic acid function is attached to an asymmetric carbon atom as in 15, the Kolbe coupling reaction leads to complete racemization [67]. Anodic oxidation of (+)-2-... 

Moorthy PN, Hayon E (1975) Free-radical intermediates produced from the one-electron reduction of purine, adenine and guanine derivatives in water. J Am Chem Soc 97 3345-3350 Mori M, Teshima S-l, Yoshimoto H, Fujita S-l, Taniguchi R, Hatta H, Nishimoto S-l (2001) OH Radical reaction of 5-substituted uracils pulse radiolysis and product studies of a common redox-ambivalent radical produced by elimination of the 5-substituents. J Phys Chem B 105 2070-2078 Morin B, Cadet J (1995) Chemical aspects of the benzophenone-photosensitized formation of two lysine - 2 -deoxyguanosine cross-links. J Am Chem Soc 117 12408-12415 Morita H, Kwiatkowski JS,TempczykA(1981) Electronic structures of uracil and its anions. Bull Chem Soc Jpn 54 1797-1801... 

Possible reaction pathways to give compounds 123 and 124 involving the generation of vinyl radicals 129 (path 1) were suggested as shown in Scheme 6. The formation of acetylenes 127 for the monocyclic 1,2,3-selenadiazoles 126 was explained by involving the retro [2+3] addition reaction (path 2) or the concerted elimination of molecular nitrogen and selenium atom from the radical intermediate 131 (path 3). The paths 2 or 3 were suppressed for the reaction of bicyclic 1,2,3-selenadiazoles 121 due to the difficulty in the formation of the transition states 130 and 132. 

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