Radicals addition to double bonds

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Radical additions to double bonds have been investigated mechanistically and, recently, also for synthetic applications (Giese, 1989). It has been established that the reactivity trends can be described properly in qualitative and quantitative terms by an FMO interpretation (Fischer, 1986 Fischer and Paul, 1987 Giese, 1983 Miinger and Fischer, 1985). 

In the case of methyl radical addition to double bonds, L. Herk, A. Stefani, and M. Szwarc [J. Am. Chem. Soc., 83, 3008 (1961)] have drawn attention to the importance of the electron-withdrawing power of the conjugated substituent in determining the reactivity of olefins. More recently, to explain a somewhat similar phenomenon, F. Minisci and R. Galli (Tetrahedron Letters, 1962, 533) have invoked the concept that CHS is nucleophilic in character. 

We now turn our attention to the second common reaction of radicals, addition to double bonds. Because an alkene contains an electron-rich, easily broken n bond, it reacts with an electron-deficient radical. 

Intramolecular 5-exo radical additions to double bonds forming five-membered rings have been widely explored over the last few decades, and are probably the methods of choice to form such rings. The radicals can be produced in different ways. The classical tributyltin-mediated formation of radicals from halides, thionocarbonates, and so on, has been widely used. More recently, diiodosamarium-mediated radical formations have been explored. In these cases, aldehydes are reacted with Sml2 to generate ketyl radicals, which can be added to activated double bonds. 

As a simple example, consider the addition of ethyl radical to the O end of the carbonyl group in allyl acetate (CH3C(0)0CH2CHCH2). The Reaction Recipe for radical-addition-to-double-bond is shown in Fig. 7. 

Radical additions to double bonds are, in general, highly exothermic processes and rates increase with increasing temperature. The rcgiospccificity of addition to double bonds and the relative reactivity of various olefins towards radicals are also temperature dependent. Typically, specificity decreases with increasing temperature (the Reactivity-Selectivity Principle applies). Flowever, a number of exceptions to this general rule have been reported. 

In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if all antioxidant materials are removed. A chain reaction is indicated here. It might be explained by steric considerations. In butadiene-based rubbers, double bonds are quite accessible. Radical addition to double bonds could give highly reactive radicals, which would be likely to add to other polymer double bonds. A chain of additions might be more likely in butadiene rubber than in the presence of hindering methyl groups in isoprene rubbers. 

The rates of chain-transfer to polymer and radical addition to double bonds of preformed polymers increase as a function of three variables (1) temperature, (2) absence of chain-transfer agent, and (3) monomer conversion. Mochel (69) showed that in the absence of a chain-transfer agent, up to 90% of the polymer formed at 30% monomer conversion was gelled. 

Other common reactions are hydrogen abstraction and radical additions to double bonds. These reactions are denoted with single-headed arrows as combinations of the two kinds of electron pushing given just above. Combining the kinds of steps shown here, with different radical reactants, will allow you to write the electron pushing for most radical reactions. 

Thiyl radical addition to double bonds has also been used in model studies of radical damage to DNA. In the work by Giese and co-workers [29], thiyl radicals were chosen to generate selectively positioned radical centers and in particular... 

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