Polar effects on free radical reactions

The understanding of polar effects on free radical reactions arose from studies of free radical polymerization where transition state effects were empha-sized. Further studies involved diacyl peroxide reactions (equation 45), hydrogen abstraction from ring-substituted toluenes, and reactions of peresters involving transition state 38 (equation 57).  

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If we define polar effects on free-radical reactions as effects due to electron-withdrawal or electron-release—rather than to accommodation of the odd electron—then there is no doubt about their existence it is the interpretation of such effects that is open to question. 

This reaction is based on the proposition that the sensitivity to polar effects in free-radical chemistry is the result of polarity and polarizability of both the radical and the substrate. This means that the polarity of the heteroaromatic base plays a key role in the process. Actually, the nucleophilic character of an alkyl radical, for example, is not so marked as to justify the addition to the N-heteroaromatic base, and in fact either no substitution occurs or low yields and selectivity are observed. 

Free radical chemistry continues to be of importance in the field and a survey has been published on polar and enthalpic effects in free radical reactions. A further short review dealt with free radicals, carbenes and nitrenes at the anomeric centre of carbohydrates. A useful survey has appeared of the application of carbohydrate based chiral auxilliaries in stereoselective syntheses which covers a range of reactions, for example cycloaddition processes, reductions and Strecker reactions. ... 

One type of solvent effect on free-radical addition reactions such as the propagation step of free-radical polymerization is the so-called polarity effect . This type of solvent effect is distinguished from other solvent effects, such as complexation, in that the solvent affects the reactivity of the different types of propagation steps without directly participat-... 

The first step of a free radical aromatic substitution, the formation of the a-com-plex, is also an addition step. The o,m,p-product ratio therefore also responds to steric effects. This is shown for the free radical phenylation and dimethylamination of toluene and r.-butylbenzene in Table 8. The larger the substituent on the aromatic system and the bulkier the attacking radical, the more p-substitution product is obtained at the expense of o-substitution. In the phenylation reaction the yield of m-product also increases in contrast to the dimethylamination reaction. The substitution pattern of this latter reaction is, in addition to the steric effect, governed heavily by polar effects because a radical cation is the attacking species113. ... 

Chemically induced dynamic nuclear polarization (CIDNP) was first reported in 1967 in independent work from three different laboratories. The effects of free radicals on NMR spectra were revealed (Fig. 6) in studies of radicals from peroxides (equation 62) and azo compounds, as well as radicals generated from the reaction of alkyl halides and organolithium compounds. ... 

Beranek I, Fischer H (1989) Polar Effects on Radical Addition Reactions An Ambiphilic Radical. In Minisci F (ed) Free Radicals in Synthesis and Biology. Kluwer, Amsterdam, p 303... 

Based on the product analysis of the reaction of Me3SnLi with various 1,4-dihalo bicyclo[2.2. l]heptanes, it was proposed that in this system a polar mechanism can compete effectively with free radical chain processes. The HME pathway is predominant for the dibromo, bromoiodo and diiodo compounds, a competition between HME and ET was suggested with the chloroiodo compound and the ET reaction is important for the fluoroiodo and chlorobromo derivatives135. 

Dneprovkii AS, Miltsov SA, Arbuzov PV (1988) Mechamisms of free-radical reactions. XXIV. Quantitative description of the polar effects of substituents on the kinetics of the free-radical chlorination of aliphatic compounds by N-chloropiperidine. Zh Org Khim (Russ) 24(10) 2026-2037... 

Studies show that the measured composition of the product mixture at constant temperature depended on the water density (Fig. 7.7). This was taken as an indication that these products could be formed by competing ionic and free-radical reaction pathways. Usually in gas-phase kinetics the product composition changes with temperature because of the different activation energies and, to a minor extent with pressure, mainly because of the concentration effect on bimolecular elementary reaction steps. In water, the drastic dependence on pressure is likely a consequence of the competition between reactions with different polarity. Free radical reaction rates (involving large free radicals beyond the RRKM high-pressure limit, see, for example, [25]) should decrease with pressure as a result... 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mostly owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (31). Application of frontier molecular orbital theory may be more appropriate to explain certain reactions (32,33). Radical reactivities have been studied by esr spectroscopy (34-36) and modeling based on general reactivity and radical polarity (37). Recent radical trapping studies have provided considerable insight into the course of free-radical reactions, particularly addition polymerizations, using radical traps such as 2,4-diphenyl-4-methyl-l-pentene (a-methylstyrene dimer, MSD) (38-44) and 1,1,3,3-tetramethyl-2,3-dihydro-liT-isoindol-2-yloxyl (45-49). 

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