Free radical aromatic substitution

A second theoretical index, and one for which there appears to be more justification in its application to free-radical reactions, is the atom localization energy. This index is a measure of the energy required to localize one electron of the 7r-electron system in the aromatic molecule at the point of attack of the radical. The formation of the intermediate adduct in a free-radical aromatic substitution may be regarded as the sum of two processes one, the localization of an electron at the point of attack and the other, the pairing of this... 

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. ... 

Table 8. Steric substituent effects in free radical aromatic substitutions...

Since benzenesulfonyl peroxide was used as an initiator in polymerization reactions, it was thought that a free radical aromatic substitution of benzene by the benzenesulfonoxy radical takes place. A detailed study by Dannley and Knipple reveals that attachment of the sulfonoxy group derived from a bis(arylsulfonyl) peroxide to the aromatic ring occurs by electrophilic aromatic substitution (equation 5) °. 

Free-Radical Induced Bond Formation and Aromatization. Based on current understanding of free-radical aromatic substitution reactions at low temperatures ( 7), a major pathway for polymerization and crosslink formation in coal systems is expected to be,... 

Alkylmercury halides or carboxylates undergo a free radical aromatic substitution reaction with pyridine, pyridinium ions or dialkylanilines. Addition of R to these substrates produces the easily oxidized radicals... 

J. G. Traynham, Ipso Substitution in Free Radical Aromatic Substitution Reactions , Chem. Rev., 1979, 79, 323. 

The first examples of selective substitution of heteroaromatic bases by carbon-centered radicals were reported by Minisci and his group at the end of the 1960s (Scheme 1) [1,2]. Those manuscripts, which can be considered the first accounts of the so-called "Minisci reaction" [3], played a decisive role to the chemical community in reconsidering the potentiality of free-radical aromatic substitutions. In fact, until then the homolytic process had been considered of poor interest compared with the electrophilic and nucleophilic ionic substitutions, due to the discouraging results achieved witii homolytic arylation, the reaction most studied so far [4]. The low selectivity observed for that reaction was erroneously assumed as a general feature of homolytic aromatic substitution and this misinterpretation was overcome only when it was recognized that the polar effect could play a key role in free-radical reactions as well. 

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