Free radical aromatization

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 newly formed free radical may terminate by abstraction of a hydrogen atom, or it may continue cracking to give ethylene and a free radical. Aromatic compounds with side chains are usually dealkylated. The produced free radicals further crack to yield more olefins. 

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) °. 

Intramolecular cyclization of N-alkenyl-2-bromoindoles has been described. The formation of five as well as six-membered ring in the radical cyclization proceeds in good yield [95TL4857]. A novel intramolecular free radical aromatic ipso substitution reaction has been reported [95CC1353]. Treatment of 138 with tributyltin hydride generates an alkyl radical which attacks the alkene at the carbon bearing the sulfur substituent. This is followed by elimination of the sulfur substituent to provide the cyclized products 139 in moderate yields. The efficiency of the reaction is dependent on the size of the ring formed and on the sulfur substituent. 

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. 

Examples of some important initiators are ethyl methyl ketone peroxide, cyclohexanone peroxide, benzoyl peroxide and cumene peroxide. Accelerator is a reducing agent, such as cobalt octoate, which is added in very small quantity to catalyze decomposition of the initiator into free radicals. Aromatic amines such as dimethyl aniline or dimethyl /j-toluidine are added to promote that reaction. The two components are combined prior to application, ensuring even distribution of initiator in the system. The dosage of initiator, accelerator, promoter and inhibitor will determine the pot life, the longest period of time during which mixture is still usable and can be applied. 

Free-radical aromatic nitromethylation is promoted by different cerium(IV) salts (Kurz and Ngoviwatchai, 1981). The reaction is carried out by refluxing a mixture of an aromatic compound, nitromethane, and a cerium(IV) salt in acetic acid (scheme 46). The mechanism in-... 

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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