Meta directing groups radicals

If the salt is decomposed in an aromatic solvent containing a meta-directing group, the isolation of a meta-substituted product supports a heterolytic process, while an ortho- or para-substituted product favors a radical mechanism. 

In the pyrrole series, ester groups a to nitrogen are more readily hydrolyzed by alkali, but those in a /3 position more readily by acid. A methoxycarbonyl group in the 2-positlon is meta directing thus bromination yields mainly 4-bromo-2-methoxycarbonylpyrrole. Free radical chlorination with f-butylhypochlorite gives the 5-chloro derivative. 

In a study aimed at the identification of products of free radical reactions with polystyrene- and aromatic-based PEMs using model compounds, Hiibner and Roduner observed the addition of free radicals to the aromatic rings, preferentially in the ortho position to alkyl- and RO-substituents (in polystyrene- and aromatic-based PEMs, the para position is blocked by the presence of the sulfonic acid group). This study demonstrated the combined ortho-activation by these substituents and the meta-directing effect... 

Since OH is strongly electrophilic, the OH group directs OH into its ortho- and para-positions [reactions (51)—(53)]. One of the ortho-positions is occupied by the somewhat bulky carboxylate group which renders reaction (51) less likely than reaction (52). An addition to the meta-position can be largely neglected. Upon oxidation of the OH-adduct radicals, cyclohexadienones are formed [reactions (54)-(56)] which either decarboxylate [reaction (57)] or rearrange into the corresponding phenols [reactions (58) and (59) e.g., Bausch et al. 1976]. Product yields from hydroxybenzoic acids are compiled in Table 3.5 from salicylic acid in Table 3.6. 

A homobimetallic rhodium catalyst derived from a P,N-ligand was found to allow for intermolecular direct arylations of unfunctionalized arenes [24]. Interestingly, aryl iodides, bromides-and even chlorides-could be employed as electrophiles, and a variety of valuable functional groups was tolerated by the catalytic system (Scheme 9.12). The C—H bond functionalization of toluene yielded ortho-, meta- and para-substituted regioisomers in a ratio of 71 19 10. Based on this observation and a Hammett correlation, a mechanism proceeding through radical intermediates was suggested. 

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