Intermolecular arylations, radical-based

The mechanism is believed to begin by SET from the base to the aryl iodide to form a radical anion. Inter-molecular reaction can occnr through a HAS, the radical variant of the more commonly known EAS reaction. For intermolecular reaction, addition of the radical to the coupling partner resnlts in a resonance-stabilized radical. Deprotonation with base forms a biaryl radical anion which propagates the mechanism by donating an electron to another aryl haUde. For intra-molecular reaction, the aryl radical likely adds to the ipso position of the tether. Subsequent ring expansion and re-aromatization yields the prodnct. ... 

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. 

Many neutral organic radicals are based on heavily chlorinated aryl derivatives (Fig. 1), such as the perchlorophenalenyl or the polychlorotriphenyl radicals [12, 13]. When described, the X-ray crystal structures of such compounds inevitably show intermolecular Cl- -Q contacts, without identifying particularly short ones. In perchlorophenalenyl, for example, steric interactions between the Cl atoms in peri position in the molecules lead to a strong distortion from planarity, which induces a ruffled molecular structure [13]. As a consequence, the shortest intermolecular Cl Cl distances within the stacks amount to 3.78 A, above the sum of the van der Waals radii (3.50 A). The bromo or iodo analogs of such complexes were not described, probably for stability reasons. 

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