Base-Promoted Homolytic Aromatic Substitution BHAS

4 Base-Promoted Homolytic Aromatic Substitution (BHAS) 

OvCT the recent years, there have been many reported syntheses of biaryl compounds using BHAS [108], thereby avoiding the use of stoichiometric amounts of tin or silicon reagents, radical initiators, or transition metals. In 2008, Itami showed that KOBu caused the addition of the aryl part of Arl or ArBr to pyrazine and other election-poor arenes under elevated temperatures or MW irradiation (Eq. 9.28) [109]  

Furthermore, different research groups have reported independently the construction of biaryl compounds from unactivated aromatic compounds by direct C—H activation using NaOEu or KOEu and 1,10-phenanthroline [110, 111] or DMEDA [112] as ligands (Eqs. 9.29 and 9.30)  

The facts that the reactions require a large excess of acceptor, the appearance of the reduced haloarene with an H/D isotope effects, and inhibition by TEMPO or other radical scavengers are consistent with radicals being involved. 

A number of authors have suggested that ET from a complex 134 of NaOBu or KOBu with a ligand to Phi is the initiation step in the radical process to give the radical cation complexes 135 and the radical anion of Phi 136 (Eq. 9.31) [110, 111, 120, 124]  

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