An Assay for Aryl Radicals Using BHAS Coupling

Aryl radicals are intermediates in many reactions, but determining their presence unambiguously is often challenging. As we recently reported, reaction of 2-iodo-1,3-dimethylbenzene 7 in benzene with KOtBu and a suitable organic additive, leads to a base-induced homolytic aromatic substitution (BHAS) coupling reaction giving 2,6-dimethylbiphenyl 12 and biphenyl 3 as coupled products, together with xylene. Biphenyl arises from a radical translocation and is the major coupling product. This paper now investigates that reaction, using deuterium isotope studies to provide detailed mechanistic support. The reaction gives very similar outcomes with two different radical generation sources, and in doing so, shows its ability to act as an assay for aryl radicals. An advantage of such a BHAS process is that it involves a chain reaction that can amplify radical activity.