An assay for aryl radicals using BHAS coupling

Clark, Kenneth F. and Tyerman, Seb and Evans, Laura and Robertson, Craig M. and Murphy, John A. (2024) An assay for aryl radicals using BHAS coupling. Organic and Biomolecular Chemistry, 22 (5). pp. 1018-1022. ISSN 1477-0520 (https://doi.org/10.1039/D3OB01743E)

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Abstract

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 (13). In this case, biphenyl arises from a radical translocation and is the major coupling product. This paper now quantitatively investigates that reaction, which shows a very similar ratio for 3 : 12 [ca. 4 : 1] when using different sources of radical initiation. Deuterium isotope studies provide detailed mechanistic support for the proposed mechanism; when carried out in C6D6 vs. C6H6, the reaction is characterised by a strong isotope effect for formation of 3-d10 vs. 3, but not for formation of 12-d5 vs. 12. These distinctive properties mean that the transformation can act as an assay for aryl radicals. An advantage of such a BHAS process is its sensitivity, since it involves a chain reaction that can amplify radical activity.

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