Monitoring Radical\nIntermediates in Photoactivated\nPalladium-Catalyzed Coupling of Aryl Halides to Arenes by an Aryl\nRadical Assay

An aryl radical assay\nis used to provide information about the\nformation of aryl radicals from aryl halides in coupling reactions\nto arenes in the presence of palladium sources and under LED irradiation\n(λ = 456 nm). The assay uses 2-halo-<i>m</i>-xylenes\nas substrates. Aryl radical formation is indicated both by a defined\nproduct composition and by signature deuterium isotope effects. Comparison\nwith our recently published results for corresponding ground-state\npalladium-catalyzed reactions shows three principal differences: (i)\nin the photoactivated reactions, evidence supports the formation of\naryl radical intermediates with all the phosphine ligands tested,\nin contrast to thermal ground-state chemistry where only specific\nligands had encouraged this pathway, while others had promoted a nonradical\ncoupling mechanism; (ii) oxidative addition complexes that are formed\nfrom the reaction of Pd(0) sources with aryl halides react under photoactivation\nto form biaryl coupled products through radical intermediates, in\ncontrast to their behavior under thermal activation – so Ar–Pd\nbonds are homolyzed under LED irradiation; (iii) the photoreactions\nwork well with mild bases like Cs<sub>2</sub>CO<sub>3</sub>, while\nthe thermal reactions required KO<sup>t</sup>Bu as the base due to\nthe different roles for base under the thermal versus photochemical\nmechanisms.