Monitoring Radical Intermediates in Photoactivated Palladium-Catalyzed Coupling of Aryl Halides to Arenes by an Aryl Radical Assay

An aryl radical assay is used to provide information about the formation of aryl radicals from aryl halides in coupling reactions to arenes in the presence of palladium sources and under LED irradiation (λ = 456 nm). The assay uses 2-halo-<i>m</i>-xylenes as substrates. Aryl radical formation is indicated both by a defined product composition and by signature deuterium isotope effects. Comparison with our recently published results for corresponding ground-state palladium-catalyzed reactions shows three principal differences: (i) in the photoactivated reactions, evidence supports the formation of aryl radical intermediates with all the phosphine ligands tested, in contrast to thermal ground-state chemistry where only specific ligands had encouraged this pathway, while others had promoted a nonradical coupling mechanism; (ii) oxidative addition complexes that are formed from the reaction of Pd(0) sources with aryl halides react under photoactivation to form biaryl coupled products through radical intermediates, in contrast to their behavior under thermal activation - so Ar-Pd bonds are homolyzed under LED irradiation; (iii) the photoreactions work well with mild bases like Cs₂CO₃, while the thermal reactions required KO^tBu as the base due to the different roles for base under the thermal versus photochemical mechanisms.