Monitoring Radical Intermediates in Photoactivated Palladium-Catalysed 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 (lambda = 456 nm). The assay uses 2-haloxylenes as substrates. Aryl radical formation is indicated both by a defined product composition and by signature deuterium isotope effects. Comparison with recently published results for corresponding ground state palladium-catalysed 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 non-radical coupling mechanism; (ii) oxidative addition complexes that are formed from reaction of Pd(0) sources with aryl halides reacted under photoactivation to form biaryl coupled products through radical intermediates, in contrast to their behaviour under thermal activation;. (iii) the photoreactions work well with mild bases like Cs2CO3, while the thermal reactions required KOtBu as base due to the different roles for base under the thermal versus photochemical mechanisms.