Exogenous Ligand-Free Nickel-Catalyzed Carboxylate O-Arylation: Insight into NiI/NiIII Cycles

Nickel-catalyzed cross-coupling reactions have become a powerful methodology to construct C-heteroatom bonds. However, many protocols suffer from competitive off-cycle reaction pathways and require non-equimolar amounts of coupling partners to suppress them. Here, we report on mechanistic examination of carboxylate O-arylation under thermal conditions, in both the presence and absence of an exogeneous bipyridine-ligand. Furthermore, spectroscopic studies of the novel ligand-free carboxylate O-arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of an off-cycle Ni-I/Ni-III dimer upon reduction. This study provides insights into the competition between productive catalysis and deleterious pathways (comproportionation and protodehalogenation) in the commonly proposed self-sustained Ni-I/Ni-III catalytic cycle. Thereby we show that for productive nickel-catalyzed carboxylate O-arylation a choice must be made between either mild conditions or equimolar ratios of substrates.

Automated summary

This study investigates the mechanistic examination of carboxylate O-arylation under thermal conditions and provides insights into the competition between productive and deleterious pathways in the commonly proposed self-sustained Ni-I/Ni-III catalytic cycle. The spectroscopic studies reveal the resting state of the nickel catalyst and the crucial role of alkylamine base in the reaction process. The study highlights the need to choose between mild conditions or equimolar ratios of substrates for productive nickel-catalyzed carboxylate O-arylation.