Exogenous Ligand-Free Nickel-Catalyzed Carboxylate O-Arylation Insight into Ni^I/Ni^III 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^II 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.