Relevance ofNi(I) in Catalytic Carboxylation Reactions

Efforts to incorporate CO2, an abundant and inexpensive feedstock chemical, into organic molecules to form valuable carboxylic acids have included the development of nickel-catalyzed carboxylation reactions. These reactions span from electrochemical methods to more recently developed transformations where electrochemical cells have been replaced with solid reductants or an organometallic reagent. Although carboxylation of a very wide variety of substrates is now possible with high selectivities and with good substrate scope, there remain open questions regarding certain mechanistic aspects of these reactions. Herein, we aim to provide the context for emphasis on Ni(I) as a key oxidation state during the CO2 insertion step of many carboxylation reactions, and in doing so, discuss electrochemical and stoichiometric evidence for Ni(I). The challenges and opportunities facing mechanistic studies of such reactions are also discussed.