Transformative ligand effects in Fe-photocatalyzed Giese-type additions

Visible-light-promoted Fe photocatalysis is a sustainable strategy for synthetic chemistry. Yet, the adoption of Fe photocatalytic reactions proceeding through a ligand-to-metal charge transfer event has been limited to simple substrates in many transformations. An outstanding challenge in the field is the selection of tunable ligand scaffolds providing general reactivity. In this work, we describe the use of aliphatic amines as ligands in Fe-promoted decarboxylative Giese-type additions. Unlike prior photoredox reports, this method enables the coupling of substrates with free amines, alcohols, and a boronic ester. To gain insight into the role of diethylenetriamine (L1), ligated Fe salts were investigated using ultraviolet-visible spectroscopy, thermal gravimetric analysis, and density functional theory. These studies support the formation of a photoactive octahedral carboxylate (L1)Fe(OCOR)₃. As an increasing number of photocatalytic reactions proceed using earth-abundant metals, the use of these ligands expands the possibility of applying Fe to a broad array of transformations.