RESUMEN
This study presents a Ni-photoredox method for indole N-arylation, broadening the range of substrates to include indoles with unprotected C3-positions and base-sensitive groups. Through detailed mechanistic inquiries, a Ni(I/III) mechanism was uncovered, distinct from those commonly proposed for Ni-catalyzed amine, thiol, and alcohol arylation, as well as from the Ni(0/II/III) cycle identified for amide arylation under almost identical conditions. The key finding is the formation of a Ni(I) intermediate bearing the indole nucleophile as a ligand prior to oxidative addition, which is rare for Ni-photoredox carbon-heteroatom coupling and has a profound impact on the reaction kinetics and scope. The pre-coordination of indole renders a more electron-rich Ni(I) intermediate, which broadens the scope by enabling fast reactivity even with challenging electron-rich aryl bromide substrates. Thus, this work highlights the often-overlooked influence of X-type ligands on Ni oxidative addition rates and illustrates yet another mechanistic divergence in Ni-photoredox C-heteroatom couplings.
RESUMEN
This perspective details advances made in the field of Ni-catalyzed C-N bond formation. The use of this Earth abundant metal to decorate amines, amides, lactams, and heterocycles enables direct access to a variety of biologically active and industrially relevant compounds in a sustainable manner. Herein, different strategies that leverage the propensity of Ni to facilitate both one- and two-electron processes will be surveyed. The first part of this Perspective focuses on strategies that facilitate C-N couplings at room temperature by accessing oxidized Ni(III) intermediates. In this context, advances in photochemical, electrochemical, and chemically mediated processes will be analyzed. A special emphasis has been put on providing a comprehensive explanation of the different mechanistic avenues that have been proposed to facilitate these chemistries; either Ni(I/III) self-sustained cycles or Ni(0/II/III) photochemically mediated pathways. The second part of this Perspective details the ligand designs that also enable access to this reactivity via a two-electron Ni(0/II) mechanism. Finally, we discuss our thoughts on possible future directions of the field.