Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis

A new class of chiral
iron catalysts is introduced that contains
exclusively achiral ligands with the overall chirality being the result
of a stereogenic iron center. Specifically, iron­(II) is cis-coordinated to two N-(2-pyridyl)-substituted N-heterocyclic
carbene (PyNHC) ligands in a bidentate fashion in addition to two
monodentate acetonitriles, and the dicationic complex is complemented
by two hexafluorophosphate ions. Depending on the helical twist of
the PyNHC ligands, the metal center adopts either a Λ or Δ
absolute configuration. Importantly, the two PyNHC ligands are constitutionally
and configurationally inert, while the two acetonitriles are labile
and allow asymmetric transition metal catalysis. This is demonstrated
with an enantioselective Cannizzaro reaction (96% yield, 88% ee) and
an asymmetric Nazarov cyclization (89% yield, >20:1 dr, 83% ee).