Asymmetric Catalysis with an Inert Chiral-at-Metal Iridium Complex

The development of
a chiral-at-metal iridium­(III) complex for the
highly efficient catalytic asymmetric transfer hydrogenation of β,β′-disubstituted
nitroalkenes is reported. Catalysis by this inert, rigid metal complex
does not involve any direct metal coordination but operates exclusively
through weak interactions with functional groups properly arranged
in the ligand sphere of the iridium complex. Although the iridium
complex relies only on the formation of three hydrogen bonds, it exceeds
the performance of most organocatalysts with respect to enantiomeric
excess (up to 99% ee) and catalyst loading (down to 0.1 mol %). This
work hints at an advantage of structurally complicated rigid scaffolds
for non-covalent catalysis, which especially relies on conformationally
constrained cooperative interactions between the catalyst and substrates.