Base-Free Transfer Hydrogenation of Ketones Using Cp*Ir(pyridinesulfonamide)Cl Precatalysts

N-(2-(Pyridin-2-yl)­ethyl)­benzenesulfonamide
derivatives
and 1,1,1-trifluoro-N-(2-(pyridin-2-yl)­ethyl)­methane­sulfonamide
(1–4), along with three-legged piano
stool Cp*IrIIICl complexes (5–11) (Cp* = pentamethyl­cyclopentadienyl) bearing pyridinesulfonamide
ligands with varying electronic parameters, were synthesized. These
ligands and air-stable complexes were characterized by 1H and 13C­{1H} NMR spectroscopy, elemental analysis,
and single-crystal X-ray diffraction. Precatalysts, 5–11, were assessed for transfer hydrogenation
of aryl, diaryl, dialkyl, linear, cycloaliphatic, and α,β-unsaturated
ketones, diones, β-ketoesters, and a biomass-derived substrate
with 2-propanol, using 1 mol % precatalyst. Catalysis was also efficient
using a 0.1 mol % loading. Remarkably, all catalysis experiments can
be conducted in air without dried and degassed substrates, and basic
additives and halide abstractors are not required for high activity
in transfer hydrogenation. Control experiments and a mercury poisoning
experiment support a homogeneous catalyzed pathway. Overall, the fastest
reactions are observed using electron-poor substrates and precatalysts
bearing electron-rich ligands.