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

N-(2-(Pyridin-2-yl)ethyl)benzenesulfonamide derivatives and 1,1,1-trifluoro-N-(2-(pyridin-2-yl)ethyl)methanesulfonamide (1-4), along cat. with three-legged piano stool Cp*(IrCl)-Cl-III complexes (5-11) (Cp* = pentamethylcyclopentadienyl) bearing pyridinesulfonamide ligands with varying electronic parameters, were synthesized. These ligands and air-stable complexes were characterized by H-1 and C-13{H-1} 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 alpha,beta-unsaturated ketones, diones, beta-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.