A New Mechanism of Metal-Ligand Cooperative Catalysis in Transfer Hydrogenation of Ketones.

We report iridium catalysts IrCl(η-Cp*)(κ-(2-pyridyl)CHNSOCHX) (, X = CH and , X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes ( and ) derivatize to a series of catalytic intermediates: Ir(η-Cp*)(κ-(CHN)CHNSOAr) (), IrH(ηCp*)(κ-(2-pyridyl)CHNSOAr) (), and Ir(η-Cp*)(κ-(2-pyridyl)CHNSOAr) (). The structures of and were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step ( + RCHOH ⇄ + RCO) is suggested by kinetic isotope effects, Eyring parameters (Δ = 29.1(8) kcal mol and Δ = -17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle.