A new mechanism of metal-ligand cooperative catalysis in transfer hydrogenation of ketones.

We report a new metal-ligand cooperative mechanism for transfer hydrogenation of ketones with alcohols enabled by iridium catalysts 1-Me and 1-F. The reaction proceeds through the nine-membered cyclic transition state, TS , which is stabilized by an extra alcohol molecule that shuttles the proton from the alcohol to the catalyst. We report iridium catalysts IrCl(η5-Cp⁎)(κ2-(2-pyridyl)CH 2 NSO 2 C 6 H 4 X) (1-Me , X = CH 3 and 1-F , 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 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η5-Cp⁎)(κ2-(C 5 H 4 N)CHNSO 2 Ar) (2), IrH(η5-Cp⁎)(κ2-(2-pyridyl)CH 2 NSO 2 Ar) (3), and Ir(η5-Cp⁎)(κ3-(2-pyridyl)CH 2 NSO 2 Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R 2 CHOH ⇄ 3 + R 2 CO) is suggested by kinetic isotope effects, Eyring parameters (Δ H ≠ = 29.1(8) kcal mol−1 and Δ S ≠ = −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. [ABSTRACT FROM AUTHOR]