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

Abstract

We report iridium catalysts IrCl(η5-Cp⁎)(κ2-(2-pyridyl)CH2NSO2C6H4X) (1-Me, X = CH3 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-(C5H4N)CHNSO2Ar) (2), IrH(η5-Cp⁎)(κ2-(2-pyridyl)CH2NSO2Ar) (3), and Ir(η5-Cp⁎)(κ3-(2-pyridyl)CH2NSO2Ar) (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 + R2CHOH ⇄ 3 + R2CO) is suggested by kinetic isotope effects, Eyring parameters (ΔH≠ = 29.1(8) kcal mol−1 and ΔS≠ = −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.