Abstract
We report iridium catalysts IrCl(η(5)-Cp*)(κ(2)-(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(η(5)-Cp*)(κ(2)-(C(5)H(4)N)CHNSO(2)Ar) (2), IrH(η(5)Cp*)(κ(2)-(2-pyridyl)CH(2)NSO(2)Ar) (3), and Ir(η(5)-Cp*)(κ(3)-(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 (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.