Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts

Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H ₂NCHPhCHPhN(SO₂C₆H₄CH ₃)^-) (1H(H)) with O₂ generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H₂O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O₂]), resulting in an overall rate constant of 0.024 ± 0.013 M^-2 s^-1. Isotopic labeling revealed that the rate of the reaction of 1H(H) + O₂ was strongly affected by deuteration at the hydride position (k_HH2/k_DH2 = 6.0 ± 1.3) but insensitive to deuteration of the amine (k_HH2/k_HD2 = 1.2 ± 0.2); these values are more disparate than for conventional transfer hydrogenation (Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001). The temperature dependence of the reaction rate indicated Δ ^‡ = 82.2 kJ/mol, ΔS^‡ = 13.2 J/mol·K, and a reaction barrier of 85.0 kJ/mol. A CH₂Cl ₂ solution under 0.30 atm of H₂ and 0.13 atm of O ₂ converted to H₂O in the presence of 1 and 10 mol % of H(OEt₂)₂BAr_F⁴ (BAr^F ₄^- = B(C₆H₃-3,5-(CF ₃)₂)₄^-). The formation of water from H₂ was verified by ²H NMR for the reaction of D ₂ + O₂. Solutions of 1 slowly catalyze the oxidation of amyl alcohol to pentanal; using 1,4-benzoquinone as a cocatalyst, the conversion was faster. Complex 1 also catalyzes the reaction of O₂ with RNH₂BH₃ (R = H, t-Bu), resulting in the formation of water and H₂. The deactivation of the catalyst 1 in its reactions with O₂ was traced to degradation of the Cp* ligand to a fulvene derivative. This pathway is not observed in the presence of amine-boranes, which were shown to reduce fulvenes back to Cp*. This work suggests the potential of transfer hydrogenation catalysts in reactions involving O ₂.