Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts

Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)(-)) (1H(H)) with O-2 generate Cp*Ir(TsDPEN-H) (1) and 1 equiv of H-2O. Kinetic analysis indicates a third-order rate law (second order in [1H(H)] and first order in [O-2]), 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) + O-2 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 Delta H-double dagger = 82.2 kJ/mol, Delta S-double dagger = 13.2 J/mol center dot K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl2 solution under 0.30 atm of H-2 and 0.13 atm of O-2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)(2)BAr4F (BAr4F- = B(C6H3-3,5-(CF3)(2))(4)(_)). The formation of water from H2 was verified by H-2 NMR for the reaction of D-2. + O-2. 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 O-2 with RNH2BH3 (R = H, t-Bu), resulting in the formation of water and H-2. The deactivation of the catalyst 1 in its reactions with O-2 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.