Homogeneous Catalytic Reduction of Dioxygen Using Transfer Hydrogenation Catalysts.

Solutions of Cp*lrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (1 H(H)) with O2 generate Cp*lr(TsDPEN) (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 1 H(H) + O2 was strongly affected by deuteration at the hydride position (kHH2/kDH2 = 6.0 ± 1.3) but insensitive to deuteration of the amine (kHH2/kHD2 = 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 ΔH* = 82.2 kJ/mol, ΔS* = 13.2 J/mol·K, and a reaction barrier of 85.0 kJ/mol. A CH2CI2 solution under 0.30 atm of H2 and 0.13 atm of O2 converted to H2O in the presence of 1 and 10 mol % of H(OEt2)2BArF4 (BArF4 = B(C6H3-3,5-(CF3)2)4). The formation of water from H2 was verified by 2H NMR for the reaction of D2 + 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 (A = 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 O2. [ABSTRACT FROM AUTHOR]