Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts

Zachariah M. Heiden, Thomas B. Rauchfuss

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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 (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 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)2BArF4 (BArF 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.

Original languageEnglish (US)
Pages (from-to)14303-14310
Number of pages8
JournalJournal of the American Chemical Society
Issue number46
StatePublished - Nov 21 2007

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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