title: Homogeneous catalytic reduction of dioxygen using transfer hydrogenation catalysts.
abstract: Solutions of Cp*IrH(rac-TsDPEN) (TsDPEN = H2NCHPhCHPhN(SO2C6H4CH3)-) (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 DeltaH = 82.2 kJ/mol, DeltaS = 13.2 J/mol K, and a reaction barrier of 85.0 kJ/mol. A CH2Cl2 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)2BAr(F)4 (BAr(F)4- = 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 (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 O2.
authors.affiliation: School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Zachariah M
authors.initials: ZM
authors.lastname: Heiden
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Rauchfuss
mesh_terms: D000588:Amines / Q000737:chemistry; D016227:Benzoquinones / Q000737:chemistry; D002384:Catalysis; D003903:Deuterium / Q000737:chemistry; D005029:Ethylenediamines / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006865:Hydrogenation; D007495:Iridium / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014050:Toluene / Q000031:analogs & derivatives* / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D000588:Amines; D016227:Benzoquinones; D005029:Ethylenediamines; C526626:N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine; D009942:Organometallic Compounds; D014867:Water; D014050:Toluene; D007495:Iridium; D012428:Ruthenium; D006859:Hydrogen; D003903:Deuterium; D010100:Oxygen