Hydride Transfer in the Oxidation of Alcohols by 2+. A kH/kD Kinetic Isotope Effect of 50

Article abstract of DOI:10.1021/ja00237a019

The kinetics of oxidation of a series of alcohols by <(bpy)2(py)Ru^IV(O)>²⁺ (bpy is 2,2'-bipyridine and py is pyridine) have been studied in aqueous solution and in acetonitrile.The reactions are first order in both alcohol and Ru^IV=O²⁺, pH independent from pH 1.0 to 6.8, and slightly slower in acetonitrile than in water.Rate constants for oxidation increase with substituent as follows: methyl < primary < secondary < allylic -1 s^-1 for the oxidation of methanol (ΔH = 14 +/- 2 kcal/mol, ΔS = -26 +/- 6 eu) to 2.43 +/- 0.03 M^-1 s^-1 for the oxidation of benzyl alcohol (ΔH = 5.8 +/- 0.4 kcal/mol, ΔS = -38 +/- 1 eu).Large C-H kinetic isotope effects are observed, but solvent isotope effects are negligible.For CH3OH compared to CD3OH, k_H/k_D = 9 at 25 deg C and for benzyl alcohol, C6H5CH2OH compared to C6H5CD2OH, k_H/k_D = 50 +/- 3 at 25 deg C.Spectral evidence, in conjunction with the isotope effect data, suggests that oxidation of alcohols by <(bpy)2(py)Ru^IV(O)>²⁺ occurs by a two-electron, hydride transfer.