Activation of Cobalt-Nitro Complexes by Lewis Acids: Catalytic Oxidation of Alcohols by Molecular Oxygen

Article abstract of DOI:10.1021/ja00402a044

Lewis acid dramatically enhance the oxidation power of cobalt-nitro complexes.Thus, in the presence of BF3*Et2O or LiPF6, cobalt-nitro complexes such as pyCo(saloph)NO2 or pyCo(TPP)NO2 oxidize primary alcohols to aldehydes and secondary alcohols to ketones.No reaction is observed in the absence of Lewis acid.The effect of Lewis acids is attributed to their association with the nitro ligands, thereby increasing its electrophilicity.The results strongly suggest that the oxidation proceeds via an "ester-like" intermediate, which in a nonradical pathway collapses to the carbonyl product, water, and the corresponding nitrosyl complex.Importantly, it has been found that the reoxidation of the nitrosyl complexes by molecular oxygen is facile in the presence of Lewis acids.This finding facilitated the conversion of the stoichiometric oxidation of alcohols into a catalytic system using molecular oxygen as the oxidant.Initial oxidation rates are rapid.However, the rates decline as the byproduct, water, accumulates in the reaction mixture.