Autoxidation of Phenols Catalyzed by Copper(II)-Ethylenediamine Complexes: The Reaction Mechanism

Article abstract of DOI:10.1021/jo00197a024

The mechanism of the autoxidation of phenols with molecular oxygen catalyzed by Cu(II)-ethylenediamine complexes is elucidated.The kinetics are of first order in both and and of one-half order in .Those results imply the formation of a oxygen-bridged complex in the course of the reaction.The C-C coupling of two phenoxy radicals is facilitated within the coordination sphere of Cu(II) and affords the primary oxidation products, i. e. 2,2'-biphenyldiols.The bulkiness of substituents determines the type of the secondary products and either benzofuran or dioxepin is produced.Taking into account the results of oxidation of 2,2'-biphenyldiols, the mechanism of the production of benzofuran and dioxepin is also clarified.In the Cu(II) complex with sterically crowded biphenyldiolate, the oxidation activity of Cu(II) has been enhanced and the coordinating biphenyldiolate is immediately oxidized to benzofuran.On the contrary, the biphenyldiolate-Cu(II) complex with square-planar geometry could not produce benzofuran.After the complex is forced into the active tetrahedral configuration by the access of phenoxy radical-Cu(II) complex, the biphenyldiolate is oxidized to biphenyloxy radical and suffers C-O coupling with the phenoxy radical to afford dioxepin.