CATALYTIC-OXIDATION WITH DINUCLEAR CU(I) MACROCYCLIC DIOXYGEN COMPLEXES AS INTERMEDIATES

The investigation of dinuclear copper complexes which interact with di oxygen has been extensive because of their relevance to biological sys tems and oxygen active multicopper sites are found in hemocyanin, tyro sinase and polynuclear copper oxidases. Oxygenated model copper dinucl ear complexes have characteristics similar to those of the dinuclear c opper sites of oxytyrosinase and oxyhemocyanin. In this paper the dinu clear Cu(I) complexes of macrocyclic ligands are described and the rea ctivities of their dioxygen complexes for the oxidation of various sub strates will be examined. Macrocyclic ligands were prepared by the 2 2 condensation of an aromatic dialdehyde with diethylenetriamine and with ditrimethylene-triamine. The dinuclear Cu(I) complex of the macro cyclic ligand formed by the 2 + 2 condensation of isophthaldehyde and diethylenetriamine, forms a dioxygen adduct which rapidly hydroxylates one of the benzene rings of the macrocyclic ligand. Oxygen insertion was not possible for the Cu(I) dioxygen complex of the macrocyclic lig and with furane bridging groups, and the oxygen complex was found to b e stable at room temperature. In this complex the coordinated dioxygen is activated so that it readily oxidizes various substrates, such as phenols and catechols. When the Cu(II) complex thus formed oxidizes th e same substrate, a catalytic system results. The substrate was oxidiz ed by the Cu(I) dioxygen complex, which was converted to a Cu(II) comp lex. The latter was in turn reduced to the Cu(I) complex by oxidation of the substrate, which then combined with oxygen to continue the cata lytic cycle. Similar results were obtained with the Cu(I)/Cu(II) compl exes of the ligand prepared by the 2 + 2 condensation of pyridine-2,6- dialdehyde and diethylenetriamine. Results obtained with analogous din uclear-Cu(I)/Cu(II) complexes with larger macrocyclic rings are descri bed. Examples of substrates that undergo catalytic oxidation, with tur novers from 2 to 30, are 2,6-dimethoxyphenol, 2,6-ditertiarybutylpheno l, hydroquinone, tertiarybutylhydroquinone and 3,5-ditertiarybutylcate chol. Schemes illustrating catalytic cycles for the oxidation of 2,6-d itertiarybutylphenol and 3,5-ditertiarybutylcatechol by molecular oxyg en with the dinuclear Cu(I)/Cu(II) complexes of macrocyclic ligands ar e presented. An interesting aspect of the present work is that the cat alytically active Cu(I) dioxygen complexes with macrocyclic ligands ar e stable enough at room temperature, and have long enough lifetimes, t o carry out two-electron oxidation of various phenolic and catecholic substrates. These models of tyrosinase are unique in that they functio n at room temperature. Copper dioxygen complexes reported previously d ecompose above -70 degrees C with very few exceptions. Also, the Cu(I) dioxygen complexes described in this paper are the first tyrosinase m odels to oxidize substrates catalytically, a process that requires tha t the Cu(II) complexes also oxidize the same substrates.