Stoichiometry, product and kinetics of catalytic oxidation of 2,6-dimethylphenol by bromo (N,N′-diethylethylenediamine)copper complexes in methylene chloride

Copper(I) dimer [(DEED)CuBr](2) (4, DEED = N,N'-diethylethylenediamine) is rapidly oxidized by O-2 to mixed valence peroxocopper complex [(DEED)CuBr](4)O-2 (1) in CH2Cl2 at -50 to 30 degrees C. The long half-life for conversion of (1) into oxocopper(II) complex [(DEED)CuBr](2)O (3) allows (1), (3) and their carbonate derivative of [(DEED)CuBr](2)CO3 (5) to be compared as oxidants of 2,6-dimethylphenol (DMPOH) to the corresponding diphenoquinone (DPQ) over a range of concentrations and temperatures. DPQ production is: 1) less than stoichiometric with deficits or slight excesses of DMPOH, but 2) mildly catalytic at moderate [DMPOH], as found with tetranuclear oxohalo(pyridine)copper(II) oxidants. This behaviour is attributed to 1) co-product water destruction of initiators, and 2) inhibition by water of copper(I) reoxidation to complete the catalytic cycle. These inhibiting factors apparently are ameliorated by water incorporation in hydrogen-bonded phenol clusters in aprotic solvents. Initial rate measurements show that (1), (3) and (5) form monophenolate complexes with DMPOH in methylene chloride. The rate-determining step for conversion of these complexes to DPQ is fastest for oxocopper(II) complex (3) which is expected to be the strongest protic base. Highest rates with (3) and activation parameter comparisons suggest that the ability of phenolatocopper complexes to accept protons from coordinated phenolate is an important factor in determining overall copper-catalyzed phenolic oxidative coupling rates.