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

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 conversi on of (1) into oxocopper(II) complex [(DEED)CuBr](2)O (3) allows (1), (3) a nd their carbonate derivative of [(DEED)CuBr](2)CO3 (5) to be compared as o xidants 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(p yridine)copper(II) oxidants. This behaviour is attributed to 1) co-product water destruction of initiators, and 2) inhibition by water of copper(I) re oxidation to complete the catalytic cycle. These inhibiting factors apparen tly are ameliorated by water incorporation in hydrogen-bonded phenol cluste rs in aprotic solvents. Initial rate measurements show that (1), (3) and (5 ) form monophenolate complexes with DMPOH in methylene chloride. The rate-d etermining step for conversion of these complexes to DPQ is fastest for oxo copper(II) complex (3) which is expected to be the strongest protic base. H ighest rates with (3) and activation parameter comparisons suggest that the ability of phenolatocopper complexes to accept protons from coordinated ph enolate is an important factor in determining overall copper-catalyzed phen olic oxidative coupling rates.