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

Abstract

Copper(I) dimer [(DEED)CuBr]2 (4, DEED=N,N′-diethylethylenediamine) is rapidly oxidized by O2 to mixed valence peroxocopper complex [(DEED)CuBr]4O2 (1) in CH2Cl2 at −50 to 30°C. The long half- life for conversion of (1) into oxocopper(II) complex [(DEED)]CuBr]2O (3) allows (1), (3) and their carbonato derivative of [(DEED)CuBr]2CO3 (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 oxo-halo(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.