Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 96

In addition to forming dicopper(III) species with the
bidentate ethylene diamine ligands, Stack and co-workers have also shown
that certain of these systems can form dicopper side-on peroxo complexes
which are interconvertible and in equilibrium with their dicopper(III)
isomer. 149 Generally, the rates of formation for dicopper side-on peroxo
complexes and for their isomeric counterparts are equal, as are their
thermal decomposition rates, presumably due to the fact that the rate of
isomer interconversion is faster than their formation or decomposition rates.
This has made it difficult to study the differential reactivity between side-on
peroxo and complexes. Recently, however, they
established a system which exhibits slower isomer interconversion rates in the
solvent 2-methyl tetrahydrofuran (2-MeTHF), such that the decay of these
species occur at measurably different rates (see diagram). This has made it
possible to study differences in the reactivities of the side-on peroxo and
complexes towards substrates such as PPh 3 or 2,4-di- tert-
butylphenol. Preliminary results indicate that the side-on peroxo isomer
decays more quickly with addition of triphenylphospine, indicating that it is a
better oxygen-atom transfer agent than the isomer.
Because of the differences in core structures between the isomers (Cu-
Cu distance in dicopper side-on peroxo complexes ~ 3.6 Å vs. ~2.7 Å in
compounds), accessibility of the substrate is thought to be a
key factor in the observed differential reactivities of the two isomers towards
PPh 3 . When the mixture of isomers are treated with 2,4-di- tert-
butylphenol, the side-on peroxo complex also decays more quickly than the
form. However, the formation of the expected oxidative
coupling product 3,3',5,5'-tetra- -butyl 2,2'-biphenol does not track with

the decay of the side-on peroxo. Therefore, the authors conclude that the
side-on peroxo isomer decays via the displacement of the peroxide by the
phenol to form a phenoxide-bound Cu(II) complex instead of via oxidation of
the phenol substrate. Conversely, although slower than the decay of the
peroxo isomer, the degradation of the

tert

form does correlate

with the oxidative coupling of the phenol. Therefore, the authors conclude