Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 99

While these studies show that both the
and the cores are capable of hydroxylating arenes in
model systems, the question still remains of whether the enzyme tyrosinase
hydroxylates its substrates directly with its observed
core or if O-O bond breaking occurs first before C-H bond

activation.

3.2 Recent Tyrosinase Models

Casella and co-workers 101,102 have recently reported on catechol
oxidase and phenol hydroxylase activities of a series of binuclear copper
complexes with benzimidazole-containing ligands (Chart 4). These model
compounds catalyze the oxidation of 3,5-di- tert -butylcatecholn (DTBC) to
3,5-di- tert -butylquinone as well as the ortho hydroxylation of methyl 4-
hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The catalytic
oxidations of DTBC are biphasic; the first phase is a fast stoichiometric step
involving electron transfer from the bridging catechol anion to the
dicopper(II) centers, which is affected by the reduction potential of the
Cu(II)/Cu(I) couple. Among the four dicopper(II) complexes,
has the highest activity due to its high redox potential. The second phase
involves the oxygenation of the dicopper(I) species, binding of the catechol to
the copper-dioxygen intermediate and the electron transfer between the
catechol anion and the dioxygen moiety. Various factors can affect the
efficiency of this second phase reaction. For the case, the slow
oxygenation reaction is the rate-determining step, therefore limiting the
efficiency of this complex. Contrarily, the reactions of dioxygen with
dicopper(I) complexes of L-55 and EBA are very fast, thus the binding of the
catechol and the subsequent electron transfer reaction become the rate-