Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 101

reaction times lead to Michael adducts as the major product, claimed to arise
from a slower oxidative coupling of phenol and catechol. Recently, the
authors report that under the same reaction conditions, the oxygenation of
methyl-4-hydroxybenzoate mediated by the dicopper(I) complex
gives only catechol, with no trace of Michael adduct, even at
room temperature. Only when the reaction times are on the order of 20 h is
the formation of the ADDUCT observed with the depletion of catechol, which
remains as the major product.
A controversy exists, however, pertaining to the identity of the
primary reaction product and course of reaction. Sayre and Nadkarni 103 also
carried out experiments using but they determined that the
ADDUCT forms as a Michael addition product of phenol and 4-
carbomethoxy-1,2-benzoquinone, nothing that 4-carbomethoxy-1,2-catechol
is inert to oxidation under the conditions employed. Thus, Sayre concludes
that the benzoquinone is the direct product of oxygenation in the tyrosinase
like reaction, Such a
reaction thus proceeds like a dioxygenase, regenerating catalyst and not
requiring external reductant. It is notable that Kitajima 62 previously suggested
a similar pathway and mechanism for tyrosinase reaction, based on observed
reaction tendencies of phenol substrates with his own
complex. In fact, very recent investigations 104, 105 suggest
that in the enzyme itself, catechols are produced in an indirect route not
involving enzymatic reactions. These workers uphold the view that tyrosinase
does not act as a tyrosinase hydroxylase, but only produces o-quinones either
from mono- or diphenol substrates. If correct, this may require rethinking of
the detailed mode of dioxygen activiation in binuclear complexes, at least
with respect to phenolic substrates.
Other reviews cover earlier tyrosinase models and proposed
mechanisms. 9,106

3.3 TPQ Biogenesis

A carbonyl-containing cofactor has been long known to exist in amine
oxidases, identified by Klinman and co-workers as topaquinone (TPQ). 27
This cofactor derives from a protein tyrosine precursor and it is thought that a
copper(II) ion is required for its generation to TPQ. A mechanism for the
TPQ biogenesis has been proposed 28,107 involving the initial formation of the
[Cu(I)-Tyr] radical species and binding to copper ion yielding a
copper(II)-superoxide species which subsequently oxidizes the phenol of the
cofactor to

-quinone mediated
by copper(II) ion forms the reduced triol form of the TPQ followed by aerobic
oxidation to TPQ (Scheme 7).
A recent study 108 on catechol autoxidation reported by Sayre and co-
workers demonstrated, however, that conjugate addition of water to

o -quinone. Conjugate addition of water to the o

o -quinone