Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 161

spite of the extensive works on the enzymes including recent crystallographic
analysis, the oxygenation mechanisms are left to be clarified. It is expected that
model studies will give decisive information about reaction mechanisms.
Characterization of active species in nonheme iron systems has been
supposed more difficult than that in heme systems. However, in addition to
isolation of stabilized species in the model systems, developments in the
spectroscopic methods for detection of unstable species and in the quantum
chemical methods have given a rich information about the structures and reactivity
of nonheme iron oxygen species. High-valent iron-oxo species have been proposed
in the nonheme systems to explain reactivity from analogy to
heme species. No spectroscopic evidence for the unstable species has been
obtained, 46 but recent evidences for monoiron peroxo species and their recitivities 47-
52 supported strongly the formation of the high-valent iron-oxo species. Formation
of peroxo iron species is also highly probable in the methane monooxygenase
systems.
This chapter focuses mainly on functional model oxygenations by nonheme
iron complexes and summarizes the progresses since 1996 to avoid repetition of the
contents in Vol. 19 in the series in 1997. 4 In the studies on dioxygenase-model
oxygenations, a remarkable progress has been attained both in the intra- and
extradiol cleavage of catechols by nonheme iron complexes with various types of
ligands. Other dioxygenases such as acid-dependent dioxygenases and
lipoxygenase have little studied for development of functional model oxygenations.
In the studies on monooxygenase-model oxygenations, detection, isolation, and
characterization of mono- and diiron nonheme iron-oxygen species, such as
are the highlight of the recent bioinorganic chemistry.
Reactivities of these species for hydroxylation, ketonization, and epoxidation are
also the most interesting topics.

and

3. FUNCTIONAL MODEL STUDIES ON NONHEME IRON
DIOXYGENASES
3.1 Catechol Dioxygenases
Catechol dioxygenases play a key role in the metabolism of aromatic
compounds. They were first classified as dioxygenase in 1955 by Hayaishi who
discovered the oxygen insertion into muconic acid in the oxygenative cleavage of
pyrocatechol catalyzed by pyrocatechase. 53 Two types of oxygenations are known,
i.e., intradiol and extradiol cleaving oxygenations as shown in Fig. 1. The intradiol-
cleaving oxygenases involve as an active center, that is ligated by 2 His and 2
Tyr residues. 54-57 The extradiol cleavage is mainly catalyzed by enzymes containing