Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 160

transfer to the organic substrates were first shown by Groves and co-workers. 9, 10
Since then, numerous example of oxygen transfer to alkanes and alkenes from these
oxygen donors and the spectroscopic characterization of a green oxo-ferryl
porphyrin radical species have been reported. 7, 11-22 This type of model
systems can be applied to asymmetric or shape selective oxidations. 6 The model
complexes are placed in the different environments for achievement of specificity,
e.g., in artificial membrane, 23-26 on the polymers, 27, 28 on the solid supports, 29-36 in the
dendolimers, 37 etc. On the other hand, synthetic model complexes for peroxo-
iron(III) species, ferric porphyrin peroxo complexes, are known to catalyze
epoxidation of olefins. 38-45 Nucleophilic epoxidation reactions are accelerated by the
presence of DMSO, which coordinates in the axial position. 44
Compared with heme iron oxygenases, which function mostly as
monooxygenases, various types of oxygenations are catalyzed by nonheme iron
mono- and di-oxygenases. 4 Types and numbers of amino acid residues as ligands
depend on individual enzymes. Soluble methane monooxygenase (sMMO) has
attracted much attention in recent years, and a great progress has been made in
clarification of structures and reaction mechanisms of specific di-iron enzymes and
model systems. In the other monooxygenases, pterin-dependent hydroxylases
(phenylalanine hydroxylase, tyrosine hydroxylase, tryptophan hydroxylase) are
interesting mono-iron enzymes, but many problems about enzyme structures and
mechanisms are left unclear. Contribution of model studies for this type of enzymes
must be said poor. Isopenicillin N synthase is also interesting mono-iron enzyme by
which the C-C bond formation rather than oxygen insertion is catalyzed, but barrier
is rather high for bioinorganic chemists to work on reactions that require treatment
of organic substrates with complicated structures.
The most popular dioxygenases in nonheme iron oxygenases are catechol
dioxygenases that are the key enzymes in the metabolism of aromatic compounds.
Two types of cleavage of the aromatic C-C bond, i.e. intradiol and extradiol
cleavages, are catalyzed by two different types of oxygenases involving and
respectively. -containing oxygenases have been extensively studied from
both sides of enzyme and model in these two decades. -containing oxygenases
are becoming a research target in recent years, but functional model studies by using
-complexes should overcome a problem how the state is recycled after a
catalytic turnover under the oxygenation conditions. acid-dependent
dioxygenases are also interesting dioxygenases that insert only one atom of oxygen
to substrates accompanied by the transfer of another atom of oxygen to acid.
Chemical approach to this type of enzyme is poor, but the enzyme will be the next
important object to be studied.

Lipoxygenase is another important oxygenase. In