Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 205

enzymes and their models, which involve porphyrin ligands suitable for stabilization
of the high-valent state of iron. It is easy to assume and expect formation of the
similar iron oxygen species in the nonheme iron systems, but at the same time we
realize high barriers for characterization of these species spectroscopically or by
isolation. Participation of these species in the oxygenations by nonheme iron
complexes was first shown by selectivity of product, e.g. alcohol/ketone ratio or
C2/C3 ratio in the alkane or adamantane oxygenations, but it was not necessary for
us to wait so long before getting spectroscopic evidence for various types of
nonheme iron-oxygen species. Some iron-oxygen species were characterized after
isolation. However, we encounter paradoxical facts that stabilized iron-oxygen
species exhibit poor reactivity.
Many kinds of diiron complexes have been synthesized in relevance to
methane monooxygenase. In general, the activities of functional model
oxygenations are poor and the complexes are labile to be converted to monoiron
species in solutions. The characteristic diiron structure, however, were applied to
the diiron-substituted Keggin-type silicotungstate (Fig. 14), that exhibits a high
catalytic activity under without addition of reducing reagent. On the other hand,
some types of monoiron complexes have been developed to perform enzyme-like
oxygenations producing alcohols selectively. In addition, some monoiron
complexes are expected to become useful in oxidation of pollutants or bleaching of
dyes, regardless of usage of molecular oxygen or activated oxygen such as hydrogen
peroxide. Recently, however, reactivity of peroxo adducts of mono- and diiron
complexes were compared for the oxidation of sulfides to sulfoxides. The dinuclear
catalyst was found to be more reactive and (enantio)selective than its mononuclear
counterpart, suggesting that a second metal site affords specific advantages for
stereoselective catalysis. 436 This result may be helpful for the design of future
enantioselective iron catalysts.
Nonheme iron oxygenases are characteristic for the abundant types of
dioxygenation, compared with heme oxygenases. Progresses in chemistry of
catechol dioxygenases are remarkable in these ten years, but little development has
been achieved in chemistry of other types of dioxygenases. The selective cleavage
of aromatic rings with molecular oxygen is an attractive reaction that converts
aromatic to aliphatics. The mechanism of the selectivity control in the oxygen
insertion process is an interesting subject in bioinorganic chemistry. It involves
chemistry of Fe(III) and Fe(II) for activation of catechols and oxygen. The fruits in
that chemistry will be applied to other types of dioxygenases.
Recent great progress in the X-ray crystallographic analysis brings about
structural information about metalloproteins directly. However, structural model