Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 182

methane monooxygenases (sMMO), that bear diiron centers and are able to catalyze
monooxygenations of not only methane to methanol, but also other alkanes and
alkenes to alcohols, ketones and epoxides. 4 The other important types are monoiron
enzymes, i.e. pterine-dependent hydroxylases; phenylalanine hydroxylase which
catalyzes para -hydroxylation of an aromatic ring of phenylalanine, tyrosine
hydroxylase which catalyzes ortho -hydroxylation of an aromatic ring of tyrosine to
form a catechol moiety, and triptophan hydroxylase which catalyzes hydroxylation
of an aromatic ring of triptophan. Isopenicillin N synthase is also an important
monooxygenase, that catalyzes the ring closure reactions of
(hydroxycarbonyl)pentanolyl]-L-cysteinyl-D-valine (ACV) without incorporation of
oxygen into tiazolidine rings of isopenicillin N.
Functional model chemistry for nonheme iron oxygenases has developed
remarkably in recent years. 5, 192, 193 Not only iron but also other metals were found
to catalyze the oxygenase-like oxygenations. Examples shown here will be limited
to those of nonheme iron model complexes. Enzymatic monooxygenations require
molecular oxygen as an oxygen source, but both of molecular oxygen and activated
oxygen, e.g. hydrogen peroxide, can be used in the functional model systems.
Continuous challenging works have brought about new iron complexes that can
incorporate one oxygen atom to substrates to give alcohols, ketones, epoxides, etc.,
and various types of ligands were designed for enhancement of activity, selectivity
control, and clarification of mechanisms. In the oxygenation mechanisms,
formation and structures of active iron-oxygen species are most interested. In the
so-called metal-based mechanisms iron-oxo or iron-peroxo species take part in the
oxygen transfer to substrates and participation of either the

or

manifolds has been discussed. In these efforts,
and complexes have been isolated and their reactivities with
substrates have been studied. It is well assumed that the postulated iron-oxo species
and are too unstable to be detected or isolated, but non-
spectroscopic evidence for these species came to appear in recent years.
On the other hand, participation of free radicals should be considered in most
cases of oxygenations. Both of carbon and oxygen centered radicals can take part in
the oxygenation processes. Apart from the apparent autoxidation process, it is not
easy to differentiate explicitly metal-based mechanisms from free radical
mechanisms. Different types of the radical-clock reagents have been developed for
detection of radicals of different lifetime, especially in the discussions on the
radical-rebound mechanisms.
One of important aims of functional model chemistry is to develop efficient
oxygenation catalysts that are useful in industries and applied fields. Selective and