Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 4

decomposition of hydroperoxides via so-called Haber-Weiss pathways (eqs.
6 and 7) 1 .
Such radical reaction pathways do not involve directly at
the metal. Much more selective oxidation processes can be realized by
activation of the through coordination to a metal center; the coordination
is generally followed by transfer of electrons from the metal to the oxygen
moiety (see below). The reactions shown in eqs. (1) - (4) can be exemplified
by specific enzyme systems that incorporate such and these will
be discussed briefly, but with greater emphasis on the mono-oxygenase type
(eq. 2) because: (a) this type of activity is better understood, and (b) the
majority of reported studies on model oxygenation systems has involved
attempts at mimicking mono-oxygenase activity. A volume entitled
“Oxygenases and Model Systems” has appeared within this series 2 , and
discusses in detail reactions of the types shown in eqs. 1 and 2. As noted
below, Fe-porphyrin complexes embedded in a protein environment are
responsible in human systems for binding and transport of dioxygen, as well
as for incorporation of one or two O-oxygen atoms into organic substrates
via mono- or dioxygenase activity. Interest in the second-row analogues, Ru-
porphyrins, stems from this general “oxygen chemistry” but, before
discussing the Ru systems in subsequent sections, a brief introduction to the
enzyme chemistry of eqs. 1-4 is appropriate.
Dioxygenase activity (eq. 1). Two examples of dioxygenase systems
are provided by a pyrocatechase, which effects oxygenation of catechol to
cis -muconic acid (eq. 8), and tryptophan dioxygenase, which catalyzes ring
cleavage of tryptophan derivatives to the N-formyl products, called
formylkynurenines (eq. 9).
The active site within protocatechuate 3,4-dioxygenase (the enzyme
that effects reaction 8, with as determined crystallographically,
is high spin, 5-coordinate Fe(III) containing 2 tyrosine and 2 histidine
ligands, and a coordinated water or hydroxide (( 1 ), see Fig. 1) , and a
postulated mechanism, based largely on EXAFS and EPR data, for the