ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
PL4. High-Valent Iron(IV)-Oxo Complexes of Heme and Nonheme Ligands
in Dioxygen Activation Chemistry
W. Nam
Department of Chemistry and Nano Science, Center for Biomimetic Systems, Ewha Womans University, Seoul
120-750, Korea
e-mail: wwnam@ewha.ac.kr
Heme iron enzymes catalyze a diverse array of important metabolic transformations that require the binding and
activation of dioxygen. One primary goal in cytochrome P450 research is to understand the mechanistic details
of dioxygen activation and oxygen transfer reactions by the enzymes. Extensive mechanistic studies with the
enzymes and iron porphyrin models resulted in proposing oxoiron(IV) porphyrin cation radical as a sole active
oxidant that effects metabolically important oxidative transformations. Recent studies from a number of
laboratories, however, have provided experimental evidence that in addition to the high-valent iron-oxo species,
other oxidizing species are involved in oxidation reactions. For example, a hydroperoxo-iron(III) porphyrin
intermediate has been proposed as a second electrophilic oxidant in cytochrome P450-catalyzed oxidations, on
the basis of site-directed mutagenesis and radical clock experiments. The involvement of multiple oxidants has
also been proposed in iron porphyrin reactions, mainly on the basis of competitive oxidation experiments.
Computational studies, however, concluded that the oxidation reactions by the hydroperoxo-iron(III) porphyrin
intermediate are energetically unfavorable, ruling out the existence of a second electrophilic oxidant. Thus there
is an intriguing, current controversy on the involvement of multiple oxidizing species in oxygen transfer
reactions by cytochromes P450 and iron porphyrin models.
Mononuclear nonheme iron enzymes comprise an important group of dioxygen-activating enzymes that are
involved in many metabolically important oxidative transformations. The mechanistic details of dioxygen
activation and oxygen atom transfer reactions by the enzymes and their model compounds have been extensively
studied over the past two decades, thereby proposing high-valent iron(IV)-oxo intermediates as the active
oxidizing species. Recently, we have succeeded in obtaining a high-resolution crystal structure of an Fe(IV)=O
intermediate with a nonheme macrocyclic ligand and reported the synthesis and reactivity studies of a number of
nonheme iron(IV)-oxo complexes. With nonheme iron(IV)-oxo complexes firmly established by
crystallography, significant progress has been made in the chemistry of nonheme iron(IV)-oxo intermediates
over the past five years; ~15 nonheme iron(IV)-oxo complexes appeared in literature at the present time. In this
presentation, I will present our recent results on the determination of the nature of active oxidant(s) in nonheme
iron complex-mediated oxygen atom transfer reactions and the generation and reactivity studies of mononuclear
nonheme oxoiron(IV) complexes having different axial ligands. In addition, the reactivities of mononuclear
nonheme iron(IV)-oxo complexes in a variety of oxygenation reactions will be discussed (see below).
Aromatic hydroxylation
O
S
S-oxidation
OH
S
R
P-O
P-oxidation
P
Aliphatic hydroxylation
R
C OH
C H
Nonheme Iron(IV)-Oxo Complex
H3C N
H
N + HCHO
N-dealkylation
Alkene epoxidation
O
C O
H
C OH
Alcohol oxidation
Alkylaromatic oxidation
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