Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 158

1. INTRODUCTION
Catalytic oxidations and oxygenations with molecular oxygen are among the
most important topics in recent years in the large fields of chemistry and biology,
and great efforts have been made for understanding mechanisms of reactions and
development of chemical model systems in both fields of industries and academic
laboratories. Insertion of oxygen to hydrocarbons as functional groups converts
them to value-added materials that are used in the many different ways. However,
molecular oxygen, that is a simple molecule and present in air, is the molecule that
is hard to be controlled in the reactivity. This is based on the triplet state of oxygen
in the ground state while the ground state of most materials is the singlet state. Once
oxygen is activated, e.g. to superoxide ion, it reacts readily with various compounds
in the way of out of control of the selectivity in some cases. Thus, various types of
activate oxygen species, e.g., hydrogen peroxide, alkylperoxides, peracids, etc., have
been used in place of molecular oxygen for the selective oxidations and
oxygenations under the controlled conditions. These activated oxygen species,
however, require special cares in handling and storage. Thus, people challenge to
develop the selective reaction systems with using molecular oxygen.
Various metal enzymes, on the other hand, are known to activate oxygen and
to oxygenate materials with or without incorporation of molecular oxygen.
Remarkably, oxygenases oxygenate various substances in the highest selectivity in
spite of the usage of molecular oxygen. Interestingly most of these oxygenases
involve iron or copper as an active center, that is common to other metalloproteins
participating in the dioxygen transfer and storage ( e.g. hemoglobin, myoglobin,
hemocyanin, hemerythrin, etc .) and other catalytic processes by oxidases
(cytochrome c oxidase, etc. ), peroxidases (horseradish peroxidase etc. ), catalases,
and superoxide dismutases. Iron and copper are the most popular metals for
mankind from ancient times, but these have not been used efficiently as artificial
catalysts for catalytic oxygenations and oxidations. Chemists pose questions why
metalloenzymes use these metals and how selectivity is controlled, and start to
challenge to develop good catalysts that work not only in the similar fashion to
enzymes, but also more efficiently to various substrates than enzymes. For this
purpose, clarification of structures of active-center environments is important and a
lot of heme and nonheme complexes have been synthesized as structural model
complexes. No doubt information obtained from these complexes is great, but
recent remarkable progress in X-ray crystallographic analyses has made possible to
clarify directly structures of metalloproteins. Since X-ray crystallographic analyses
are useless for structures of enzymes and complexes, various spectroscopic data are
important for clarification of structures in solution. These structural model