Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 3

1. INTRODUCTION: OXYGENASE AND OXIDASE

ACTIVITY

Molecular oxygen is the most abundant and inexpensive
oxygenating/oxidizing agent, which can effect in the presence of an
appropriate catalyst a variety of useful oxidation reactions. These range from
so-called di- or monooxygenase systems where two or one O-atoms of the
are incorporated into the organic substrate (eqs. 1 and 2, respectively) to
oxidative-dehydrogenation systems, where H-atoms are removed from
organics as or (eqs. 3 and 4, respectively). In terms of strict,
modern nomenclature, “oxygenase activity” (or O-atom incorporation) is
effected by enzymatic di- or monooxygenases, and is represented by eqs. 1
and 2, while “oxidase activity” (H-atom abstraction) is effected by oxidases,
and is represented by eqs. 3 and 4. Many studies with models (i.e. non-
protein systems) aim to mimic the enzyme systems, particularly their high
selectivity (formation of a single product) and operation under ambient
conditions.

Dioxygen is clearly an attractive oxidant (whether as an oxygenation or
oxidizing agent), and is highly desirable when environmental requirements
are considered as any inorganic co-product is typically water (eqs. 2 and 4).
This may be contrasted with a classical stoichiometric oxidant such as
dichromate (e.g. for alcohol oxidations), where undesirable Cr(III) is the co-
product. However, because of its biradical nature, non-coordinated reacts
with organic substrates preferably according to a radical-chain mechanism,
which generally operates with low selectivity. A typical example is
of cyclohexene to give ene-one, ene-ol and epoxide via the
intermediate cyclohexyl hydroperoxide, formed by attack of on the allylic
radical, itself produced by hydrogen-atom abstraction (eq. 5): such reactions
are catalyzed by transition metal salts and complexes which initiate