Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 63

retrospect, the systems become akin to some that have been known for
decades, involving the use of simple, commercially available Ru compounds
such as hydrated and Nevertheless, the studies have led to
advances in the basic coordination chemistry of Ru-porphyrins, albeit it
fairly exotic and utilizing expensive ligands.
The catalytic oxidative-dehydrogenations of phenols, alcohols and
amines are chemically and mechanistically complicated by the formation of
water, the co-product; this can certainly react with the species,
and can play a role as an axial ligand when conversions to coordinated
hydroxo and oxo are possible. In the case of certain primary amine systems,
the water can hydrolyze the imine product, in a process that might also be
metal-catalyzed. Studies on the alcohol systems have led to novel alkoxide
complexes, which are of interest in their own right, while the amine systems
are of more interest (versus the net dehydrogenation catalysis) regarding
better understanding of some reductase enzymes and the metabolism of
natural amines; and again the discoveries of the oxo-imido, bis(amido),
bis(imido), bis(imine) and mixed imine/amine complexes are significant
contributions to coordination chemistry in general. The transfer of the imido
(nitrene) group to alkenes and alkanes is analogous to O-atom transfer, and
systems based on Ru-porphyrins are discussed.
An emphasis throughout this Chapter (and indeed in much of the
literature on homogeneous catalytic oxidations) is on selectivity in product
formation using O-atom transfer mechanisms, and indeed this is a worthy
aim. However, it should be noted that the major industrial homogeneous
catalytic process (from an economic standpoint) currently in operation is the
of p -xylene to terephthalic acid (the Mid-Century Amoco
process), which is catalyzed by an inexpensive mix of Co and Mn acetates
and bromides in acetic acid, the system operates at 225°C under 15 atm
This is a radical-initiated process that has been in operation for over 30
years, and the details of the mechanistic pathways continue to be studied 226 .
In essence, the system has been fine-tuned from an empirical point of view,
and is a classic case of the practical industrial approach versus the basic
academic approach: a mechanistically messy process but one that is
commercially extremely important
The investigations of Ru-porphyrins for catalytic oxidations, especially
for hydrocarbon substrates, have necessarily impinged on aspects of the
organometallic chemistry of these compounds, and this is mentioned
occasionally in the Chapter. In a more general sense, the studies have
contributed substantially to such organometallic chemistry which is now
quite extensively developed 49,l08,227,228 .
More generally, the development of catalyzed by metal
complexes is perhaps at the stage that homogeneously catalyzed