Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 62

feasibility for asymmetric epoxidation, but ee values of only ~70% have
been achieved with small total turnovers (<50) because of catalyst
decomposition. Use of O-atom donors instead of generally gives
somewhat better turnovers (up to ~1000) and, in chiral porphyrin systems, ee
values up to ~80% have been realized. Within O-atom donor systems
generally, alternative mechanisms appear to be operating, involving catalytic
species such as where L is the O-atom donor. The
asymmetric olefin epoxidation systems are not economically competitive on
a laboratory practical scale with the chiral Schiff-base systems based usually
on Mn 223 or the recently reported Fe-based, chiral, chelating amine
systems 224 . More commercially attractive oxidation catalysts are likely to be
based on cheaper, non-porphyrin ligand sets, Schiff-base ligands
or purely O-based ligand sets as in zeolites or heteropolyacids; the
latter have been considered as the oxidatively resistant, inorganic analogues
of metalloporphyrins 225 .
Oxygenation of saturated hydrocarbons via the systems
under typical catalytic conditions (e.g. ~ 1 mM in Ru) is marginally
detectable, and even this activity may be in part photo-initiated. Remarkably
active for thermal oxygenation of saturated hydrocarbons are O-atom
transfer systems based on an O-atom donor such as a pyridine oxide, and
these probably operate via a catalysis cycle and again the O-
atom donor may be a trans- ligand; in these systems, a precursor
is not essential and Ru(porphyrin)-carbonyls and -dihalo complexes (even
with the non-sterically hindered TPP) may be used. Total turnovers close to
20,000 have been seen for oxidation of adamantane. Commercial application
of these systems is limited by formation of the stoichiometric co-product
(e.g. the pyridine) with the oxygenated hydrocarbon, and their development
rests on efficient regeneration of the O-atom donor via an oxidation process
using preferably or In related systems using a chiral porphyrin,
enantioselective hydroxylation of benzylic C-H bonds leads to secondary
alcohols in up to 76% ee, while generation of chiral tertiary alcohols (with
up to 54% ee ) from racemic tertiary alkanes is noted for the first time.
Attempts to improve stability (by reducing self-destruction) of the
porphyrin catalysts for oxygenation of hydrocarbons, by
incorporation of substituents such as halogens into the meso -phenyl
substituents (the so-called 2nd generation catalysts) and then also into the
pyrrole rings (3rd generation catalysts), has led to the findings that such
halogenated species are very active for decomposition of trace
hydroperoxides present in the systems, with the result that extremely active,
and generally non-selective, free-radical oxidations are initiated within
olefinic and saturated hydrocarbons. Again, the dioxo species is not a
mandatory precursor - the nature of the axial ligands seems incidental. In