Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 146

any advantages over the earlier described ruthenium- and palladium-based
systems.

6. OTHER METALS AS CATALYSTS FOR OXIDATION
WITH

In addition to ruthenium, other late and first-row transition elements
are capable of dehydrogenating alcohols via an oxometal pathway. Some are
used as catalysts, in combination with or for the oxidative
dehydrogenation of alcohols (see later). By analogy with ruthenium, one
might expect that regeneration of the active oxidant with dioxygen would be
possible. For example, one could easily envisage alcohol oxidation by
oxovanadium(V) followed by reoxidation of the resulting vanadium(III) by
dioxygen.
However, scant attention appears to have been paid to such
possibilities. The aerobic oxidation of 1-propanol to 1-propanal (94-99%
selectivity), in the gas phase at 210°C over a catalyst modified with an
alkaline earth metal oxide (10m%), was described 78 in 1979. However, to
our knowledge vanadium-catalyzed aerobic oxidation of alcohols have not
been further investigated, in the liquid or gas phase 79 .
Chromium(VI) catalyzes the oxidation of alcohols with alkyl
hydroperoxides 80 . Chromium-incorporated molecular sieves, in particular
chromium-substituted aluminophosphate-5 (Cr-APO-5) were shown 81 to be
effective for the aerobic oxidation of secondary alcohols to the
corresponding ketones (Reaction 19). This, and related catalysts, were first
believed to be heterogeneous but more detailed investigations 82 revealed that
the observed catalysis is due to small amounts of soluble chromium that are
leached from the framework by reaction with hydroperoxides. Reaction 19
may involve initial chromium-catalyzed free radical autoxidation of the
alcohol to the hydroperoxide followed by chromium-
catalyzed oxygen transfer with the latter and/or (formed by its

dissociation) via an oxochromium(VI)-chromium(IV) cycle.