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The best selectivities to various allylic oxygenated products (11 - 24 %)
were achieved at a ratio of 1. Substitution of Co(II) by
Mn(II) up to 30% had no effect on the conversion, but shifted the selectivity
significantly towards alcohols and acetates. This was ascribed to a decrease
in the overall redox potential, which suppressed further oxidation and side
reactions of the primary products.
Cobalt(II) acetate is slightly soluble in boiling nonane and decane and
promotes autoxidation to ketones as major products
It is proposed that the reaction takes place
via
a cobalt(III)-peroxy
species which reacts with the alkanes to form cobalt(II)-alkylhydroperoxide
complexes. These complexes decompose to form preferentially ketones, with
concomitant regeneration of cobalt(II) acetate
194
.
16.
MISCELLANEOUS COBALT CATALYSTS
N
-hydroxyphthalimide, a radical catalyst, has been reported to promote
the oxidation of various organic substrates such as diols, alkylbenzenes,
cycloalkanes and adamantanes by dioxygen under mild conditions
195-199
. The
oxidation takes place both in the absence and presence of
or
Isobutane is converted to
t
-butyl alcohol with high selectivity at 100°C
and an air pressure of 10 atm in benzonitrile over 8 hours
200
. Acetone and
t-
butyl hydroperoxide are formed in smaller amounts, the latter being an
intermediate of the reaction as indicated by the maximum observed in its
concentration. The catalyst used is a combination of
N
-hydroxyphthalimide
(NHPI ) and or may also be used as the active
cobalt species, the former being the most active. NHPI is a source of
phthalimidoxyl free radicals (PINO), produced by H-atom abstraction from
NHPI by the superoxocobalt(III) species or the
complex PINO in turn abstracts an H-atom from isobutane to
produce an isobutyl radical, which is trapped by dioxygen to afford the
t-
butylhydroperoxy radical (A) and ultimately
t
-butyl hydroperoxide. The
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