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the Ru(TDCPP)CO/TBHP system
150
. The catalyst retains 67% activity after
11,700 turnovers under the selected reaction conditions: 1 mmol of alkene,
1.1 mmol of ~ 0.3 mmol of HC1, 50 mg of 0.4 wt.% Ru/M-41(
m
)
in 5 ml of The loss of activity was attributed to decomposition of
the catalyst. The possibility of being an active
intermediate was ruled out based on product distribution studies for
and cyclohexene oxidation, and on Hammett correlation
studies for the stoichiometric, epoxidation of
p-
and
m-
substituted styrenes; a species was proposed as a key intermediate,
particularly as a paramagnetic species, assumed to be Ru(III) (presumably
formed after O-atom transfer from a moiety) was detected in the
reaction mixture after the catalysis (see also Fig. 20, Section 3.4)
151
.
The longevity of the catalyst
150,151
is not the only advantage of using
Ru-porphyrins supported on mesoporous materials. Differences in selectivity
for the heterogeneous and homogeneous catalysis may also be an asset. Thus
oxidation of (+)-limonene catalyzed by gives the 8,9-
epoxide as major product (61%) and 27 and 10% of the
cis-
and
trans-
1,2-
epoxides, respectively (Fig. 14).
Under
homogeneous conditions,
in
gives preferential formation of
1,2-epoxides (55 and 19% of
cis-
and
trans
-l,2-epoxides) and 22% 8,9-
epoxide
151
. Restricted space in the M-41(
) channels likely makes the more
sterically hindered, trisubstituted C=C bond less accessible to the active
metal center, compared with the exocyclic methylene. Another example of
the influence of steric constraint of M-41(
m
) on selectivity is oxidation of
the bulky 3,4,6-tri-
O
-acetyl-
D
-glucal. Under homogeneous conditions, the
catalyst system affords stereoselectively the correspondent glycal epoxide
(this undergoes methanolysis with an inversion of configuration, leading to
methyl
m
(77%))
151
. In contrast, under
heterogeneous conditions, a 3 : 1 mixture of
was formed
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