R. Bernini et al. / Tetrahedron Letters 44 (2003) 8991–8994
8993
Table 2. Baeyer–Villiger oxidation of the ketones 1 and 17 in [bmim]BF4 using CH3ReO3 as a recyclable catalyst
Entry
Ketone
Lactone
Conversions and yields (%)a
Run no. 1b
Run no. 2b
Run no. 3b
Run no. 4b
Run no. 5b
Run no. 6b
1
2
1
17
2
18
>98 (>98)
85 (>98)
95 (>98)
85 (95)
95 (>98)
85 (95)
95 (>98)
85 (95)
88 (92)
82 (90)
65 (80)
60 (85)
a Yields of the lactones 2 and 18 are given in parentheses. They were measured by gas chromatography after chromatographic purification of the
reaction mixture.
b After the first run, successive runs were performed adding only fresh substrate and oxidant to the ionic liquid under the same experimental
conditions.
to the ionic liquid under the same experimental condi-
tions. It was noteworthy that the g-butyrolactone 2 was
isolated in the same yield without addition of
CH3ReO3, indicating that both the ionic liquid and the
catalyst were recyclable and reusable. This catalytic
system was stable and efficient for five recycling experi-
ments (Table 2, entry 1). Under the same experimental
conditions, the cyclopentanones 3, 5 and 7 reacted in
good yield to give the corresponding lactones 4, 6, 8
(Table 1, entries 2–4), the cyclohexanones 9, 11 and 13
being less reactive (Table 1, entries 5–7). The strained
adamantanone 15 presented a high reactivity (Table 1,
entry 8). This new oxidative methodology was tested on
the methylated flavanones naringenin 17 and hesperetin
19 (Table 1, entries 9 and 10) which had been previ-
ously lactonized under heterogeneous conditions.9
Experimental data indicated that the catalytic system
CH3ReO3/H2O2 in [bmim]BF4 was more efficient than
CH3ReO3 supported on poly(4-vinylpyridine) polymers/
H2O2 in t-butanol. In fact, the lactonization proceeded
under milder conditions (40°C and room temperature
versus 80°C, 2 equiv. versus 6–10 equiv. of oxidant),
faster (2 h versus 6–12 h) and more efficiently (yields
85% and 82% versus 42–80% and 35–72% according to
the polymers used).9 The experimental data for recy-
cling of the catalyst in [bmim]BF4 starting from narin-
genin 17 confirms the recyclability of the catalyst in the
same reaction medium (Table 2, entry 2).
In conclusion, the oxidative catalyst CH3ReO3 can be
immobilized in ionic liquids and used to perform an
efficient Baeyer–Villiger oxidation of cyclic ketones
with hydrogen peroxide. This new catalytic system per-
mits recycling both of the catalyst and of the reaction
medium. Work is in progress in our laboratory to
evaluate the possible chemo- and regioselectivity of this
reaction in different ionic liquids.
Acknowledgements
We are grateful to Consorzio INCA and to Italian 5%
MURST-CNR ‘Progetto Reflui Oleari’ for their finan-
cial support.
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Scheme 1. Mechanistic pathway for the oxidation of cyclo-
butanone 1 catalysed by CH3ReO3.
10. Unpublished results.