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K. Mandai et al. / Tetrahedron 71 (2015) 9403e9407
coupling reactions.12 Metal-porphyrin catalysts anchored on L-
The effects of aldehydes and solvents in the L-BIOX-catalyzed
Baeyer-Villiger oxidation of cyclohexanone are summarized in
Table 2. We first carried out the reaction with various aldehydes in
benzene for 1 h since the structure of the aldehyde can influence
BIOX have also been shown to be catalytically active in the syn-
13
thesis of carbonates from epoxides and carbon dioxides. L-BIOX
can be exposed to diverse heating and work-up conditions. One of
the derivatives of L-BIOX is magnetic iron oxide covered with sili-
2
6e28
the reactivity in Baeyer-Villiger oxidation.
While the reaction
1
4e16
cate,
which is used as a support for magnetically recyclable
in benzene without aldehyde did not proceed (<2% yield, entry 1),
the reaction with benzaldehyde gave 2a in 48% yield (entry 2). The
reaction with 3-chlorobenzaldehyde provided 2a with diminished
yield (10%, entry 3) as observed in the literature for aerobic Baeyer-
5
immobilized enzyme catalyst. Silica microtubules with a trace
amount of an iron component can also be obtained by heat and acid
treatment and reactions.
1
7,18
Nevertheless, the functions of L-BIOX
2
7,29
itself in organic synthesis have not yet been explored. Therefore, we
investigated the catalytic ability of L-BIOX in iron oxide-catalyzed
Baeyer-Villiger reactions, which were developed by Murahashi
Villiger oxidation.
Aliphatic aldehydes such as heptanal and
isobutyraldehyde were less effective than benzaldehyde, and pro-
vided 2a in yields of 4% and 8%, respectively (entries 4 and 5); these
19
19,26
et al. in 1992 using a molecular oxygen/aldehyde system. Herein,
we report that L-BIOX, which has hitherto been treated as industrial
waste, is capable of promoting the Baeyer-Villiger reaction using
molecular oxygen as a readily accessible oxidant and an aldehyde
under ambient reaction conditions. To the best of our knowledge,
this is the first example of the application of an iron oxide of bac-
results are consistent with previous reports.
Further in-
vestigation of the amount of benzaldehyde revealed that the re-
action with 3 equivalents of benzaldehyde proceeded efficiently
(see Supplementary data, Table S1). In the reactions with other
solvents, a trace amount of lactone 2a was obtained in DCM
(dichloromethane) or acetonitrile (6% yield in entry 7 and 7% yield
in entry 9), whereas the reaction in 1,2-DCE (1,2-dichloroethane)
proceeded more efficiently to give 2a in 43% yield (entry 8). No
reaction, at least within 1 h, was observed in toluene, THF (tetra-
hydrofuran) or DMF (N,N-dimethylformamide) (entries 6, 11, and
2
0e24
terial origin itself as a catalyst in organic synthesis.
Compari-
son of L-BIOX and typical iron compounds in the reaction and
additional experiments were also performed to gain insight into
the catalytic activity of L-BIOX.
1
2). Whereas the reaction in less toxic acetonitrile performed at
ꢀ
2
. Results and discussion
50 C for 3 h proceeded efficiently to obtain 2a in >98% yield (entry
0), benzene was a solvent of choice to facilitate the L-BIOX-cata-
1
Initially, we set out to screen various iron compounds including
lyzed Baeyer-Villiger oxidation under milder conditions for the
L-BIOX in the Baeyer-Villiger oxidation of cyclohexanone in the
subsequent easy screening according to the results in entries 6e9,
3
0
ꢀ
presence of benzaldehyde in benzene at 25 C under an oxygen
11 and 12.
19
atmosphere, referring to Murahashi’s report. (Table 1). The iron
compounds that were tested in the reaction by way of comparison
2 3 2 3 3 4
were commercially available a-Fe O , g-Fe O , Fe O , and a-Fe. The
Table 2
reaction of 1a with L-BIOX bearing an elemental composition of
Fe:Si:P¼73:22:5 gave the desired lactone 2a in 50% yield (entry 1),
while L-BIOX with Fe:Si:P¼78:10:12 gave 2a in 40% yield (entry 2),
suggesting that the elemental composition of L-BIOXs may in-
Baeyer-Villiger oxidation of cyclohexanone with various iron compounds and
aldehydes
a
8
fluence the catalytic activity (vide infra). The reactions with Fe (III)
oxides such as
6%, respectively (entries 3 and 4). The reactions with Fe
Fe provided the product in 44% and 36% yield, respectively (entries
a
-Fe
2
O
3
and
g
-Fe
2
O
3
gave 2a in yields of 18% and
3
3
O
4
and
a-
Entry
Solvent
Aldehyde
Yield (%)b
5
and 6). L-BIOX with Fe:Si:P¼73:22:5 definitely enhanced the re-
1
2
3
4
5
6
7
8
9
Benzene
Benzene
Benzene
Benzene
Benzene
Toluene
DCM
1,2-DCE
Acetonitrile
Acetonitrile
THF
None
<2
48
10
4
8
0
6
43
7
>98
0
0
action compared to catalyst-free conditions (entry 1 vs entry 7). L-
Benzaldehyde
3-chlorobenzaldehyde
Heptanal
BIOX enhanced the reaction better than the other iron compounds
2
5
2 3
investigated, including two types of Fe O .
Isobutyraldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Table 1
Baeyer-Villiger oxidation of cyclohexanone with various iron compoundsa
1
1
1
0c
1
2
DMF
a
b
c
Same conditions as those in Table 1. L-BIOX with Fe:Si:P¼73:22:5 was used.
Determined by GC analysis.
ꢀ
Entry
1c
2
3
4
5
6
7
Iron compound
Yield (%)b
The reaction was carried out at 50 C for 3 h.
L-BIOX
L-BIOX
50 (57, 56, 38)
40 (47, 40, 34)
18 (25, 20, 10)
36 (48, 38, 24)
44 (55, 41, 36)
36 (55, 32, 22)
25 (35, 22, 18)
d
a-Fe
g-Fe
Fe
a-Fe
No catalyst
2
O
3
The range of the applicable substrates in L-BIOX-catalyzed
2
O
3
Baeyer-Villiger oxidation was extended to a variety of cyclic and
acyclic ketones (Table 3). Adjustment of the optimal reaction con-
ditions for each substrate allowed Baeyer-Villiger oxidation prod-
ucts to form in 23e>98% yield with high regioselectivity of the
oxygen insertion position at either a more substituted alkyl group
or an aryl group adjacent to the carbonyl carbon. Cyclohexanone,
which was used to optimize the reaction conditions, gave the lac-
tone 2a in >98% yield within 3 h in the oxidation with 1 mol % Fe
3
O
4
a
Reaction conditions: 1a (1 mmol), iron compounds (1 mol % Fe), benzaldehyde
ꢀ
(
3 mmol) in 3 mL of benzene at 25 C under an O
2
atmosphere (1 atm).
b
Average yields of three runs. The yield of each run is in parentheses. Determined
by GC analysis using dodecane as internal standard.
c
L-BIOX was obtained from a freshwater purification plant in Joyo city, Kyoto,
Japan. Fe:Si:P¼73:22:5 (except O).
(
2
entry 1). Notably, the reaction with L-BIOX furnished the lactone
a effectively under milder reaction conditions in aldehyde/O
d
L-BIOX was obtained from a cultivation tank at Okayama University, Okayama,
2
Japan. Fe:Si:P¼78:10:12 (except O).