Selective hydroxylation of cyclohexene in water as an environment-friendly solvent with hydrogen peroxide over febipyridine encapsulated in y-type zeolite

Article abstract of DOI:10.1246/cl.2012.713

The selective hydroxylation of cyclohexene to 2-cyclohexen- 1-ol with hydrogen peroxide in water was successfully achieved using [Fe(bpy) ₃]²⁺ complexes encapsulated into Y-type zeolite.

Full text of DOI:10.1246/cl.2012.713

doi:10.1246/cl.2012.713
Published on the web June 30, 2012
713
Selective Hydroxylation of Cyclohexene in Water as an Environment-friendly Solvent
with Hydrogen Peroxide over Fe­Bipyridine Encapsulated in Y-type Zeolite
Syuhei Yamaguchi, Tomohiro Fukura, Chiharu Fujita, and Hidenori Yahiro*
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering,
Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577
(
Received March 12, 2012; CL-120212; E-mail: hyahiro@ehime-u.ac.jp)
The selective hydroxylation of cyclohexene to 2-cyclo-
hexen-1-ol with hydrogen peroxide in water was successfully
achieved using [Fe(bpy)3]2 complexes encapsulated into Y-type
zeolite.
the reaction of Fe-Y with bipyridine (bpy) (Scheme S2).19 This
compound was characterized by XRD, UV­vis., and elemental
analysis. As a model reaction for evaluating the catalytic
activity, the partial oxidation of cyclohexene was carried out.
Each catalyst (7.9 ¯mol Fe atoms in catalysts), cyclohexene
+
(
7.9 mmol), 30% H2O2 (65 ¯L, 0.8 mmol), and solvent (aceto-
Recently, government projects and new legislation demand
large investment in green chemistry procedures and environ-
mentally friendly technologies. One of the most interesting areas
in catalysis is the development of inorganic­organic hybrid
materials for catalytic oxidation reactions.1 The selective
oxidation of hydrocarbons is still a challenge in chemical
industries and academic fields. As a test reaction for catalytic
activity, the oxidation of cyclohexene with hydrogen peroxide
using homogeneous Fe complexes has been reported by many
nitrile (10 ¹ x mL), water (x mL)) were stirred at 50 °C under Ar.
The yield was determined periodically by GC analysis.
2+
Elemental analysis of C, H, N, and Fe for [Fe(bpy)3] @Y
indicated that the ratio of bpy ligands/Fe ions was ca. 3
­3
19
(Table S1 ), suggesting that free and uncomplexed Fe(II) ions
hardly remained. This result also suggests that approximately
2+
one [Fe(bpy)3] complex per two supercages in zeolite was
2+
contained in [Fe(bpy)3] @Y.
It has been reported that the relationship between the
relative peak intensities of the 220 (I220) and 311 (I311)
reflections in the XRD pattern is closely related to the formation
of a large metal complex ion in the supercage of faujasite-type
zeolites: I220 > I311 for the original Y-type zeolite and I220 < I311
4
,5
researchers. Que et al. reported that cyclohexene oxide and
cyclohexane diol were formed by the oxidation of cyclohexene
with hydrogen peroxide using Fe complexes controlled by
4
designed tetradentate ligands. However, there is little informa-
2
,20­24
tion about the selective hydroxylation of cyclohexene with
hydrogen peroxide using Fe complexes to 2-cyclohexen-1-ol,
for the zeolite containing large complexes.
As shown in
1
9
Figure S1, I220 at 2ª = 10° was greater than I311 at 12° for
Fe-Y, while I220 is lower than I311 for [Fe(bpy)3] @Y. This is
5
2+
due to the subsequent oxidation of 2-cyclohexen-1-ol to 1-one.
In recent years, the immobilization of transition-metal com-
plexes on the surface of supports or the encapsulation of
transition-metal complexes inside porous materials has received
considerable attention. Such an application can result in a
significant enhancement of novel catalytic activities that
homogeneous catalysts have not exhibited yet.6 Utilizing
clear evidence for the formation of complex ions within the
supercages.
Figure 1 shows UV­vis diffuse reflectance spectra of
2+
[Fe(bpy)3] @Y, [Fe(bpy) ](ClO ) + Na-Y, and Fe-Y. The
3
4 2
absorption spectrum of [Fe(bpy)3](ClO4)2 + Na-Y gave three
bands at 533 and 360 nm assigned to a metal-to-ligand (d­³*)
­8
2
5­27
water as an alternate to toxic and harmful organic solvents
charge-transfer (MLCT),
and 295 nm assigned to a ³­³*
is important and has been extensively studied.9
­14
Thus, the
transition of the bpy ligand, similar to that of [Fe(bpy) ]-
(ClO4)2 in CH3CN (Figure S2).
27
3
1
9,28
2+
combination of catalysts with iron as a ubiquitous metal and
water as a solvent seems to be very attractive from a viewpoint
of green sustainable chemistry.1
[Fe(bpy)3] @Y exhibits
5­17
The Y-type zeolite belonging to the faujasite family has
large cavities, so-called supercages, with a diameter of
1
8
ca. 13.0 ¡. These supercages are connected to each other by
tunnels or windows with a widest diameter of ca. 7.4 ¡. The
estimated diameter of [Fe(bpy)3]2 complex (ca. 12 ¡) is larger
+
2+
than a window size of supercage so that the [Fe(bpy)3]
complex synthesized in supercage cannot go out from the cavity
1
9
2+
(
Scheme S1 ). Therefore, [Fe(bpy)3] complexes encapsulated
2+
into Na-Y ([Fe(bpy)3] @Y) have been reported as a heteroge-
neous oxidation catalyst.2
,20­23
However, there is no information
about the activity of [Fe(bpy)3]2 @Y catalyst for selective
oxidation with hydrogen peroxide as a oxidant in water solvent.
In this report, [Fe(bpy)3]2 @Y was characterized by several
methods and solvent effects on its catalytic activity for oxidation
of cyclohexene with hydrogen peroxide was investigated.
Fe ion-exchanged Y-type zeolite (Fe-Y) was prepared from
Na-Y with FeSO4 solution. [Fe(bpy)3]2 @Y was obtained by
+
+
Figure 1. UV­vis diffuse reflectance spectra of (a) [Fe-
2+
(bpy)3] @Y, (b) [Fe(bpy) ](ClO ) + Na-Y, (c) Fe-Y, and
3
4 2
(d) Na-Y, where the amount of Fe contained in each sample
was held constant except for Na-Y.
+
Chem. Lett. 2012, 41, 713­715
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

7
14
Table 1. Oxidation of cyclohexene with hydrogen peroxide over Fe-containing catalystsa
Product/10 mol (Selectivity/%)b
¹
5
2 2
H O
H2O2
conv./%
Catalyst
Solvent
Cyclohexene
efficiency
Cyclohexen-1-ol
Cyclohexen-1-one
Othersd
c
oxide
/%
Fe(bpy)3]2 @Y
+
CH3CN
CH3CN
CH3CN
58
94
92
27.4 (91)
22.0 (58)
24.5 (91)
2.1 (7)
11.8 (31)
1.3 (5)
0.6 (2)
3.2 (9)
1.0 (4)
0
66
51
37
[
[
Fe(bpy)3](ClO4)2
0.9 (2)
0
Fe-Y
[
[
Fe(bpy)3]2+@Y
Fe(bpy)3](ClO4)2
H O
H2O
H O
2
47
98
52
19.5 (>99)
2.9 (25)
3.4 (>99)
0
0
0
0
0
53
15
8
2
6.1 (52)
0
2.6 (22)
0
Fe-Y
a
b
Reaction condition; Fe (7.9 ¯mol), cyclohexene (7.9 mmol), H2O2 (0.79 mmol), solvent (10 mL), 50 °C, 24 h under Ar. Selectivity
c
d
(
1
%) = (product [mol])/(all products [mol]) © 100. H2O2 efficiency (%) = (all products [mol])/(consumed H2O2 [mol]) © 100. Others:
,2-cyclohexanediols (cis and trans) assigned with standard reagents.
similar MLCT band (531 nm) but a slight blue-shift of the ³­³*
transition band at 290 nm. In spite of the same amount of
2
+
[
Fe(bpy)3] , the intensity of the band assigned to ³­³*
2+
transition in [Fe(bpy)3] @Y was larger than that in [Fe-
bpy) ](ClO ) + Na-Y, because the intermolecular ³­³ stack-
(
3
4 2
ing interaction of bpy ligands as shown in crystal structures of
1
9
[
Fe(bpy)3](ClO4)2 (Figure S3 ) was inhibited by immobiliza-
29
tion of complexes into supercages. These results support
that [Fe(bpy)3] complex ions exist within the supercages of
zeolite Y.
2+
Table 1 summarizes the catalytic activities of [Fe-
bpy)3]2 @Y and Fe-Y for the oxidation of cyclohexene with
+
(
hydrogen peroxide in CH3CN and H2O solvents. The result of
Fe(bpy)3](ClO4)2 as a homogeneous catalyst is also shown in
this table. No catalytic activity for oxidation of cyclohexene
with H2O2 was observed with Na-Y and without catalyst under
the same experimental conditions. In the case of CH3CN as an
[
Figure 2. Effect of water addition into solvent for catalytic
activity of (a) [Fe(bpy)3] @Y and (b) [Fe(bpy) ](ClO ) ( ;
3 4 2
2+
2-cyclohexen-1-ol, ; 2-cyclohexen-1-one, ; cyclohexene-
oxide). Reaction conditions: Fe in catalysts (7.9 ¯mol), cyclo-
hexene (7.9 mmol), 30% aqueous H2O2 (0.8 mmol), solvent
(total 10 mL; CH3CN (10 ¹ x mL) + water (x mL)), 50 °C, Ar
atmosphere.
organic solvent, the product yield and the selectivity of
+
2
-cyclohexen-1-ol generated on [Fe(bpy)3]2 @Y were similar
to those on Fe-Y. As can be seen in Table 1, the selectivity to
+
2
-cyclohexen-1-ol for [Fe(bpy)3]2 @Y- and Fe-Y-catalyzed
oxidations of cyclohexene was ca. 90%, which was much
higher than that for [Fe(bpy)3](ClO4)2-catalyzed oxidation.
Interestingly, [Fe(bpy)3]2 @Y showed much higher H2O2
efficiency than [Fe(bpy) ](ClO ) and Fe-Y. The recycle test
and selectivity of 2-cyclohexen-1-ol than [Fe(bpy)3]2+@Y.
Although the selectivity to 2-cyclohexen-1-ol on Fe-Y catalyst
+
2+
was similar to that on [Fe(bpy)3] @Y catalyst, the amount of
2-cyclohexen-1-ol and H O efficiency on the former catalyst
3
4 2
2
2
2+
of [Fe(bpy)3] @Y in CH3CN demonstrated no degradation of
either the catalytic activity or selectivity to 2-cyclohexen-1-ol
during at least three runs (Table S2;19 after the first reaction, the
catalyst was filtered, washed with CH3CN three times, dried
under air, and supplied to the next catalytic run).
was extremely lower than those on the latter catalyst. This result
2+
suggests that the combination of [Fe(bpy)3] and zeolite cage is
indispensable for obtaining high 2-cyclohexen-1-ol yield with
2+
high selectivity. Thus, it was proven that [Fe(bpy)3] @Y is a
potential heterogeneous catalyst for oxidation of cyclohexene
with hydrogen peroxide to 2-cyclohexen-1-ol in water. Our
results are very important from the viewpoint of green chemistry
because H2O is an environment-friendly solvent.
Figure 2 shows the effect of H O addition to CH CN
2
3
solvent with [Fe(bpy)3]2 @Y and [Fe(bpy)3](ClO4)2 catalysts.
+
When water was added into CH3CN solvent, the product yield of
2
-cyclohexen-1-ol with [Fe(bpy)3](ClO4)2 gradually decreased
The oxidation of cyclohexene in the presence or absence of
2,6-di-tert-butyl-4-methylphenol as a radical-trapping reagent
gave us mechanistic information. In the presence of 2,6-di-tert-
with increasing the amount of water added (Figure 2b), while
the products yields with [Fe(bpy)3]2 @Y hardly decreased
+
2+
(Figure 2a). As can be seen in Table 1, the product yield of
butyl-4-methylphenol, [Fe(bpy)3] @Y and [Fe(bpy)3](ClO4)2
exhibited no catalytic activity. This result suggests that a radical
-cyclohexen-1-ol over [Fe(bpy)3]2 @Y in water solvent
+
2
2+
(
[
x = 10 mL) was about five times larger than that over
Fe(bpy)3](ClO4)2. [Fe(bpy)3](ClO4)2 as homogeneous catalyst
species generated from [Fe(bpy)3] complexes and H2O2 plays
an important role in the oxidation mechanism. The reaction
might proceed via Fenton pathway (generation of hydroxyl
radicals) due to the absence of a labile site for H2O2 in the
showed higher H O2 conversion than the other tested Fe
catalysts, although it exhibited much lower H2O2 efficiency
2
Chem. Lett. 2012, 41, 713­715
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

7
15
complex occupied by three bipyridine ligands.30­33 Furthermore,
the zeolite is likely to stabilize the radical species active in
water, as some radical species were directly observed in zeolite
15 N. Aoyama, K. Manabe, S. Kobayashi, Chem. Lett. 2004,
33, 312.
16 C. Ogawa, S. Kobayashi, Chem. Lett. 2007, 36, 56; L.
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3
4­37
by ESR.
2
The reason for such a high selectivity of
-cyclohexen-1-ol over [Fe(bpy)3]2 @Y and Fe-Y may be due
+
+
to coordination of hydroxy groups in generated products to Na
ions contained in Y-type zeolite. In this case, the oxidation of
2+
2-cyclohexen-1-ol with hydrogen peroxide over [Fe(bpy)3] @Y
and Fe-Y to 2-cyclohexen-1-one is thought to hardly operate.
The catalytic activity of the mixture of [Fe(bpy) ](ClO )
and zeolite was similar to that of the [Fe(bpy)3](ClO4)2 under
19 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
3
4 2
homogeneous conditions. This result may support the fact that
the iron complexes in [Fe(bpy)3]2 @Y are present inside of
supercages. However, it is still not clear that the reaction
proceeds inside or outside of the cage, because the size effect for
a large substrate has not been examined.
In summary, we have developed mild and efficient selective
hydroxylation of cyclohexene with hydrogen peroxide in water
over Fe complexes encapsulated in Y-type zeolite. This discov-
ery is expected to be a breakthrough in organic synthesis
because organic solvents and/or additives are not required.
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2
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Chem. Lett. 2012, 41, 713­715
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/