Selective epoxidation of cyclohexene by a square-planar Ru complex immobilized into mesoporous silicate FSM-16

Article abstract of DOI:10.1246/cl.2007.122

The Ru complex, [Ru^II(babp)(dmso)₂] (1) (H ₂babp = 6,6′-bis(benzoylamino)-2,2′-bipyridine), was immobilized into mesoporous silicate FSM-16. The heterogeneous catalyst demonstrated a more selective epoxidation compared with the homogeneous 1. Copyright

Full text of DOI:10.1246/cl.2007.122

122
Chemistry Letters Vol.36, No.1 (2007)
Selective Epoxidation of Cyclohexene by a Square-planar Ru Complex
Immobilized into Mesoporous Silicate FSM-16
Takeshi Okumura,1 Hideki Takagi,2 Yasuhiro Funahashi,1 Tomohiro Ozawa,1
Yoshiaki Fukushima,2 Koichro Jitsukawa,ꢀ1 and Hideki Masudaꢀ1
1Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555
2TOYOTA Central R&D Labs, Inc., Yokomichi, Nagakute 480-1192
(Received October 19, 2006; CL-061237; E-mail: jitsukawa.koichiro@nitech.ac.jp, masuda.hideki@nitech.ac.jp)
The Ru complex, [RuII(babp)(dmso)2] (1) (H2babp = 6,60-
experiment was enough to accommodate cyclohexene and
TBHP molecules (the sizes of both molecules are estimated to
˚
be below 10 A). On the basis of X-ray photoelectron spectro-
scopic and elemental analyses, sulfur atom was not detected
in FSM-1. This result indicates that the dmso ligand of 1 has
been replaced by Si–OH groups existing on the inner surface
of FSM, although Si–O–Ru bond formation could not be charac-
terized directly.
bis(benzoylamino)-2,20-bipyridine), was immobilized into
mesoporous silicate FSM-16. The heterogeneous catalyst
demonstrated a more selective epoxidation compared with the
homogeneous 1.
High-valent metal–oxo species have been proposed as one
of the active intermediates not only in metalloenzymes but also
in the oxidative transformation of organic compounds catalyzed
by transition-metal complexes.1 In metalloenzymes, the second-
ary coordination sphere contributes to high specificity and
reactivity.2 There are many reports of the immobilized transi-
tion-metal complex catalysts.3 The second coordination sphere
of the complex immobilized into a mesopore must contribute
as a regulated reaction field to form the enzyme–substrate com-
plex.4 Mesoporous silica, FSM (Folded Sheet Mesoporous
Material), is expected to give a suitable reaction field for the cat-
alysis, because it has regularly arranged 2D hexagonal cylin-
ders.5 Easy immobilization of transition-metal complex into
FSM through the coordination of the axial ligand may afford
an efficient catalysts. We previously reported that Ru complexes
with a square-planar ligand, [RuII(babp)(dmso)L] (babp = 6,60-
bis(benzoylamino)-2,20-bipyridine, L = dmso (1)), proceed the
catalytic oxidation for various olefins, in which the axial ligands
L controlled the oxidation ability. Selective epoxidation of cy-
clohexene performed by a Ru complex with t-BuOOH (TBHP)
is still difficult because cyclohexene has weak C–H bond at allyl-
ic position. Here, we describe preparation of the heterogeneous
catalyst containing the Ru complex 1, and its ability to catalyze
selective oxidation of cyclohexene. Under the coexistence of
TBHP as an oxidant, the immobilized Ru complex catalyst
showed higher selectivity for epoxidation of cyclohexene in
comparison with the corresponding homogeneous reaction sys-
tems.
Immobilization of 1 into FSM-16 (pore size is about 3 nm)
in solution phase was carried out under an Ar atmosphere.7
Using less polar solvent, such as dichloromethane, efficient ad-
sorption of the Ru complex 1 into FSM was observed through
disappearance of color in the solution, whereas adsorption of 1
into FSM was not detected when a polar solvent such as metha-
nol and acetone was employed. Adsorption of 1 reached to equi-
librium within 1 h in dichloromethane to give orange powder of
the immobilized complex, FSM-1. The complex 1 once adsorbed
into FSM-16 did not leach out in the less polar solvent.8 The pore
size distributions of FSM-1, which was estimated by BJH analy-
sis of N2 adsorption isotherm (Figure S1),9 revealed that 1 was
immobilized not on outer surface but into pores of FSM-16.
The oxidation reaction of cyclohexene was carried out by
using 1 and FSM-1 in the presence of PhIO or TBHP as an
oxidant in 1,2-dichloroethane at 40 ꢁC under an Ar atmosphere.
Generally, oxidation of cyclohexene gives two types of products
through the different processes; one is epoxidation by an electro-
philic attack of an active oxygen species to the double bond
to give cyclohexene oxide, and another is allylic oxidation by
nucleophilic or radical attack to the ꢀ-position of the double
bond to give 2-cyclohexen-1-ol and 2-cyclohexen-1-one. When
PhIO was employed as an oxidant (Table 1, Entries 1 and 2), het-
erogeneous FSM-1 showed a reactivity similar to homogeneous
1 for the oxidation of cyclohexene, where RuV=O species was
spectroscopically characterized as a reaction intermediate.6,10
In the presence of dry TBHP (content of active oxygen was
93 wt %), the epoxidation selectivity decreased compared with
the cases of PhIO. When TBHP was added at once into the
reaction solution of 1, a large amount of 1-(tert-butylperoxy)-
2-cyclohexene was obtained as a result of the allylic peroxida-
tion (Table 1, Entry 3). There have been some reports on the
formation of such a peroxy ether, which may be generated from
Table 1. Oxidation product distributions for cyclohexenea
Turnover number
OH
O
OOtBu
Selec-
tivityg
/%
O
Entry Catalyst Oxidant
1
2
3
4
5
6
7
8
9
PhIO
PhIO
19
20
9
22
26
19
26
23
19
trace
trace
5
3
9
5
9
19
4
3
nd
nd
20
20
1
7
3
2
3
79
87
21
45
67
38
74
79
1
FSM-1b
1
TBHP
TBHP
TBHPf
TBHPf
TBHPf
TBHPf
TBHPf
4
2
3
5
2
1
2
FSM-1b
FSM-1b
1
FSM-1c
FSM-1d
FSM-1e
4
69
aReaction conditions: Ru:TBHP (93.1 wt %):Cyclohexene =
1:100:100 mM. All reactions were performed in CH2ClCH2Cl
under Ar atmosphere at 40 ꢁC. The amount of 1 immobilized into
FSM is 36.3 mmol/100 mg of FSM. The yields of reaction
products were determined by GC at 4 h for PhIO and 8 h for
TBHP. b1st use (Fresh). c2nd use. d3rd use. e4th use. fSuccessive
addition of 12.5 equiv. at every 1 h. gSelectivity of epoxidation,
which is based on the sum of oxidation products.
˚
The average pore size (26.5 A) of FSM-1 used for oxidation
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.1 (2007)
123
In summary, we have prepared the immobilized epoxidation
catalyst, FSM-1, by adsorption of a square-planar Ru complex 1
into FSM-16 through the liquid phase adsorption method. This
easy preparation of immobilized complex gave the reusable ox-
idation catalyst. Moreover, FSM-1 showed higher selective ep-
oxidation of cyclohexene using TBHP as an oxidant in compar-
ison with the case of 1 in the homogenous system. Especially,
successive addition of TBHP to immobilized FSM-1 catalyst
was quite effective for selective epoxidation of olefins.
30
25
20
15
10
5
O
O
OOtBu
OH
0
0
120 240 360 480
Reaction time /min
References and Notes
Figure 1. Time course for the product distributions of cyclo-
hexene oxidation, catalyzed by FSM-1 (2nd use) and successive
addition of TBHP (12.5 equiv. per hour).
1
a) R. A. Sheldon, J. Y. Kochi, Metal-Catalyzed Oxidations of
Organic Compounds, Academic Press, New York, 1981. b) B.
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a coupling reaction of t-BuOOꢂ and cyclohexenyl radicals.11
In the heterogeneous system catalyzed by FSM-1, however, the
ratio of epoxide to allylic oxidation products increased (Table 1,
Entry 4). In general, the oxidation reaction catalyzed by RuIV=O
species causes C–H bond activation12 and that by RuV=O
species promotes epoxidation of cyclohexene.1b,13 Therefore,
the increase of epoxidation selectivity in FSM-1 indicates that
the generation of RuV=O species was promoted. This may be
explained as follows; first, coordination of Si–O(H) groups to
the Ru complex at the axial position accelerates the reactivity
of RuV=O species, as reported in metallo–porphyrin complexes
that the heterolytic O–O bond cleavage of coordinated peroxide
is promoted by the ‘‘push effect’’ of donor ligands.14 Second, a
hydrophilic reaction pocket in mesopores of FSM15 provides a
polar environment to promote the ionic pathway leading to the
heterolytic O–O bond cleavage of TBHP.
In Entries 3 and 4 (Table 1), the active intermediate species
might react with not only cyclohexene but also excess TBHP,
generating 1-(tert-butylperoxy)-2-cyclohexene via formation of
t-BuOOꢂ by H atom abstraction reaction. So, successive addition
of TBHP was examined in order to clarify the genuine reactivity
of Ru=O species for oxidation of cyclohexene. In Entry 5
(Table 1), the selectivity of epoxidation increased and that of
allylic peroxidation decreased clearly. While, in homogeneous
1 (Table 1, Entry 6), the amount of 2-cyclohexen-1-one increas-
ed as much as epoxide. As described above, the successive
addition of TBHP to the immobilized Ru complex catalysts is
a good method for epoxidation of olefins.
2
3
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In typical procedure, FSM-16 (100 mg) was added to a CH2Cl2
solution of 1 (26.0 mg, 40 mmol) under an Ar atmosphere, and
stirred for 1 h. The adsorbed amount of 1 was determined by
measuring UV–vis spectrum of supernatant solution. The result-
ant orange powder was filtrated and dried in vacuo.
The amount of leached catalyst was monitored by measuring the
absorption intensity of the supernatant solution at 400 nm, which
was estimated to be below 1% based on the immobilized Ru
complex.
4
5
6
7
8
9
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
10 K. Jitsukawa, Y. Oka, S. Yamaguchi, H. Masuda, Inorg. Chem.
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16 The solid FSM-1 was exposed to air for one week. The oxidation
of Ru ion was confirmed by detecting the LMCT band of babp2ꢃ
to RuIII that appeared in the longer wavelength region more than
500 nm in UV–vis reflectance spectrum.
During the reaction under an Ar atmosphere, Ru species was
not eluted from FSM-1 and the catalyst could be recovered after
the reaction.8 Notably, increase in the selectivity for epoxidation
was observed in the recycle use of the catalyst in comparison
with the case of the fresh use (Table 1, Entries 5 and 7). In the
second use of FSM-1, as shown in Figure 1, the yield of cyclo-
hexene oxide increased drastically and linearly with the succes-
sive addition of TBHP every 1 h, although that of the allylic
oxidation products did not increase so much. This finding may
indicate that oxidation of RuII to RuIII through the reaction
promotes the generation of RuV=O species by two-electron
oxidant, TBHP. After oxidation reaction, trivalent Ru ion was
confirmed by detection of LMCT band from babp2ꢃ to RuIII in
the region >500 nm and of characteristic ESR signals to RuIII
species. Similar improvement of epoxidation was also observed
in preoxidized FSM-1 by treatment with oxygen.16 In the fourth
run (Table 1, Entry 9), the turnover number (TON) of epoxida-
tion was maintained at ca. 20.
Published on the web (Advance View) December 2, 2006; doi:10.1246/cl.2007.122