Highly selective epoxidation of cycloaliphatic alkenes with aqueous hydrogen peroxide catalyzed by [PO4{WO(O2) 2}4]3-/imidazole

Article abstract of DOI:10.1002/cctc.201402268

In the presence of imidazole as an additive, a phosphorus-containing tetranuclear peroxotungstate, THA₃[PO₄{WO(O ₂)₂}₄] (I, THA=tetra-n-hexylammonium), could act as an efficient catalyst for epoxidation of cycloaliphatic alkenes with 30% aqueous hydrogen peroxide (H₂O₂). Compound I showed higher catalytic activity and selectivity to epoxide than other tungstates. By using the I/imidazole system, various kinds of cycloaliphatic alkenes could be highly selectively converted into the acid-sensitive epoxides including industrially important diepoxides in high to excellent yields under the almost stoichiometric conditions. The ¹H NMR spectroscopy showed that imidazole would work not only as a proton acceptor but also as a Lewis base to remarkably suppress the acid-catalyzed ring opening of epoxides.

Full text of DOI:10.1002/cctc.201402268

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DOI: 10.1002/cctc.201402268
Highly Selective Epoxidation of Cycloaliphatic Alkenes
with Aqueous Hydrogen Peroxide Catalyzed by
3
À
[
PO {WO(O ) } ] /Imidazole
4
2 2 4
[a]
[a]
[a]
[a]
[b]
Keigo Kamata, Kosei Sugahara, Ryo Ishimoto, Susumu Nojima, Motoya Okazaki,
Takaya Matsumoto, and Noritaka Mizuno*
[b]
[a]
In the presence of imidazole as an additive, a phosphorus-con-
liphatic alkenes could be highly selectively converted into the
acid-sensitive epoxides including industrially important diepox-
ides in high to excellent yields under the almost stoichiometric
taining tetranuclear peroxotungstate, THA [PO {WO(O ) } ] (I,
3
4
2 2 4
THA=tetra-n-hexylammonium), could act as an efficient cata-
lyst for epoxidation of cycloaliphatic alkenes with 30% aque-
ous hydrogen peroxide (H O ). Compound I showed higher
1
conditions. The H NMR spectroscopy showed that imidazole
would work not only as a proton acceptor but also as a Lewis
base to remarkably suppress the acid-catalyzed ring opening
of epoxides.
2
2
catalytic activity and selectivity to epoxide than other tung-
states. By using the I/imidazole system, various kinds of cycloa-
Introduction
Epoxides are important chemical intermediates in the laborato-
such as amines, carboxylates, and acids enhance selectivities to
epoxides and/or catalytic activities. However, they have some
disadvantages: i) applicability to production of industrially im-
portant cycloaliphatic diepoxides has scarcely been investigat-
ed, ii) excess amounts of highly concentrated H O , urea/H O
[
1]
ry as well as chemical industry. Especially, epoxy resin is one
of the most versatile classes of polymeric materials with di-
verse applications such as metal can coatings, automotive
primer, printed circuit boards, and semiconductor encapsu-
2
2
2
2
[
2]
lants. In addition to the most commercially important glycidyl
ether based epoxy resins (e.g., prepared from bisphenol A and
epichlorohydrin), cycloaliphatic (di)epoxide-based ones have
attracted attention owing to the complete absence of chlorine,
which is particularly useful for electronic applications. However,
these (di)epoxides are industrially produced by stoichiometric
adduct (UHP), and multiple additives are used, and iii) strictly
[8,9]
controlled reaction conditions are required.
Therefore, de-
velopments of effective catalysts for H O -based selective ep-
2
2
oxidation of cycloaliphatic dienes are still challenging subjects.
Among the tungstate-catalyzed oxidation systems with
H O , the Ishii–Venturello system based on phosphorus-con-
2
2
[
2]
epoxidation of cycloaliphatic alkenes with peracids. In this
context, catalytic epoxidation of cycloaliphatic alkenes with
aqueous hydrogen peroxide (H O ) as an oxidant is a reliable
taining tungstates has extensively been investigated. Ishii and
co-workers have reported effective epoxidation of terminal al-
kenes with H O catalyzed by H PW O in the presence of ce-
2
2
2
2
3
12 40
[10a]
solution, which replaces the synthetic process with low atom
efficiency.
tylpyridinium chloride as a phase-transfer agent.
The Ishii
system could be applied to H O -based selective oxidation of
2
2
As cycloaliphatic alkenes and their epoxides are susceptible
to allylic oxidation and acid-catalyzed ring opening, respective-
ly, effective H O -based catalytic systems applicable to the syn-
allylic alcohols, monoterpenes, a,b-unsaturated carboxylic
[10]
acids, alcohols, amines, alkynes, and sulfides. Venturello and
co-workers have reported the structural characterization and
catalytic application of a phosphorus-containing tetranuclear
peroxotungstate, THA [PO {WO(O ) } ] (I, THA=tetra-n-hexy-
2
2
thesis of acid-sensitive cycloaliphatic epoxides have still been
[
1]
[3]
[4]
[5]
limited. Several effective systems based on Ti, Mn, Fe,
3
4
2 2 4
[
6]
[7]
[11]
W, and Re catalysts have been reported, and some additives
lammonium, Figure 1). On the basis of the spectroscopic and
kinetic studies, I is a catalytically important species among vari-
ous peroxotungstates generated by the reaction of H PW O
[a] Dr. K. Kamata, K. Sugahara, Dr. R. Ishimoto, S. Nojima, Prof. Dr. N. Mizuno
3
12 40
[13]
[
12]
Department of Applied Chemistry
School of Engineering, The University of Tokyo
with excess H O . Although many effective homogeneous
2
2
[14]
and heterogeneous systems based on I or its related com-
pounds have been developed, effects of an organic base as an
additive on H O -based epoxidation catalyzed by polyoxometa-
7
-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81) 3-5841-7220
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
2
2
[b] M. Okazaki, Dr. T. Matsumoto
lates including I has not yet been investigated. During the
Fine Chemicals R&D Group, Chemicals Research Laboratory
Research & Development Division
JX Nippon Oil & Energy Corporation
course of our investigation on the oxidation catalysis by tung-
[15]
states, we have found that the addition of imidazole remark-
ably suppresses ring opening of epoxides for epoxidation of
8
Chidoricho, Naka-ku, Yokohama 231-0815 (Japan)
cycloaliphatic alkenes with 30% aqueous H O catalyzed by I.
In this communication, we report a simple catalytic system
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402268.
2
2
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[
11a]
Figure 1. Molecular structure of the anion part of I.
based on I and imidazole applicable to the synthesis of cycloa-
liphatic epoxides including industrially important diepoxides.
To the best of our knowledge, the noteworthy increase in
yields of cycloaliphatic diepoxides by the presence of imida-
zole has not been reported to date for tungstate-catalyzed ep-
a
Figure 2. a) Molecular structures and pK values of nitrogen bases. b) Plots
of yield of 2a, E , and reaction rate (R ) against pK of additives for epoxida-
ox
0
a
[
6,8–14]
2 2
tion of 1a with H O catalyzed by I in the presence of additives. Reaction
oxidation of cycloaliphatic dienes with aqueous H O .
2
2
conditions: I (25 mmol), additive (50 mmol), 1a (2 mmol), 30% aqueous H
2 mmol), acetonitrile (5 mL), 323 K, 24 h. Yield was determined by GC analy-
sis. Eox (%)=[products (mol)/consumed H (mol)]ꢁ100.
2 2
O
(
2
O
2
Results and Discussion
Effects of additives on epoxidation of cyclohexene with
H O catalyzed by I
2
2
First, epoxidation of cyclohexene (1a) as a model substrate
with H O catalyzed by I in the absence and presence of addi-
2
2
tives was performed under the stoichiometric conditions (I:ad-
ditive:1a:H O =1:2:80:80, Figure 2, see also Table 1). In each
2
2
case, formation of allylic oxidation products such as 2-cyclo-
hexen-1-ol (3a) and 2-cyclohexen-1-one (4a) was hardly ob-
served. In the absence of additives, 1,2-epoxycyclohexane (2a)
and trans-1,2-cyclohexanediol (5a) were obtained in 68% and
7
% yields, respectively. To suppress the successive ring open-
ing of 2a, effects of nitrogen bases [pyrazole (pK of conjugate
a
[
16a]
[16a]
[16a]
acid in water: 2.49),
pyridine (5.23),
imidazole (6.99),
Figure 3. Plots of yield and R
tion of 1a with H catalyzed by I in the presence of imidazole. Reaction
conditions: I (25 mmol), imidazole (0–100 mmol), 1a (2 mmol), 30% aqueous
(2 mmol), acetonitrile (5 mL), 323 K, 7 h. Yield was determined by GC
0
against molar ratio of imidazole/I for epoxida-
[
16b]
N,N-dimethyl-4-aminopyridine (DMAP, 9.70),
and 1,8-
2
O
2
[
16c]
diazabicyclo[5.4.0]undec-7-ene (DBU, 12.00) ] as additives
were investigated. The catalytic performance was strongly de-
pendent on the kinds of additives. Plots of reaction rates (R0),
yields of 2a, and efficiencies of H O utilization (E ) against the
2 2
H O
analysis.
2
2
ox
pK values of additives are shown in Figure 2. The R value
of catalysts and solvents on epoxidation of 1a with H O cata-
a
0
2
2
reached a maximum at pK 5 and decreased with increase in
lyzed by I were investigated (Table 1). For epoxidation of 1a
catalyzed by tetra-n-butylammonium salt of
[PO {WO(O ) } ] , [(n-C H ) N] [PO {WO(O ) } ], the yield, se-
a
the pK value. On the other hand, the E value dramatically de-
a
a
ox
3
À
[17]
creased at pK 10 and higher probably caused by the non-pro-
a
4
2 2 4
4
9 4
3
4
2 2 4
ductive decomposition of H O in the presence of a strong
lectivity to 2a, and R value were almost the same as those of
2
2
0
base. Among the additives tested, imidazole with the moder-
I (Table 1, entry 2), suggesting that the kind of cation does not
ate pK value of 6.99 was the most effective, and the yield of
affect the catalytic performance for the present epoxidation.
a
2
a and Eox value were 91% and 95%, respectively.
Effects of the amount of imidazole with respect to I on ep-
Compound I exhibited higher yield of 2a and larger R value
0
2À [12c]
than the other tungstates such as [HPO {WO(O ) } ] ,
4 2 2 2
2
À [15e]
2À [15g]
oxidation of 1a with H O were investigated (Figure 3). The R
[{WO(O ) } (m-O)] ,
[SeO {WO(O ) } ] ,
and [g-SiW O -
10 34
2
2
0
2 2 2
4
2 2 2
4À
[15a]
value decreased with increase in the amount of imidazole,
whereas the yields of 2a were highest with one and two
equivalents of imidazole. In the presence of imidazole, effects
(H O) ] (entries 1–6).
In the absence of catalysts, the reac-
2
2
tion did not proceed (entry 7). Among the solvents tested, ace-
tonitrile was the most effective (entries 1 and 8–14). After the
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efficiently proceeded to give the corresponding diepoxides in
83%, 75%, and 84% yields, respectively (entries 5, 7, and 9).
The present I/imidazole system also exhibited high catalytic ac-
tivities and selectivities for epoxidation of substituted cyclo-
hexenes. By using one equivalent of H O , 4-substituted cyclo-
2 2
Table 1. Effects of catalysts and solvents on epoxidation of 1a with H O
catalyzed by I in the presence of imidazole.
[
a]
2
2
hexenes such as 3-cyclohexene-1-carboxylic acid methyl ester
1 f) and 4-vinyl-1-cyclohexene (1g) gave the corresponding
(
Entry Catalyst Solvent Conv.
Yield [%]
2a 3a 4a
R
0
À1
cycloaliphatic monoepoxides (2 f and 2g) in 92% and 88%
yields, respectively (entries 13 and 15). Not only 4-substituted
cyclohexenes but also 1-substituted ones could be selectively
epoxidized. 1-Methyl-1-cyclohexene (1h) and (+)-3-carene (1i)
were selectively converted into the corresponding epoxides
[%]
5a
[mmmin ]
1
2
3
4
5
6
7
8
9
1
I
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
96
96
90
68
84
60
<1
83
87
83
81
84
53
35
91 nd
90 nd
86 nd
nd
nd
nd
2
3
1
2.19
2.18
0.89
PW4
PW2
W2
SeW2
SiW10
67 nd <1
nd 1.02
65 nd
40
1
1
3
2
nd
2.42
0.36
–
(
2h and 2i) in good yields (entries 17 and 19). To further con-
1
firm effectiveness of the present I/imidazole system, larger
scale epoxidation of 1b, 1e, and 1 f was performed
without CH
nd nd
82 nd
86 nd
82 nd
79 nd
79 nd
52 nd
nd
I
I
I
I
I
I
I
CHCl
DMC
EtOAc
MEK
tBuOH
toluene
DMF
3
nd <1 2.59
nd
nd
1
1
1
1
1.16
0.91
0.60
(
Scheme 1). Analytically pure 2b, 2e, and 2 f could be isolated
0
in 81% (2.04 g), 76% (1.92 g), and 83% (2.60 g) yields, respec-
tively, demonstrating that the present system is applicable to
gram-scale reactions.
1
1
12
13
14
nd <1 0.27
nd <1 0.17
15
1
2
4
0.15
The role of imidazole on the present epoxidation was inves-
1
[
(
a] Reaction conditions: catalyst (W: 100 mmol), imidazole (50 mmol), 1a
2 mmol), 30% aqueous H (2 mmol), solvent (5 mL), 323 K, 24 h. Con-
tigated by H NMR spectroscopy (Figure 4a and b). The
1
2
O
2
H NMR spectrum of imidazole exhibited the downfield shift of
version and yield were determined by GC analysis. Conv. of 1a (%)=[1a
consumed (mol)/initial 1a (mol)]ꢁ100. Yield (%)=[product (mol)/initial
the signal of the H2 proton from d=7.55 to 7.89 ppm by addi-
tion of 0.5 equivalents of I with respect to imidazole, thus indi-
cating the protonation of imidazole described in Ref. [18]. The
1
a (mol)]ꢁ100. R
low conversion (ꢁ10%) of 1a and H
as follows: [(n-C N] [PO {WO(O
{WO(O ] (PW2), [(n-C N] [{WO(O
{WO(O (SeW2), and [(n-C N]
c] Solvents were abbreviated as follows: dimethyl carbonate (DMC), ethyl
0
values were determined from the reaction profiles at
2
2
O . [b] Tungstates were abbreviated
1
4
H
9
)
4
3
4
2
)
2
}
4
]
(PW4),
(m-O)] (W2), [(n-C
[g-SiW10 O)
[(n-C
4
H
H
9
9
)
)
4
N]
N]
2
-
-
H NMR spectra of imidazole in the presence of I, aqueous
[
[
[
HPO
SeO
4
2
)
2
}
2
4
H
9
)
4
2
2 2 2
) }
4
4
2
H O , or H O showed smaller downfield shifts (Dd=0.01–
2
2
2
4
2
)
2
}
2
]
4
H
9
)
4
4
O
34(H
2
2
]
(SiW10).
0.13 ppm) of the H2 proton signal than that (Dd=0.38 ppm)
acetate (EtOAc), methyl ethyl ketone (MEK), tert-butyl alcohol (tBuOH),
and N,N-dimethyl formamide (DMF).
in the presence of I and aqueous H O (Figure 4c and Support-
2
2
ing Information, Figure S1), indicating that H O and/or H O
2
2
2
[18,19]
probably work as proton sources in the presence of I.
In
addition to the 7.89 and 7.04 ppm signals of a protonated imi-
dazole, new signals appeared at d=8.14, 7.27, and 7.26 ppm
with the respective intensity ratio of 1:1:1 likely assignable to
the imidazole coordinated to the Lewis acidic tungsten
epoxidation of 1a in the presence of imidazole for 6 h under
the reaction conditions in Figure 2, 1a (2 mmol) and 30%
aqueous H O (2 mmol) were added into the reaction solution.
2
2
[17,20,21]
The epoxidation again proceeded with almost the same yield
and selectivity to 2a (83% yield and ꢀ99% selectivity) as
those (84% yield and 98% selectivity) observed for the first
run. In this case, the total turnover number (TON) reached up
to 134, and the I/imidazole system is intrinsically recyclable.
The present I/imidazole system could be applied to epoxida-
tion of various kinds of structurally diverse cycloaliphatic
mono- and dienes with 30% aqueous H O (Table 2). In each
center.
In contrast, such downfield shifts and/or appear-
ance of new signals were not observed for pyrazole, suggest-
ing no interaction of weakly basic pyrazole with I (Figure 4d
[22]
and e). In the case of strongly basic DBU, only the signals
likely assignable to a protonated DBU were observed in agree-
[23,24]
ment with the low Eox value (Figure 4 f and g).
Therefore,
imidazole would work not only as a proton acceptor but also
as a Lewis base to suppress the acid-catalyzed ring opening of
epoxides, resulting in high selectivities to epoxides. Upon addi-
2
2
case, the conversions of substrate and yields of epoxide in the
absence of imidazole were much lower than those in the pres-
ence of imidazole, showing that imidazole plays an important
role in the completeness of the reaction as well as the increase
in selectivity to epoxide. By using the I/imidazole system, 3-cy-
clohexenylmethyl 3-cyclohexene-1-carboxylate (1b) could se-
lectively be converted into the commercially important diepox-
ide, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxy-
late (2b), in 82% yield with only two equivalents of H O with
4À
tion of [g-SiW O (H O) ] , which exhibited lower catalytic ac-
10
34
2
2
tivity and selectivity than I, into the [D ]acetonitrile solution
3
containing imidazole, many unidentified signals appeared, thus
demonstrating the specific interaction of imidazole with I.
Conclusions
The simple I/imidazole system exhibited high catalytic activity
and selectivity for epoxidation of cycloaliphatic alkenes with
30% aqueous H O as a sole oxidant. Various kinds of structur-
2
2
respect to 1b (Table 2, entry 1). The yield of 2b could be in-
creased to 91% by using three equivalents of H O (entry 3).
2
2
2
2
Epoxidation of other cycloaliphatic dienes of 4,7,8,9-tetrahy-
droindene (1c), dicyclopentadiene (1d), and 1-methyl-1,4-cy-
clohexadiene (1e) with two to three equivalents of H O also
ally diverse cycloaliphatic mono- and dienes could be convert-
ed into the corresponding epoxides in high to excellent yields
under the almost stoichiometric conditions.
2
2
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purchased from ACROS and used as received. Tungstic
2 2
Table 2. Epoxidation of various kinds of cycloaliphatic alkenes with H O catalyzed by
I in the presence and absence of imidazole.
acid (Wako Chemical), H PO4 (Kanto Chemical, 85%
[
a]
3
aqueous solution), tetra-n-hexylammonium chloride (Al-
drich), H O2 (Kanto Chemical, 30% aqueous solution),
Entry Substrate
Product
Im Conv. Yield
%] [%]
2
[
and solvents (benzene, chloroform, dimethyl carbonate,
ethyl acetate, methyl ethyl ketone, tert-butyl alcohol,
and N,N-dimethylformamide (Kanto Chemical)) were pur-
chased and used as received. 3-Cyclohexenylmethyl 3-cy-
clohexene-1-carboxylate (1b) was purchased from
Ningbo Huana Chemical Co., Ltd. Tungstates ([(n-C H ) N]
1
2
1b
1b
2b
2b
+
96 82
58
À
9
[
[
b]
b]
3
4
+
>99 91
4
9 4
[12c]
3
À
80 22
[
17]
[
PO {WO(O ) } ],
[(n-C H ) N] [HPO {WO(O ) } ],
4 2 4 2 2 2
4
2 2 4
9 4
15e]
[
[
[
c]
c]
[(n-C H ) N] [{WO(O ) } (m-O)],
4
9 4
2
2 2 2
[(n-C H ) N] [SeO -
4
9 4
2
4
5
6
1c
2c
+
>99 83
>99 57
[15g]
[15a]
{WO(O ) } ],
and [(n-C H ) N] [g-SiW O (H O) ]
)
2
2 2
4
9 4
4
10 34
2
2
À
were synthesized and characterized according to the lit-
erature procedures.
[
b]
b]
7
8
1d
2d
+
>99 75
[
À
+
92 29
[
b]
[
9
1
1e
1a
1 f
2e
2a
2 f
>99 84
b]
d]
Instruments
0
À
92 17
[
1
1
+
96 91
94 68
IR spectra were measured on a Jasco FT/IR-460 spec-
trometer Plus using KCl disks. Raman spectra were re-
corded on a Jasco NR-1000 spectrometer with excitation
at 532.36 nm using JUNO 100 green laser (Showa Op-
tronics Co., Ltd.). UV/Vis spectra were recorded on
a JASCO V-570 spectrometer. NMR spectra were recorded
[
d]
12
À
+
[
e]
e]
1
1
3
4
97 92
42 34
[
À
[
[
e]
e]
1
1
5
6
1g
1h
1i
2g
2h
2i
+
89 88
61 45
À
+
1
13
on a JEOL JNM-EX-270 spectrometer ( H, 270.0 MHz; C,
[
c,d]
c,d]
31
183
1
1
7
8
95 94
61 35
67.8 MHz; P, 109.3 MHz; W, 11.2 MHz) by using 5 mm
[
À
+
1
13
31
183
tubes (for H and C) or 10 mm tubes (for P and W)
[
d]
d]
1
13
1
2
9
0
89 85
51 40
or on a JEOL ECA-500 spectrometer ( H, 495.1 MHz; C,
24.5 MHz) by using 5 mm tubes. Chemical shifts (d)
were reported in ppm downfield from SiMe (solvent,
[
À
1
4
[
a] Reaction conditions: I (25 mmol), imidazole (Im, 50 mmol), substrate (1 mmol), 30%
aqueous H (2 mmol), acetonitrile (5 mL), 323 K, 24 h, under Ar (1 atm). Conversion
and yield were determined by GC analysis. Compound I, substrates, and 30% aqueous
were soluble in acetonitrile, and the system was not two-phase. [b] 30% aque-
ous H (3 mmol). [c] Imidazole (25 mmol). [d] Substrate (2 mmol). [e] 30% aqueous
(1 mmol).
1
13
CDCl ) for H and C NMR spectra, 85% H PO for
P NMR spectra, and 2m Na WO (solvent, D O) for
2
O
2
3
3
4
31
183
W
2
4
2
H
2
O
2
NMR spectra, respectively. GC analyses were performed
on Shimadzu GC-2014 with a flame ionization detector
equipped with an InertCap 5 capillary column (internal
diameter=0.25 mm, length=60 m) or with a Stabilwax
capillary column (internal diameter=0.25 mm, length=
0 m). Cold-spray ionization (CSI)–MS spectra were recorded on
2
O
2
2 2
H O
3
a JEOL JMS-T100-CS spectrometer. Typical measurements were as
follows: Orifice voltage 85 V for positive ions; sample flow
À1
0
.1 mLmin ; solvent acetonitrile; concentration 0.1 mm; spray
temperature at 263 K; ion source at RT.
Synthesis and characterization of [(n-C H ) N] -
6
13 4
3
[
PO {WO(O ) } ]
4 2 2 4
The
tetra-n-hexylammonium
(THA)
salt
derivative
of
3À
[PO {WO(O ) } ] was synthesized according to the literature pro-
4 2 2 4
[11a]
31
cedure.
Yield: 1.39 g (50%). P NMR (109.25 MHz, CD CN, 298 K,
3
2
183
H PO ): d=4.5 ppm ( J =18.5 Hz); W NMR (11.20 MHz, CD CN,
3
4
W-P
3
2
2
98 K, Na WO ): d=À588.2 ppm ( J =18.4 Hz, Dv =7.3 Hz);
2
4
W-P
1/2
Scheme 1. Larger-scale epoxidation of 1b, 1e, and 1 f with 30% aqueous
catalyzed by I in the presence of imidazole. Reaction conditions: I
1.25 mol% with respect to 1), imidazole (2.5 mol% with respect to 1), 30%
(30 mmol for 1b and 1 f, 60 mmol for 1e), acetonitrile (20 mL), 323 K.
À1 3 À1
UV/Vis (CH CN) l
(e)=254.2 nm (1268 (mol of W) m cm ); IR
3
max
2 2
H O
(
5
3
KCl): n˜ =977, 853, 843, 797, 757, 728, 660, 649, 603, 591, 573, 549,
(
À1
25, 444 cm ; Raman: v=990, 864, 821, 655, 597, 580, 543, 391,
2 2
H O
À1
33, 305, 266, 237 cm ; positive ion CSI–MS (CH CN): m/z: 2569
3
+
[(THA) PO {WO(O ) } ] ; elemental analysis calcd (%) for
4
4
2 2 4
C H N O PW ((THA) [PO {WO(O ) } ]): C 39.05, H 7.10, N 1.90, P
72
156
3
24
4
3
4
2 2 4
Experimental Section
1.40, W 32.22; found: C 38.82, H 6.97, N 1.36, P 1.36, W 33.28.
Materials
Procedure for catalytic epoxidation
Acetonitrile (Kanto Chemical) and toluene (Kanto Chemical) were
purified by The Ultimate Solvent System (GlassContour Company)
The catalytic epoxidation of various cycloaliphatic alkenes was per-
formed in a 30 mL glass vessel containing a magnetic stir bar. All
products were identified by the comparison of GC retention time,
[25]
prior to use. Substrates were purified according to the reported
[26]
procedures. Deuterated solvents (CD CN, CDCl , and D O) were
3
3
2
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standard (chlorobenzene), and 30% aqueous H O (2 mmol) were
2
2
charged in the reaction vessel. The reaction was initiated by the
addition of I (25 mmol) and the reaction solution was periodically
analyzed. Remaining H O after the reaction was analyzed by the
2
2
3
+/4+
[27]
Ce
titration. The products are known and identified by com-
parison of their H and C NMR signals with the literature data.
1
13
[28]
Larger scale production of epoxides
Into a glass reactor were successively placed substrate [10 mmol
1b) or 20 mmol (1e and 1 f)], imidazole (2.5 mol% with respect to
substrate), 30% aqueous H O [30 mmol (1b and 1 f) or 60 mmol
(
2
2
(1e)], and acetonitrile (20 mL). The reaction was initiated by addi-
tion of I (1.25 mol% with respect to substrate), and the reaction
mixture was stirred at 323 K. The GC yields of 2b, 2e, and 2 f
reached up to 96%, 89%, and 92% for 12 h, 8 h, and 6 h, respec-
tively. The isolation of 2b, 2e, and 2 f was performed by column
chromatography on silica gel (Silica Gel 60N, spherical, neutral, 63–
2
10 mm, Kanto, Cat. No. 37565-79) using chloroform/ethyl acetate
95/5–80/20, v/v for 2b) or n-hexane (for 2e and 2 f) as an eluent,
giving 2.04 g of 2b (81% yield), 1.92 g of 2e (76% yield), and
.60 g of 2 f (83% yield), respectively, with >95% purity by
(
2
1
H NMR spectroscopy.
Acknowledgements
This work was supported in part by the Japan Society for the
Promotion of Science (JSPS) through its “Funding Program for
World-Leading Innovative R&D on Science and Technology (FIRST
Program)” and Grants-in-Aid for JSPS Fellows and Scientific Re-
search from the Ministry of Education, Culture, Science, Sports,
and Technology of Japan.
Keywords: alkenes · epoxidation · nitrogen heterocycles ·
peroxides · tungsten
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Figure 4. H NMR spectra of a) imidazole, b) imidazole in the presence of I
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Received: April 23, 2014
Published online on && &&, 0000
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K. Kamata, K. Sugahara, R. Ishimoto,
S. Nojima, M. Okazaki, T. Matsumoto,
N. Mizuno*
&& – &&
Highly Selective Epoxidation of
Cycloaliphatic Alkenes with Aqueous
Hydrogen Peroxide Catalyzed by
Two epoxides, one tungstate complex:
In the presence of imidazole, a phospho-
rus-containing tetranuclear peroxotung-
state, THA [PO {WO(O ) } ] (I, THA=
cycloaliphatic alkenes with 30%
aqueous hydrogen peroxide. Thus,
various cycloaliphatic alkenes can be
highly selectively converted into the
acid-sensitive epoxides including
industrially important diepoxides.
3À
[PO {WO(O ) } ] /Imidazole
4
2 2 4
3
4
2 2 4
tetra-n-hexylammonium), can act as an
efficient catalyst for epoxidation of
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&
ÞÞ
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