Efficient, regioselective epoxidation of dienes with hydrogen peroxide catalyzed by [γ-SiW10O34(H2O) 2]4-

Article abstract of DOI:10.1016/j.jcat.2004.02.027

A divacant, lacunary, Keggin-type silicotungstate, [γ-SiW ₁₀O₃₄(H₂O)₂]^4-, exhibits high catalytic performance for the epoxidation of various nonconjugated dienes using hydrogen peroxide under mild conditions, high regioselectivity to the more accessible double bonds, and high efficiency of hydrogen peroxide utilization. The high regioselectivity for the [γ-SiW₁₀O₃₄(H ₂O)₂]^4--catalyzed epoxidation would be caused by the steric hindrance of the active site.

Full text of DOI:10.1016/j.jcat.2004.02.027

Journal of Catalysis 224 (2004) 224–228
www.elsevier.com/locate/jcat
Research Note
Efficient, regioselective epoxidation of dienes with hydrogen peroxide
catalyzed by [γ -SiW10O34(H2O)2]4− ✩
Keigo Kamata,a Yoshinao Nakagawa,b Kazuya Yamaguchi,a,b and Noritaka Mizuno a,b,∗
a
Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama, 332-0012, Japan
b
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received 19 December 2003; revised 20 February 2004; accepted 20 February 2004
Abstract
A divacant, lacunary, Keggin-type silicotungstate, [γ -SiW
4−
(H O) ] , exhibits high catalytic performance for the epoxidation of
2 2
O
10 34
various nonconjugated dienes using hydrogen peroxide under mild conditions, high regioselectivity to the more accessible double bonds, and
4−
high efficiency of hydrogen peroxide utilization. The high regioselectivity for the [γ -SiW
caused by the steric hindrance of the active site.
O
10 34
(H O) ] -catalyzed epoxidation would be
2
2
2004 Elsevier Inc. All rights reserved.
Keywords: Diene; Epoxidation; Hydrogen peroxide; Polyoxometalate
1. Introduction
is still low. Therefore, efficient systems for the regioselective
epoxidation of dienes with hydrogen peroxide are previously
unknown.
Epoxidation of olefins is an important reaction in the lab-
oratory as well as the chemical industry, because epoxides
are widely used as intermediates in organic syntheses [1,2].
Generally, more electron-rich double bonds of dienes are
preferably epoxidized when stoichiometric oxidants such as
peroxyacids [3] and peroxides [4] or hydroperoxides in the
presence of molybdenum compounds [5–7] are used. The
effects of steric protection of catalysts for the regioselective
epoxidation of nonconjugated dienes have been examined.
For example, the highly regioselective epoxidation of more
accessible, but less nucleophilic, double bonds using ster-
ically hindered metalloporphyrins with NaOCl and PhIO
has been developed [8–11]. In contrast to these expensive
oxidants, hydrogen peroxide is an attractive oxidant from
economical and environmental viewpoints because it gen-
erates only water as a by-product and has a high content
of active oxygen species. Although various catalysts for the
regioselective epoxidation of nonconjugated dienes with hy-
drogen peroxide have been studied [12–26], the efficiency
of hydrogen peroxide utilization and/or the regioselectivity
Very recently, we have reported an efficient, simple route
for the epoxidation of various olefins including nonacti-
vated terminal olefins such as propylene with hydrogen
peroxide catalyzed by a divacant lacunary silicotungstate,
[γ -SiW10O34(H2O)2]4− (I) [27]. In this paper, we report
the efficient, regioselective epoxidation of various noncon-
jugated dienes with hydrogen peroxide catalyzed by I.
2. Experimental
2.1. Instruments
IR measurements were carried out with KBr pellets us-
ing a Perkin-Elmer Paragon 1000PC spectrometer. NMR
spectra were recorded at 298 K on a JEOL JNM-EX-270
(1H, 270 MHz; 13C, 67.5 MHz; 29Si, 53.45 MHz; 183W,
11.20 MHz) spectrometer. Chemical shifts (δ) were re-
ported using SiMe4 (1H, 13C, and 29Si) and Na2WO4 (183W)
as external standards. UV–vis spectra were recorded on a
Perkin-Elmer Lambda 12 spectrometer. GC analyses were
performed on Shimadzu GC-14B with a flame ionization de-
tector equipped with a TC-WAX capillary or SE-30 packed
✩
Supplementary data for this article may be found on ScienceDirect.
Corresponding author. Fax: +81-35841-7220.
*
E-mail address: tmizuno@mail.ecc.u-tokyo.ac.jp (N. Mizuno).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.02.027
2004 Elsevier Inc. All rights reserved.

K. Kamata et al. / Journal of Catalysis 224 (2004) 224–228
225
column. Mass spectra were recorded with a Perkin-Elmer
TurboMass spectrometer at an ionization voltage of 70 eV.
40 µmol), solvent (CH3CN, 6 mL), diene (5 mmol), and an
internal standard (n-decane or naphthalene) were charged in
the reaction vessel. The reaction was initiated by the addition
of 30% aqueous H2O2 (1 mmol). The reaction solution was
periodically sampled and analyzed by NMR and GC in com-
bination with mass spectroscopy. A detailed presentation of
physical data of dienes and epoxides is given in Support-
ing Information. The stoichiometric epoxidation of dienes
with m-CPBA was carried out under the following condi-
tions: m-CPBA (850 µmol), CHCl3 (6 mL), nonconjugated
diene (5 mmol), reaction temperature (305 K), reaction time
(0.5 h).
2.2. Procedure for the oxidation of dienes
The epoxidation of nonconjugated dienes was carried
out with a 30-mL glass vessel containing a magnetic stir
bar. Nonconjugated dienes and solvents were obtained
from Aldrich and Tokyo Kasei, and purified by the stan-
dard procedure [28]. 1,2-Epoxylimonene and 1,2-epoxy-4-
vinylcyclohexene were obtained from Aldrich and used as
authentic samples. The other epoxides were synthesized
via Payne oxidation [29] and/or m-CPBA oxidation, and
used as authentic samples. The products were identified by
the comparison of GC retention time, mass spectra, and
NMR spectra with those of the authentic samples. The
quantifications were carried out by calibrated GC analy-
ses. The reaction conditions (i.e., concentration of diene,
diene to oxidant ratio, reaction temperature, etc.) were
controlled to avoid double epoxidation. It was confirmed
that the internal standard was not oxidized at all under
the reaction conditions. The diene to oxidant ratio was
5/1 (diene, 5 mmol; H2O2, 1 mmol). Under the present
reaction conditions, diepoxides and glycols were hardly
produced in all cases. The carbon balance in each experi-
ment was in the range of 95–100%. The H2O2 remaining
after the reaction was analyzed by Ce4+/3+ titration [30].
A typical procedure for the catalytic epoxidation of non-
conjugated dienes was as follows: catalysts containing
80 µmol of tungsten (i.e., [γ -SiW10O34(H2O)2]4−, 8 µmol;
3. Results and discussion
Table 1 shows the epoxidation of various dienes with hy-
drogen peroxide in acetonitrile at 305 K catalyzed by the
tetrabutylammonium salt derivative of I (TBA-I). Under
the present reaction conditions, the amounts of diepoxides
and glycols produced through the hydrolysis were neg-
ligible in all cases. Under the stoichiometric conditions
(1-methyl-1,4-cyclohexadiene (1a)/H2O2 molar ratio =
1/1), the total product yield was 61% and the diepoxide
was formed. Therefore, we used the reaction conditions
in the high ratio of substrate to H2O2. For the epoxida-
tion of 1a (entry 1), the [1b]/[total epoxide] ratio was
0.89 and the more accessible double bond was much more
selectively epoxidized. The value was higher than or com-
parable to those reported for sterically hindered porphyrin
systems with NaOCl or PhIO (0.11–0.95) [3,9,11] and
[PO4{WO(O2)2}4]3−, 20 µmol; [W2O3(O2)4(H2O)2]2−
,
Table 1
a
Epoxidation of dienes catalyzed by TBA-I with hydrogen peroxide
Entry
Diene
Time (h)
Product (yield (%))
b/(b+c)
H O efficiency (%)
2 2
1
4
(83)
(10)
0.89
> 99
1a
2a
1b
2b
1c
b
2
7
7
(43)
(27)
(55)
0.61
0.38
81
2c
c
3
(34)
93
3a
4a
3b
4b
3c
4c
4
6
4
(16)
(78)
0.17
0.04
> 99
5
(4)
(94)
98
5a
5b
5c
a
Reaction conditions: TBA-I (8 µmol), diene (5 mmol), 30% aq hydrogen peroxide (1 mmol), acetonitrile (6 mL), reaction temperature (305 K). The reac-
tion conditions were controlled to inhibit the double epoxidation. Yields were determined by gas chromatographic analysis with an internal standard technique.
3+
4+
Yield (%) = product (mol)/H O used (mol) × 100. Remaining H O after the reaction was estimated by potential difference titration of Ce /Ce . H O
2
2
2
2
2 2
efficiency = products (mol)/H O consumed × 100.
2
2
b
c
Toluene was produced as a by-product (7% yield).
Acetonitrile (9 mL).

226
K. Kamata et al. / Journal of Catalysis 224 (2004) 224–228
the mono-cobalt-substitued heteropolytungstate system with
O2 in the presence of aldehydes (0.69) [31] (Table S1).
For the epoxidation of trans-1,4-hexadiene (2a, entry 2),
the terminal double bond was preferably epoxidized. The
[2b]/[total epoxide] ratio was 0.61 and the value was much
higher than those reported for Mo(CO)6/CHP (0.14) [5]
and sterically hindered porphyrin systems with NaOCl or
PhIO; Mn(TTPPP)(OAc)/NaOCl (0.35) [9], Mn(T(2ꢀ, 6ꢀ-
G1APh)P)Cl/PhIO (0.20) [3], Mn(TPP)(OAc)/NaOCl (0.03)
[9], and Mn(T(3ꢀ, 5ꢀ-G2Ph)P)Cl/PhIO (0.03) [3] (Table S2).
The epoxidation of R-(+)-limonene (3a, entry 3) easily
proceeded to afford a mixture of diastereoisomeric epoxides
and the [3b]/[total epoxide] ratio was 0.38. The value was
higher than or comparable to those reported for H2O2-based
epoxidation; peroxotungstates (<0.03) [20–23], polyox-
ometalates (<0.01) [15,16], Fe(TDCPN5P)Cl (0.27) [13],
MTO/UHP (<0.01) [18], manganese complexes (0.17) [19],
stoichiometric reactions such as dimethyldioxirane (<0.01)
[4], m-CPBA (0.09) [3], benzonitrile/K2CO3/H2O2 (0.37)
[24], and diisopropylcarbodiimide/K2CO3/H2O2 (0.18–
0.40) [25,26]. Although the regioselectivity for TBA-I is
lower than the values reported for sterically hindered por-
phyrin systems with NaOCl or PhIO (0.62–0.75) [3,11] and
various H2O2-based systems of Mn(TDCPP)Cl/imidazole
(0.59–0.67) [12], [WZnMn2(ZnW9O34)2]12− (0.50) [14],
and Ti-β (0.55) [17], these systems have disadvantages: the
use of rather expensive, nongreen oxidants such as NaOCl
and PhIO; the necessity of excess amounts of oxidant; a long
reaction time; and a high reaction temperature (Table S3).
Similarly, the high selectivity to the epoxidation of the
electron-poor terminal double bond for the epoxidation of
7-methyl-1,6-octadiene (4a, entry 4) was observed. The
[4b]/[total epoxide] ratio was 0.17 and the value was much
higher than those for the H2O2-based epoxidation systems
of Al2O3/H2O2 (<0.01) [32], CF3CH2OH/Na2HPO4/H2O2
(<0.01) [33], and (CF3)2CO/C2F5OH/NaHPO4/H2O2
(<0.01) [34] (Table S4). The epoxidation of 4-vinyl-1-
cyclohexene (5a, entry 5) proceeded exclusively at the
ring position rather than the external position, and the
[5b]/[total epoxide] ratio of 0.04 was lower than those re-
ported for sterically hindered porphyrin systems with NaOCl
or PhIO (0.20–0.73) [9,10] and comparable to dendrimer-
metalloporphyrins (0.04–0.11) [3] (Table S5). The results
of competitive epoxidation of C6-olefins which have similar
alkyl substituents to the corresponding dienes (1a–5a) are
in good agreement with the trends of regioselectivity for I
(Table S6).
Table 2
Initial and relative rates for the epoxidation of a series of C -olefins cat-
alyzed by TBA-I with hydrogen peroxide
6
a
−1
b
Entry
1
Olefin
R
(mM min
)
Relative rate
0
3.08
0.68
13.9
2
3
3.1
3.0
0.68
0.47
4
5
2.1
0.30
0.22
1.4
1.0
6
a
Reaction conditions: TBA-I (8 µmol), olefin (5 mmol), 30% aq hydro-
gen peroxide (1 mmol), acetonitrile (6 mL), reaction temperature (305 K).
R
values were determined from the reaction profiles at low conversion
0
(ꢀ10%) of hydrogen peroxide.
b
The values are ratios of R /R (trans-2-hexene).
0
0
showed the highest regioselectivity to the more accessi-
ble, but less nucleophilic, double bond. In addition, TBA-I
showed higher TONs than those for [PO4{WO(O2)2}4]3−
and [W2O3(O2)4(H2O)2]2− as shown in Fig. 1B. It was
confirmed that the other tungstate fragments such as
[W2O3(O2)4(H2O)2]2− and [SiO4{WO(O2)2}4]4− were not
formed during the catalysis by I, suggesting the stability
of I. Further, the fact that a fully occupied silicodode-
catungstate, [γ -SiW12O40]4−, was inactive for epoxidation
suggests the generation of an active oxidant on a divacant
lacunary site.
Table 2 shows the order of reactivity for epoxidation of
a series of C6-olefins catalyzed by I. The reactivity was
decreased in the order of cis-2-hexene (13.9) > 2-methyl-1-
pentene (3.1) ∼ 2-methyl-2-pentene(3.0) > 2,3-dimethyl-2-
butene (2.1) > 1-hexene (1.4) > trans-2-hexene (1.0). This
order is not consistent with that of the π(C=C) HOMO en-
ergies of C6-olefins,1 but that observed for the epoxidation
catalyzed by TS-1, where restricted transition-state shape se-
lectivity and diffusion effects were observed [43]. Therefore,
the inconsistence of the orders between the π(C=C) HOMO
energy and the reactivity would be caused by the steric con-
straints of the active site of I.
The regioselectivity for the epoxidation of various non-
conjugated dienes in the present system was compared with
those using m-CPBA as a stoichiometric oxidant or H2O2
in the presence of peroxotungstates of [PO4{WO(O2)2}4]3−
and [W2O3(O2)4(H2O)2]2−, which have been reported to be
effective catalysts for the H2O2-based epoxidation [20–23].
The results are shown in Fig. 1A. In the case of m-CPBA,
more electron-rich double bonds are preferably oxidized
in all cases. Among the tungsten catalysts tested, TBA-I
1
The π(C=C) HOMO energies of C -olefins were calculated at
6
the HF/6-311G(d,p) level and decreased in the order of 2,3-dimethyl-
2-butene (–8.67 eV) > 2-methyl-2-pentene (–8.97 eV) > trans-2-hexene
(–9.30 eV) ∼ cis-2-hexene (–9.32 eV) > 2-methyl-1-pentene (–9.44 eV) >
1-hexene (–9.72 eV). The alkyl substitution increases electron density of
the C=C double bond and raises the π(C=C) HOMO energy, resulting in
an increase of the reactivity of the olefin with electrophilic oxidants with-
out steric hindrance such as peroxyacids, peroxides, and peroxo complexes
(see Refs. [35–42]).

K. Kamata et al. / Journal of Catalysis 224 (2004) 224–228
227
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