Epoxidation of alkenes with hydrogen peroxide catalyzed by selenium-containing dinuclear peroxotungstate and kinetic, spectroscopic, and theoretical investigation of the mechanism

Article abstract of DOI:10.1021/ic902381b

The dinuclear peroxotungstate with a SeO₄^2- ligand, (TBA)₂[SeO₄{WO(O₂)₂}₂] (I; TBA=[(n-C₄H_g)₄N]⁺), could act as an efficient homogeneous catalyst for the selective oxidation of various kinds of organic substances such as olefins, alcohols, and amines with H ₂O₂ as the sole oxidant. The turnover frequency (TOF) was as high as 210 ^h-1 for the epoxidation of cyclohexene catalyzed by I with H₂O₂. The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO₄^2- ligand in I played an important role in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and spectroscopic results showed that the present catalytic epoxidation included the following two steps: (I) formation of the subsequent peroxo species [SeW_mO_n] ^o- (II; m=1 and 2) by the reaction of I with an olefin and (ii) regeneration of I by the reaction of Il with H₂O₂. Compound I was the dominant species under steady-state turnover conditions. The reaction rate for the catalytic epoxidation showed a first-order dependence on the concentrations of olefin and I and a zero-order dependence on the concentration of H₂O₂. The rate of the stoichiometric epoxidation with I agreed well with that of the catalytic epoxidation with H₂O₂ by I. All of these kinetic and spectroscopic results indicate that oxygen transfer from I to the C=C double bond is the rate-determining step. The computational studies support that the oxygen-transfer step is the rate-determining step.

Full text of DOI:10.1021/ic902381b

Inorg. Chem. 2010, 49, 2471–2478 2471
DOI: 10.1021/ic902381b
Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing
Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical
Investigation of the Mechanism
†
,‡
†
†
†
†,‡
Keigo Kamata, Ryo Ishimoto, Tomohisa Hirano, Shinjiro Kuzuya, Kazuhiro Uehara, and
,†,‡
Noritaka Mizuno*
†
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
‡
Tokyo 113-8656, Japan, and Core Research for Evolutional Science and Technology (CREST), Japan Science
and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received December 2, 2009
The dinuclear peroxotungstate with a SeO4 - ligand, (TBA) [SeO {WO(O ) } ] (I; TBA=[(n-C H ) N] ), could act
as an efficient homogeneous catalyst for the selective oxidation of various kinds of organic substances such as olefins,
2
þ
2
4
2 2 2
4 9 4
-
1
alcohols, and amines with H O as the sole oxidant. The turnover frequency (TOF) was as high as 210 h for the
2
2
epoxidation of cyclohexene catalyzed by I with H O . The catalyst was easily recovered and reused with maintenance
2
2
2-
of the catalytic performance. The SeO4 ligand in I played an important role in controlling the Lewis acidity of the
peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and
spectroscopic results showed that the present catalytic epoxidation included the following two steps: (i) formation
o-
of the subsequent peroxo species [SeW O ] (II; m=1 and 2) by the reaction of I with an olefin and (ii) regeneration of
m
n
I by the reaction of II with H O . Compound I was the dominant species under steady-state turnover conditions. The
2
2
reaction rate for the catalytic epoxidation showed a first-order dependence on the concentrations of olefin and I and a
zero-order dependence on the concentration of H O . The rate of the stoichiometric epoxidation with I agreed well with
that of the catalytic epoxidation with H O by I. All of these kinetic and spectroscopic results indicate that oxygen
2
2
2
2
transfer from I to the CdC double bond is the rate-determining step. The computational studies support that the
oxygen-transfer step is the rate-determining step.
1
Introduction
oxygen species and coproduction of only water. Over the
past few decades, homogeneous and heterogeneous transi-
2
Selective oxygen transfer to organic substances with H O2
2
3
4
5
6
7
tion-metal catalysts such as Ti, V, Fe, Mn, W, Re, and
as a terminal oxidant is an important reaction in industry and
synthetic chemistry because of its high content of active
8
Pt for H O -based oxidation have been developed. Notably,
the Lewis acid character of the metal catalysts plays an
important role in the activation of H O and/or substrate
2
2
2
2
*
To whom correspondence should be addressed. E-mail: tmizuno@
mail.ecc.u-tokyo.ac.jp. Tel.: þ81-3-5841-7272. Fax: þ81-3-5841-7220.
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r 2010 American Chemical Society
Published on Web 02/02/2010
pubs.acs.org/IC

2
472 Inorganic Chemistry, Vol. 49, No. 5, 2010
Kamata et al.
for the catalytic oxygen-transfer reactions (e.g., epoxidation
and Baeyer-Villiger reaction) and the high activity correlates
9,10
well with the high Lewis acidity of metal centers.
There-
fore, the electronic and steric control of catalytically active
centers is one of the most important points in achieving the
desired chemo-, regio-, diastereo-, and stereoselectivity to-
ward products.
Polyoxometalates are early-transition-metal oxygen anion
clusters that have been applied to various fields such as
structural chemistry, analytical chemistry, surface science,
11
medicine, electrochemistry, photochemistry, and catalysis.
Polyoxometalates have the following advantages as oxida-
tion catalysts: (a) The redox and acid-base properties can be
controlled by changes in the chemical composition, (b) they
are not susceptible to oxidative and thermal degradation in
comparison with organometallic complexes, and (c) the
catalytically active sites can be designed at the atomic and/
or molecular levels. Therefore, many catalytic H O -based
Figure 1. Proposed structure of the anionic part of I.
1
3
lacunary polyoxometalates, and transition-metal-substituted
1
4
polyoxometalates have been developed.
Recently, we have reported the highly efficient epoxidation
of homoallylic and allylic alcohols and oxidation of sulfides
2
2
12
oxidations by polyoxometalates such as peroxometalates,
with H O2 catalyzed by a selenium-containing dinuclear
2
peroxotungstate, (TBA) [SeO {WO(O ) } ] (I; TBA = [(n-
2
4
2 2 2
(
10) Catalysis: An Integrated Approach to Homogeneous, Heterogeneous
þ
15
C H ) N] ; Figure 1). The nature of the heteroatoms in the
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Catalysis; Richards, R., Ed.; Taylor and Francis Group: New York, 2006; p
4
9 4
n-
di- and tetranuclear peroxotungstates with XO
X=Se , S , As , P , Si , etc.) was crucial in controlling
ligands
4
11) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-
VI VI
V
V
IV
(
the Lewis acidity of the peroxotungstates, which significantly
affects their electrophilic oxygen-transfer reactivity. In this
paper, we report full details of the catalytic performance of I
for the H O -based oxidation of various organic substrates,
1
2
2
including olefins, alcohols, and amines, and investigate the
kinetic and mechanistic aspects of the I-catalyzed epoxida-
tion system.
Experimental Section
4
63. (j) Mizuno, N.; Kamata, K.; Uchida, S.; Yamaguchi, K. In Modern
Materials. Acetonitrile (Kanto Chemical) and dichloro-
methane (Kanto Chemical) were purified by The Ultimate
Heterogeneous Oxidation Catalysis; Mizuno, N., Ed.; Wiley-VCH: Weinheim,
Germany, 2009; p 185.
16
Solvent System (Glass Contour Company) prior to use. Sub-
strates were purified according to the reported procedure.
(
12) (a) Ishii, Y.; Yoshida, T.; Yamawaki, K.; Ogawa, M. J. Org. Chem.
17
1
988, 53, 5549. (b) Venturello, C.; D'Aloisio, R.; Bart, J. C. J.; Ricci, M. J. Mol.
Deuterated solvents (CD
chased from Acros and used as received. Tungstic acid (Wako
Chemical), H SeO (Kanto Chemical, 80% aqueous solution),
tetra-n-butylammonium nitrate (Wako Chemical), and H O
2
3 3 2
CN, CDCl , and D O) were pur-
Catal. 1985, 32, 107. (c) Dengel, A. C.; Griffith, W. P.; Parkin, B. C. J. Chem.
Soc., Dalton Trans. 1993, 2683. (d) Duncan, D. C.; Chambers, R. C.; Hecht, E.;
Hill, C. L. J. Am. Chem. Soc. 1995, 117, 681. (e) Kamata, K.; Yamaguchi, K.;
Hikichi, S.; Mizuno, N. Adv. Synth. Catal. 2003, 345, 1193. (f) Kamata, K.;
Yamaguchi, K.; Mizuno, N. Chem.;Eur. J. 2004, 10, 4728. (g) Kamata, K.;
Kuzuya, S.; Uehara, K.; Yamaguchi, S.; Mizuno, N. Inorg. Chem. 2007, 46, 3768.
2
4
2
(Kanto Chemical, 30% aqueous solution) were used as received.
Instruments. IR spectra were measured on a Jasco FT/IR-460
spectrometer Plus using KCl disks. Raman spectra were re-
corded on a Jasco NR-1000 spectrometer with excitation at
(
h) Br ꢀe geault, J.-M.; Vennat, M.; Salles, L.; Piquemal, J.-Y.; Mahha, Y.; Briot, E.;
Bakala, P. C.; Atlamsani, A.; Thouvenot, R. J. Mol. Catal. A: Chem. 2006, 250,
77.
13) (a) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi,
1
5
32.36 nm using a JUNO 100 green laser (Showa Optronics Co.,
(
S.; Mizuno, N. Science 2003, 300, 964. (b) Kamata, K.; Nakagawa, Y.;
Yamaguchi, K.; Mizuno, N. J. Catal. 2004, 224, 224. (c) Kamata, K.; Kotani,
M.; Yamaguchi, K.; Hikichi, S.; Mizuno, N. Chem.;Eur. J. 2007, 13, 639. (d)
Musaev, D. G.; Morokuma, K.; Geletii, Y. V.; Hill, C. L. Inorg. Chem. 2004, 43,
Ltd.). UV-vis spectra were recorded on a Jasco V-570 spectro-
meter. NMR spectra were recorded on a JEOL JNM-EX-270
1
spectrometer ( H, 270.0 MHz; C, 67.80 MHz; Se, 51.30
13
77
1
83
1
W, 11.20 MHz) by using 5 mm tubes (for H and
MHz;
7
2
702. (e) Prabhakar, R.; Morokuma, K.; Hill, C. L.; Musaev, D. G. Inorg. Chem.
006, 45, 5703. (f) Sartorel, A.; Carraro, M.; Bagno, A.; Scorrano, G.; Bonchio,
13
77
183
C) or 10 mm tubes (for Se and W). Chemical shifts (δ)
were reported in ppm downfield from SiMe
for H and C NMR spectra, Me
NMR spectra, and 2 M Na WO
4
(solvent, CD7C7l
3
)
M. Angew. Chem., Int. Ed. 2007, 46, 3255. (g) Carraro, M.; Sandei, L.; Sartorel,
A.; Scorrano, G.; Bonchio, M. Org. Lett. 2006, 8, 3671. (h) Phan, T. D.; Kinch,
M. A.; Barker, J. E.; Ren, T. Tetrahedron Lett. 2005, 46, 397. (i) Ishimoto, R.;
Kamata, K.; Mizuno, N. Angew. Chem., Int. Ed. 2009, 48, 8900.
1
13
2
Se (solvent, DMF-1d83) for Se
O) for W NMR
7
2
4
(solvent, D
2
spectra. Gas chromatography (GC) analyses were performed on
a Shimadzu GC-2014 gas chromatograph with a flame ioniza-
tion detector equipped with an InertCap 5 capillary column
(
14) (a) Neumann, R.; Gara, M. J. Am. Chem. Soc. 1995, 117, 5066. (b)
B €o sing, M.; N €o h, A.; Loose, I.; Krebs, B. J. Am. Chem. Soc. 1998, 120, 7252. (c)
Ritorto, M. D.; Anderson, T. M.; Neiwert, W. A.; Hill, C. L. Inorg. Chem. 2004,
(
internal diameter=0.25 mm; length=60 m) and a Shimadzu
4
2
3, 44. (d) Sloboda-Rozner, D.; Alsters, P. L.; Neumann, R. J. Am. Chem. Soc.
003, 125, 5280. (e) Sloboda-Rozner, D.; Witte, P.; Alsters, P. L.; Neumann, R.
Adv. Synth. Catal. 2004, 346, 339. (f) Zhang, X.; Chen, Q.; Duncan, D. C.;
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Mizuno, N.; Nozaki, C.; Kiyoto, I.; Misono, M. J. Am. Chem. Soc. 1998, 120,
(
15) (a) Kamata, K.; Hirano, T.; Kuzuya, S.; Mizuno, N. J. Am. Chem.
Soc. 2009, 131, 6997. (b) Kamata, K.; Hirano, T.; Mizuno, N. Chem. Commun.
2
009, 3958.
(16) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
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9
3
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722.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2473
GC-17A gas chromatograph with a flame ionization detector
equipped with a InertCap Pure-WAX capillary column (internal
diameter=0.25 mm; length=30 m).
All calculations were performed with the Gaussian03 program
package.
2
3
Synthesis and Characterization of [(n-C
4
H
9
)
4
N]
O ) } ] (I). The TBA salt derivative of [SeO {WO(O ) } ]
2
[SeO
4
{WO-
Results and Discussion
2
-
(
2
2 2
4
2 2 2
1
5
Catalytic Oxidation with H O by I. The catalytic
2
2
was synthesized according to the reported procedure. Full
details of the synthesis and characterization of I are shown in the
Supporting Information (Figures S1-S5).
Procedure for Catalytic Oxidation. Catalytic oxidation of
various organic substrates was carried out in a 30-mL glass
vessel containing a magnetic stir bar. All products were identi-
fied by a comparison of the GC retention time, mass spectra, and
NMR spectra with those of the authentic samples. A typical
procedure for catalytic oxidation was as follows: 1a (5 mmol),
activity of I for epoxidation of cyclohexene (1a) was
compared with those of the Se and W catalysts (Table 1).
Among the catalysts tested, I showed the highest yield of
1,2-epoxycyclohexane (1b): 88% yield, 97% selectivity
to 1b, and g99% efficiency of H O utilization (Table 1,
2
2
entry 1). In this case, the turnover frequency (TOF) was as
-
1 24-26
high as 210 h
also proceeded efficiently and gave 1b in 84% yield
Table 1, entry 2). Oxidation did not proceed in the absence
of the catalyst (Table 1, entry 3). The catalyst precursors of
H WO , H SeO , and a mixture of H WO and H SeO
.
Epoxidation at ambient temperature
2 2
acetonitrile (6 mL), and 30% aqueous H O (1 mmol) were
charged in the reaction vessel. The reaction was initiated by the
addition of I (10 μmol), and the reaction solution was periodi-
(
cally analyzed. H
2
O
2
remaining after the reaction was analyzed
titration. The products (10b, 11b, and 14b ) are
2
4
2
4
2
4
2
4
3þ/4þ
18
19
19
20
were almost inactive for epoxidation (Table 1, entries 4-6).
Selenium oxide, which has been applied to various oxida-
tions with H O , also showed low catalytic activity
by Ce
known and identified by a comparison of their H and C NMR
1
13
1
signals with the literature data. 10b: H NMR (270 MHz,
2
2
CDCl
3
, 298 K, TMS) δ 6.68 (t, J=5.9 Hz, 1H), 3.76 (t, J=7.0
and selectivity to 1b under the present conditions
2
7
Hz, 2H), 2.48 (dt, J=5.9 and 6.8 Hz, 2H), 1.92-1.86 (m, 2H),
(
Table 1, entry 7). The reaction rate of I was 20 times
larger than that of the simple dinuclear peroxotungstate
(TBA) [{WO(O (μ-O)] (W ), suggesting that the di-
meric peroxotungstate unit is strongly activated by connec-
1
0
.60-1.52 (m, 2H), 1.40-1.32 (m, 2H), 0.98 (t, J=7.3 Hz, 3H),
13 1
.96 (t, J=7.6 Hz, 3H); C{ H} NMR (67.5 MHz, CDCl , 298
3
1
2
)
2
2
}
2
2
K, TMS) δ 139.5, 65.2, 29.5, 28.6, 19.7, 19.1, 14.0, 13.6. 11b: H
NMR (270 MHz, CDCl , 298 K, TMS) δ 7.76 (s, 1H), 7.30-7.10
m, 4H), 4.06 (dt, J=0.8 and 7.6 Hz, 2H), 3.18 (t, J=7.3 Hz, 2H);
3
2-
tion with the SeO4 ligand (Table 1, entry 8). Epoxidation
was carried out in the presence of various peroxotungstates
(
1
3
1
C{ H} NMR (67.5 MHz, CDCl , 298 K, TMS) δ 134.2, 120.1,
3
1
29.4, 128.4, 127.7, 127.3, 125.5, 58.0, 27.8. 14b: H NMR (270
1
MHz, CDCl
with assembling ligands, (THA) [XO
3
4
{WO(O
2
)
2
}
4
] (THA
þ
3
, 298 K, TMS) δ 3.79-3.64 (m, 2H), 3.61-3.45 (m,
= [(n-C H ) N] ) [AsW , X = As; PW , X = P],
6
13 4
4
4
2
3
7
H), 1.90-1.77 (m, 3H) 1.62-1.44 (m, 1H), 1.04 (d, J=6.2 Hz,
(TBA) [HXO {WO(O ) } ] [AsW , X=As; PW , X=P],
2 4 2 2 2 2 2
1
3
1
H); C{ H} NMR (67.5 MHz, CD
0.5, 68.5, 28.3, 26.8, 19.3.
3
CN, 298 K, TMS) δ 84.4,
(TBA) [SO {WO(O ) } ] (SW ), and (TBA) [Ph SiO -
2 4 2 2 2 2 2 2 2
{WO(O ) } ] (SiW ). The reactivities were strongly depen-
2 2 2 2
dent on the kinds of heteroatoms and the number of
Recovery of Catalyst and Recycle Experiments. The reaction
conditions for the first run were as follows: I (10 μmol), 3a
5 mmol), 30% aqueous H O (1 mmol), CH CN (6 mL), 298 K.
(
2
2
3
(24) (a) Bruijnincx, P. C. A.; Buurmans, I. L. C.; Gosiewska, S.;
The corresponding epoxide 3b was obtained in 99% yield for
00 min. After epoxidation of 3a was completed, acetonitrile
Moelands, M. A. H.; Lutz, M.; Spek, A. L.; Koten, G.; Gebbink, R. J. M.
K. Chem.;Eur. J. 2008, 14, 1228. (b) Koo, D. H.; Kim, M.; Chang, S. Org.
Lett. 2005, 7, 5015. (c) van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon, R. A.
Chem. Commun. 1999, 263. (d) Srinivas, K. A.; Kumar, A.; Chauhan, S. M. S.
Chem. Commun. 2002, 2456. (e) Rudolph, J.; Reddy, K. L.; Chiang, J. P.;
Sharpless, K. B. J. Am. Chem. Soc. 1997, 119, 6189.
1
was removed by evaporation, followed by the addition of diethyl
ether (20 mL). The precipitated catalyst was collected (98%
recovery), washed with diethyl ether (ca. 20 mL ꢀ 2), and dried
in vacuo prior to being recycled. Epoxidation in the presence of
the recovered I proceeded with almost the same yield and
selectivity as those observed for the first run, and 3b was
obtained in 96% yield.
(25) (a) Berardi, S.; Bonchio, M.; Carraro, M.; Conte, V.; Sartorel, A.;
Scorrano, G. J. Org. Chem. 2007, 72, 8954. (b) Nam, W.; Oh, S.-Y.; Sun, Y. J.;
Kim, J.; Kim, W.-K.; Woo, S. K.; Shin, W. J. Org. Chem. 2003, 68, 7903. (c)
Maiti, S. K.; Malik, K. M. A.; Gupta, S.; Chakraborty, S.; Ganguli, A. K.;
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J. M.; Olmstead, M.; Mascharak, P. K. Inorg. Chem. 2001, 40, 2810. (e) van Vliet,
M. C. A.; Arends, I. W. C. E.; Sheldon, R. A. Chem. Commun. 1999, 821. (f)
Srinivas, K. A.; Kumar, A.; Chauhan, S. M. S. Chem. Commun. 2002, 2456. (g)
Brinksma, J.; Hage, R.; Kerschnerc, J.; Feringa, B. L. Chem. Commun. 2000,
Quantum Chemical Calculations. The calculations were car-
2
1
ried out at the B3LYP level of theory with 6-31þG(d,p) basis
sets for H, C, and O atoms and the double-ξ quality basis sets
2
2
with effective core potentials proposed by Hay and Wadt for
Se and W atoms. The entire structure of peroxotungstate was
used as a model in the calculations, and the overall charge of the
system was 2-. Ethylene was used as a model substrate. All of
the geometries were optimized without symmetry restrictions.
Transition-state structures were searched by numerically esti-
mating the matrix of second-order energy derivatives at every
optimization step and by requiring exactly one eigenvalue of
this matrix to be negative. The optimized geometries are shown
in Table S1 and Figures 5 and S6 in the Supporting Information.
The zero-point vibrational energies were not included.
5
37.
26) The TOF was higher than those reported for the H
systems for the epoxidation of cyclohexene (Table S2 in the Supporting
(
2 2
O -based catalytic
24,25
II
Information):
zol-2-yl)-propionate, 4 h ],
imidazole [TDCPP
78
[Fe (PrL1) ](OTf) [PrL1=propyl 3,3-bis(1-methylimida-
2 2
-
1 24a
-1 24b
WO /MCM-48 (2 h ),
3
Mn(TDCPP)Cl/
0
0
=
5,10,15,20-tetrakis(2 ,6 -dichlorophenyl)porphyrin,
-1
5a
III
III
14-
-1 14g
h
],
RRβR-(NaOH
2
)(Fe OH
2
)Fe
2
(P
2
W
15 56
O )
2
]
(2
h ),
1
7-
-1 14c
[
((MnOH
2
)Mn
2
PW
9
O
34
)
2
(PW
6
O
26)]
(8 h ),
perfluoroheptadecan-9-
-1
24c
-1 24d
-1 24e
3 3
one (48 h ), Ta-SBA-15 (41 h ), and CH ReO /pyridine (32 h ).
While the TOFs of [γ-SiW10
meso-tetrakis(pentafluorophenyl)porphinate dianion, 2160 h ),
4
-
-1 25a
O
36(PhPO)
2
]
(1200 h ), Fe(tpfpp)Cl (tpfpp=
-1 25b
[MoO-
-
1 25c
(O
2
)(BOTHA)
Fe(PaPy )(CH
ethyl-2-pyridine-2-carboxamide, 276 h ),
2
]
(BOTHA = N-benzoylhydroxamic acid, 4550
h ),
[
3
3
CN)](ClO (PaPy = N,N-bis(2-pyridylmethyl)amine-N-
4
)
2
3
(
18) Vogel, A. I. A Textbook of Quantitative Inorganic Analysis Including
Elementary Instrumental Analysis; Longman: New York, 1978.
19) Murahashi, S.-I.; Mitsui, H.; Shiota, T.; Tsuda, T.; Watanabe, S.
J. Org. Chem. 1990, 55, 1736.
-1 25d
-1 25e
MTO (500 h ),
[Cl
4 8 4
S Fe ] (Cl TPPS =tetrakis(2 ,6 -dichloro-3 -sulfonatophenyl)porphyrinato,
1 25f
8
TPP-
III
0
0
0
(
-
0 0
2 2
O(OAc) (TPTN) (TPTN=N,N,N ,N -tetrakis(2-pyridyl-
1 25g
4
20 h ), and Mn
-
methyl)propane-1,3-diamine, 217 h
)
were higher than that of I, the use of
(
(
(
(
20) Paolucci, C.; Mazzini, C.; Fava, A. J. Org. Chem. 1995, 60, 169.
21) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
2 2
excess additives and H O , specific solvents and ligands, and microwave is
required.
22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
23) Frisch, M. J. et al. Gaussian 03, revision D.02; Gaussian, Inc.:
(27) Mzochowski, J.; Brz a- szcz, M.; Giurg, M.; Palus, J. H.; W oꢀ jtowicz
Eur. J. Org. Chem. 2003, 4329.
Wallingford, CT, 2004.

2
474 Inorganic Chemistry, Vol. 49, No. 5, 2010
Kamata et al.
a
2 2
Table 1. Effect of Catalysts and Solvents on the Epoxidation of Cyclohexene (1a) with 30% Aqueous H O
selectivity (%)
2 2
H O efficiency (%)
entry
1
catalyst
solvent
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
1,2-dichloroethane
n-butyronitrile
chloroform
dichloromethane
ethyl acetate
dimethoxyethane
dimethylformamide
toluene
yield (%)
1b
97
1c
1d
1e
I
88
90
<1
8
<1
<1
5
11
35
31
18
10
4
<1
<1
<1
1
3
6
>99
99
b
2
I
without
H
H
H
SeO
W
I
AsW
SW
PW
AsW
PW
SiW
93
3
c
4
2
2
2
WO
SeO
SeO
4
4
4
99
<1
<1
82
99
98
99
99
99
99
99
99
99
89
83
78
93
77
75
94
99
99
<1
<1
<1
17
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
3
<1
<1
<1
<1
<1
<1
<1
<1
<1
99
99
75
d
5
>99
16
99
>99
96
99
92
99
>99
>99
>99
90
28
81
76
90
82
80
96
21
c d
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
,
þ H
2
WO
4
e
2
<1
<1
2
<1
<1
<1
<1
<1
<1
1
4
17
22
7
23
25
6
2
<1
<1
<1
<1
<1
<1
<1
<1
<1
4
<1
<1
<1
<1
<1
<1
<1
<1
f
f
f
f
f
f
f
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
4
2
4
2
2
2
1
2
(TBA)
(TBA)
4
[γ-SiW10
O
34(H
2
O)
]
2
]
60
16
64
62
52
36
33
19
1
g
4
[γ-H SiV
2
2
W
10
O
40
I
I
I
I
I
I
I
I
<1
<1
1
>99
a
2 2 2 2
Reaction conditions: catalyst (W: 2 mol % with respect to H O ), 1a (5 mmol), 30% aqueous H O (1 mmol), solvent (6 mL), 323 K, 40 min. Yield
c d
b
(
%) = products (mol)/H
O
2 2
used (mol) ꢀ 100. 305 K, 120 min. H WO
2
4 2 2 2 4 2 2
(1 mol % with respect to H O ). H SeO (1 mol % with respect to H O ).
e
f
SeO (1 mol % with respect to H O ). 10 min. Acetonitrile/tert-butyl alcohol (3 mL/3 mL).
g
2
2
2
W atoms, respectively. The yields of 1b decreased in the
order of I (35%) > AsW (31%) > SW (18%) > PW
4
acetonitrile was the most effective (Table 1, entries 1 and
18-25). No formation of acetamide was observed, showing
that epoxidation does not proceed via a peroxy-
carboximidic acid intermediate generated by the reaction
4
2
(
10%) > AsW (4%) > PW (2%) > SiW (1%) (Table 1,
2 2 2
entries 9-15). Such a reactivity order was also observed for
stoichiometric epoxidation with the peroxotungstates.
Therefore, the nature of the heteroatoms in the peroxotung-
states is crucial in controlling the Lewis acidity of the
peroxotungstates, which significantly affects their electro-
30
of acetonitrile with H O . Biphasic 1,2-dichloroethane,
2
2
chloroform, dichloromethane, and ethyl acetate systems
and a homogeneous n-butyronitrile system showed mode-
rate activities, while the selectivity to 1b and efficiency of
H O utilization were lower than those of the acetonitrile
1
5,28,29
philic oxygen-transfer reactivity.
ciencies of H O utilization of divacant polyoxotungstate
(
The yields and effi-
2
2
system (Table 1, entries 1 and 18-22). Toluene, in which I
was not soluble, and N,N-dimethylformamide were poor
solvents (Table 1, entries 24 and 25).
2
2
13a
TBA) [γ-SiW O (H O) ] and divanadium-substituted
4 10 34 2 2
3b
polyoxotungstate (TBA) [γ-H SiV W O ] were lower
4
2
2
10 40
than those of I under the present conditions (Table 1, entries
16 and 17). Epoxidation of 1a with H O catalyzed by I was
carried out in various solvents. Among the solvents tested,
Table 2 shows the results of oxidation of various kinds
of organic substrates with 30% aqueous H O catalyzed
2
2
2
2
by I. Epoxidation proceeded selectively to form the
corresponding epoxides without the significant formation
of allylic oxidation products and the hydrolysis, cleavage,
and rearrangement of epoxides. Cyclic and internal ole-
fins 1a-8a were oxidized to the corresponding epoxides
1b-8b in 87-99% yield with 96-99% selectivity and
91-99% efficiency of H O utilization (Table 2, entries 1,
0
(
28) For catalytic epoxidation by d transition-metal catalysts, the oxida-
tion states of the metal centers are not changed during catalysis. The metal
centers function as Lewis acids by withdrawing electrons from the O-O
bond and thus increasing the electrophilic character of the coordinated
29a-c
peroxide. Active catalysts are the metal centers with strong Lewis acidity.
The pK values of the ligands can be recognized as an index of the Lewis acidity
of transition-metal complexes. The activity (yield) order of the dinuclear
2
2
a
2, 4, 8, and 10-13). For epoxidation of cis- and trans-
29d
n-
octenes 2a, 7a, and 8a, the configuration around the CdC
moiety was retained in the corresponding epoxides
(Table 2, entries 2, 3, 12, and 13). The trans/cis ratio
peroxotungstates with XO
that of the pK values of H
has an important role for epoxidation.
29) (a) Thiel, W. R.; Eppinger, J. Chem.;Eur. J. 1997, 3, 696. (b) Arends,
4
ligands (X=Se, S, P, As, and Si) was the same as
a
n
XO , suggesting that the Lewis acidity of W atoms
4
(
(64:36) for the I-catalyzed epoxidation of 6a was similar
I. W. C. E.; Sheldon, R. A. Top. Catal. 2002, 19, 133. (c) Oyama, S. T. In
Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis;
Oyama, S. T., Ed.; Elsevier: Amsterdam, The Netherlands, 2008; pp 1-99. (d)
Nabavizadeh, S. M.; Rashidi, M. J. Am. Chem. Soc. 2006, 128, 351.
(30) Payne, G. B.; Deming, P. H.; Williams, P. J. Org. Chem. 1961, 26,
651.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2475
a
2 2
Table 2. Oxidation of Various Substrates with 30% Aqueous H O Catalyzed by I
a
Reaction conditions: I (1 mol % with respect to H
2
O
2
), substrate (5 mmol), 30% aqueous H
2 2
O (1 mmol), acetonitrile (6 mL), 323 K. Yields were
1
determined by GC and H NMR. Yield (%) = product (mol)/H
determined from the reaction profiles at low conversion (e10%) of H
-1
2 2
O
used (mol) ꢀ 100. The values in parentheses were TOF (h ). The TOF values were
b
c
d
e
2
2
O . Substrate (1 mmol). 313 K. 298 K. Recovered I (1 mol % with respect to
h
f
; see the Supporting Information). CH
Substrate (1 mmol), 305 K.
g
H
2
O
2
3 2 2 2 2
CN (9 mL). I (5 mol % with respect to H O ). Substrate (1 mmol), 30% aqueous H O (2 mmol), 305 K.
i
1
3c
to those for W and PW (Table 2, entry 11). High
2
and 18), and the selectivities to the corresponding alde-
hyde and ketone were g95%. Oxidation of bis(homo-
allylic alcohol) 14a gave the tetrahydrofuran derivative
in 91% yield, while homoallylic and allylic alcohols
were epoxidized selectively in the present system (Table 2,
entry 19). This is probably due to rearrangement of the
epoxide intermediate into the tetrahydrofuran derivative.
Such an isomerization was also observed in the m-chloro-
perbenzoic acid, methyltrioxorhenium/H O , and titano-
4
yields and selectivities to the corresponding epoxides were
also observed for epoxidation of 2a, 3a, and 4a even with 1
equiv of H O (I:substrate:H O = 1:100:100; Table 2,
entries 3, 5, and 9). Epoxidation proceeded at ambient
temperature (Table 2, entry 6). Nonactivated terminal olefin
2
2
2
2
9
a was converted to 9b in 73% yield (Table 2, entry 14).
Secondary amines such as dibutylamine (10a) and tetra-
hydroisoquinoline (11a) were oxidized to the correspond-
ing nitrones in good yield by using stoichiometric amounts
of H O with respect to the substrates (Table 2, entries 15
2
2
31
silicate/H O systems.
2
2
2
2
and 16). The present system showed moderate activity
for oxidation of alcohols 12a and 13a (Table 2, entries 17
(31) Hartung, J.; Greb, M. J. Organomet. Chem. 2002, 661, 67.

2
476 Inorganic Chemistry, Vol. 49, No. 5, 2010
Kamata et al.
7
7
Figure 3. Se NMR spectra of (a) I, (b) I after 1 equiv of 3b with respect
to I was formed, and (c) sample b with the addition of 30% H
2 2
O
3
(380 mM). Reaction conditions: I (38 mM), 3a (380 mM), CD CN
(
2.5 mL), 273 K.
as phosphorus-containing di-, tri-, and tetranuclear per-
oxotungstates, [PW O ] (x = 2-4), are also observed
z-
Figure 2. Reaction profiles for stoichiometric epoxidation of 3a with I.
a) Reaction conditions: I (38 mM), 3a (380 mM), CH CN (2.5 mL),
73 K. (b) After 20 min (i.e., 1 equiv of 3b with respect to I was formed),
0% aqueous H (380 mM) was added, as indicated by an arrow.
x
y
(
2
3
3
for stoichiometric epoxidation of 9a with PW , and PW
4
4
is regenerated by reaction of the subsequent peroxo
species with excess amounts of H O .
2 2
O
1
2d
The signals at
1071, 1053, and 1019 ppm are probably assignable to
2
2
Reaction Mechanism. The stability of I during catalytic
7
7
o-
epoxidation was investigated. The Se NMR spectra of I
both during and after the epoxidation of 3a showed a
signal at 1046 ppm, which was observed for the as-
synthesized I (Figure S7 in the Supporting Information).
In addition, I could easily be recovered in 98% yield by
the addition of an excess amount of diethyl ether
subsequent peroxo species (II) such as [SeW O ] (m=1
n
m
and 2) formed by the reaction of I with an olefin.
Compound I is regenerated by the reaction of II with
H O , and epoxidation proceeds catalytically. The ratio
3
2
2
2
of the reaction rate of II to that of I was estimated to be
0.07 ( 0.01 from the reaction profile (Figure 2a). As was
previously mentioned, I was the dominant species under
steady-state turnover conditions (Figure S7 in the Sup-
porting Information). The kinetics for epoxidation of 3a
with H O catalyzed by I were investigated. The first-
(
precipitation method) to the reaction solution. The
recovered I could be reused without loss of its catalytic
activity and selectivity (Table 2, entries 6 and 7). These
facts show that I is stable under the reaction conditions.
Stoichiometric epoxidation of 3a (1 mmol) with I
2
2
order dependence of the reaction rates on the concentra-
tions of I (0.29-2.29 mM) and 3a (0.07-0.86 M) and the
zero-order dependence of the reaction rate on the con-
centration of H O (0.04-0.36 M) were observed
(
100 μmol) at 273 K produced 198 ( 4 μmol of 3b
Figure 2a), suggesting that I has 2 equiv of active oxygen
(
for epoxidation. Such a stoichiometry is also observed for
2
2
stoichiometric epoxidation of (R)-(þ)-limonene with the
(Figures 4a-c and S8 in the Supporting Information).
The reaction rate for epoxidation of cis-2-hexene at 313 K
with 30% H O under the conditions in Table 2 was 0.46
n-
di- and tetranuclear peroxotungstates with XO
(
ligands
X=S , As , P , etc.). After 1 equiv of 3b with respect
4
VI
V
V
12h
2
2
-1
7
7
to I was formed, several Se NMR signals at 1071, 1053,
046, and 1019 ppm with an intensity ratio of 2:5:2:1 were
observed, respectively (Figure 3b). Upon the addition of
( 0.04 mM min and almost the same as that (0.43 ( 0.02
-1
1
mM min ) with 96% H O , suggesting that the reaction
2 2
rates are not dependent on the concentration of water. All
of these results suggest that I is the active species for
H O (10 equiv with respect to I), these signals at 1071,
2
2
1
1
053, and 1019 ppm almost disappeared and a signal at
046 ppm assignable to I appeared again (Figure 3c).
(32) After 2 equiv of 3b with respect to I was formed, 10 equiv of H
2 2
O
with respect to I was added to the reaction solution. The catalytic epoxida-
In this case, catalytic epoxidation of 3a proceeded at
-1
-
1
tion rate (0.18 ( 0.01 mM min ) was much smaller than that (3.72 ( 0.10
-1 77
almost the same rate (3.67 ( 0.20 mM min ) as that
mM min ) under the stoichiometric conditions, and a Se NMR signal at
046 ppm was not observed. All of these kinetic and NMR results suggest
that epoxidation with II leads to irreversible catalyst inactivation.
-
1
(
tions (Figure 2b). Several subsequent peroxo species such
3.72 ( 0.10 mM min ) under the stoichiometric condi-
1

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2477
2 2 2 2
Figure 4. Dependence of the reaction rates on the concentrations of (a) I, (b) 3a, and (c) H O and (d) Arrhenius plots for epoxidation of 1a with H O
2
catalyzedbyI. Reaction conditions for part a: I (0.29-2.29 mM), 3a (0.21 M), H
.99). Reaction conditions for part b: I (0.57 mM), 3a (0.07-0.86 M), H (0.09 M), H
Reaction conditions for part c: I (0.57 mM), 3a (0.21 M), H (0.04-0.36 M), H O (0.38 M), acetonitrile (7 mL), 305 K. Reaction conditions for part d: I
0.57 mM), 3a (0.21 M), H (0.09 M), H
O
2 2
(0.09 M), H
2
O (0.38 M), acetonitrile (7 mL), 305 K. Slope = 0.91 (R =
2
0
O
2 2
2
O (0.38 M), acetonitrile (7 mL), 305 K. Slope = 1.10 (R = 0.99).
2
O
2
2
2
(
O
2 2
2
O (0.38 M), acetonitrile (7 mL), 295-330 K. Line fit: ln k = -4794/T þ 11.96 (R = 0.98). R
0
values were
determined from the reaction profiles at low conversions (<10%) of both 3a and H O . Reaction profiles are shown in Figure S8 in the Supporting
2 2
Information.
epoxidation and that the reaction of an olefin with I is the
rate-determining step.
Scheme 1. Proposed Mechanism of the I-Catalyzed Epoxidation of
Olefin with H O
2 2
On the basis of these results, the present epoxidation of
olefins possibly proceeds as follows (Scheme 1): First, the
olefin directly attacks the peroxo oxygen groups to form the
corresponding epoxide and subsequent peroxo species II
(
with H O (step 3). The dependence of the reaction rate on
steps 1 and 2). Then, I is regenerated by the reaction of II
2
2
the reaction temperature (Arrhenius plots, 295-330 K)
was investigated (Figure 4d). The good linearity of the
Arrhenius plots was observed to afford the following
-
1
activation parameters: E = 39.8 kJ mol , ln A = 12.0,
a
q
-1
q
=-153.6 J mol-1
K ,
-1
ΔH
=37.4 kJ mol , ΔS
298 K
298 K
(
2 2
33) The kinetics for the epoxidation of 1a with H O catalyzed by I also
showed the first-order dependence of the reaction rates on the concentrations
of I (0.29-2.29 mM) and 1a (0.07-0.86 M) and the zero-order dependence of
2 2
the reaction rate on the concentration of H O (0.04-0.36 M) (Figure S9a-c
in the Supporting Information). In addition, the rate of the stoichiometric
epoxidation of 1a (160 mM) with I (8 mM) at 273 K was 0.31 ( 0.02 mM
-
1
-1
min and agreed well with that (0.32 ( 0.01 mM min ) of the catalytic
epoxidation of 1a (160 mM) with H (160 mM) at 273 K by I (8 mM). The
following activation parameters were afforded for the catalytic epoxidation
2 2
O
q
-1 33
and ΔG
energy is lower than those reported for tungsten-catalyzed
= 83.1 kJ mol
98 K
.
The present activation
2
-1
a
of 1a (Figure S9d in the Supporting Information): E =40.2 kJ mol , ln
q
-1
q
-1
-1
A=12.1, ΔH 298 K =37.7 kJ mol , ΔS 298 K=-152.8 J mol
K
, and
-1
q
-1
epoxidation (e.g., 86.4 kJ mol for Na WO , 54.2 kJ
2
4
ΔG 298 K=83.2 kJ mol . The kinetics and activation parameters with 1a
were almost the same as those with 3a, suggesting that the mechanism of
epoxidation of 1a is the same as that of 3a.
-
1
-1
mol for PW , and 55.9-68.2 kJ mol for K [{WO(O ) -
4
2
2 2
(H O)} (μ-O)] 2H O), indicating the high reactivity of I
2
2
3
2

2
478 Inorganic Chemistry, Vol. 49, No. 5, 2010
Kamata et al.
The calculated activation barriers of an ethylene attack at
O2 (TS1, 99 kJ mol ) was the lowest among those of
-
1
-
1
ethylene attacks at O1 (TS2, 146 kJ mol ), O3 (TS3,
-
1
04 kJ mol ), and O4 (TS4, 106 kJ mol ), suggesting that
-1
1
an olefin attack at O2 would be the most favorable. The
energies of the reaction steps were calculated according
to Scheme 1, and the results are summarized in Figure 5.
The formation of a subsequent peroxo species (B) and
ethylene oxide by the reaction of ethylene with the selenium-
containing dinuclear peroxotungstate (A) was calculated to
-
1
be exothermic by -130 kJ mol . The activation energy
for the oxygen-transfer reaction (TS1) was calculated to be
-
1
9 kJ mol . The H O molecule reacted with B to form A
9
and water, establishing the catalytic cycle. This step was
2 2
-
1
calculated to be exothermic by -75 kJ mol . Computational
studies support that the oxygen-transfer step is the rate-
determining step.
Figure 5. Calculated energy diagram of the epoxidation of ethylene to
ethylene oxide by I in the gas phase (energies and lengths are in kJ mol
and A, respectively). Orange, gray, red, black, and light-blue balls
represent Se, W, O, C, and H atoms, respectively.
-1
˚
Conclusion
2-
The dinuclear peroxotungstate with the SeO4 ligand, I,
1
2f,34
was synthesized and characterized. Compound I showed high
catalytic activity for the selective oxidation of various kinds
of organic substances such as olefins, alcohols, and amines
for epoxidation.
The negative value of the activation
suggests that a bimolecular transition
98 K
q
entropy ΔS
2
state (electrophilic attack of a peroxo O atom at the CdC
bond) is included in the rate-determining step.
33,35
with H O as the sole oxidant. On the basis of the kinetic,
2
2
mechanistic, and spectroscopic investigation, it was found
that I is the active species in the present epoxidation and that
oxygen transfer from I to the CdC double bond is the rate-
determining step.
In order to investigate the proposed reaction mecha-
nism, the density functional theory calculations were
carried out at the B3LYP/LanL2DZþ6-31þG(d,p) level.
First, the reactivities of four kinds of peroxo O atoms (O1,
O2, O3, and O4) in I for epoxidation were investigated.
The transition-state structures and activation barriers for
the epoxidation of ethylene with I were calculated on the
assumption that the olefin double bond directly attacks the
peroxo oxygen groups in a spiro fashion (i.e., the CCO plane
is almost orthogonal to the WOO plane). The transition-
state structures and the corresponding activation barriers
are shown in Figure S6 in the Supporting Information.
Acknowledgment. This work was supported by the
CREST program of JST, the Global COE Program
(Chemistry Innovation through Cooperation of Science
and Engineering), the Development in a New Interdisci-
plinary Field Based on Nanotechnology and Materials
Science Programs, and a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture,
Science, Sports, and Technology of Japan.
(
34) (a) Allan, G. G.; Neogi, A. N. J. Catal. 1976, 16, 197. (b) Jian, X.; Hay,
A. S. J. Polym. Sci., Part A: Polym. Chem. 1991, 29, 547.
35) Connors, K. A. Chemical Kinetics, The Study of Reaction Rates in
Solution; VCH: New York, 1990.
Supporting Information Available: Experimental details,
Tables S1 and S2, and Figures S1-S10. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(