A Na2WO4/H2WO4-based highly efficient biphasic catalyst towards alkene epoxidation, using dihydrogen peroxide as oxidant

Article abstract of DOI:10.1002/adsc.200505133

The tungsten-containing biphasic catalytic system [Na₂WO ₄/H₂WO₄/PTR/chloroacetic acid] effectively epoxidizes alkenes with 50% H₂O₂ as terminal oxidant, under organic solvent-free conditions. The catalytic process is proposed to proceed via a dinuclear tungsten peroxo species with coordinated chloroacetic acid, as suggested by ESI-MS measurements. The catalytic system is suggested to involve tungsten-peroxo and/or peracetic acid type of epoxidation catalyzed by the tungsten(VI) in the presence of an organic acid and H₂O ₂. The reaction conditions employed for various alkenes for epoxidation are mild compared to the earlier studies and result in high product selectivity and conversion rate.

Full text of DOI:10.1002/adsc.200505133

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DOI: 10.1002/adsc.200505133
A Na2WO4/H2WO4-Based Highly EfficientBiphasic Catalyst
towards Alkene Epoxidation, using Dihydrogen Peroxide as
Oxidant
Palanisamy Uma Maheswari,a Paul de Hoog,a Ronald Hage,b Patrick Gamez,a
Jan Reedijka,*
a
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA, Leiden,
The Netherlands
Fax: (þ31)-71-527-4671, e-mail:reedijk@chem.leidenuniv.nl
b
Unilever R& D Vlaardingen, Olivier van Noortlaan 120, 3133 AT, Vlaardingen, The Netherlands
Received: March 30, 2005; Accepted: June 24, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The tungsten-containing biphasic catalytic
system [Na2WO4/H2WO4/PTR/chloroacetic acid] ef-
fectively epoxidizes alkenes with 50% H2O2 as termi-
nal oxidant, under organic solvent-free conditions.
The catalytic process is proposed to proceed via a di-
nuclear tungsten peroxo species with coordinated
chloroacetic acid, as suggested by ESI-MS measure-
ments. The catalytic system is suggested to involve
tungsten-peroxo and/or peracetic acid type of epoxi-
dation catalyzed by the tungsten(VI) in the presence
of an organic acid and H2O2. The reaction conditions
employed for various alkenes for epoxidation are
mild compared to the earlier studies and result in
high product selectivity and conversion rate.
procedure is less desirable, as it requires the use of
chlorinated solvents such as 1,2-dichloroethane.[15–17]
A first breakthrough occurred in 1996 when Noyori[18]
reported a significant improvement of the original Ven-
turello system. The epoxidation of terminal alkenes
could be carried out within 4 hours, without organic sol-
vents, and with up to 49.5 TON (99% yield, based on the
olefin), using a catalytic mixture of Na2WO4 dihydrate,
(aminomethyl)phosphonic acid and the phase transfer
reagent (PTR) methyltri-n-octylammonium hydrogen
sulfate.[19] Recently, outstanding procedures involving
tungsten-containing catalysts for the production of pro-
pylene oxide[20] and the epoxidation of allylic alcohols[21]
were reported.
The new catalytic system reported here consists of
easily available, low-cost chemicals [Na2WO4/H2WO4/
Aliquat 336 (PTR)/chloroacetic acid], which can be easi-
ly handled (Scheme 1). Typically, 200 mmoles of cis-
cyclooctene were reacted with 300 mmoles of hydrogen
peroxide (50%) at 608C. The epoxidation is catalyzed
by 0.025 mol % of Na2WO4 and 0.025 mol % H2WO4
Keywords: alkenes; chloroacetic acid; epoxidation;
hydrogen peroxide; tungsten
The development of efficient and clean processes for the with 0.2 mol % of chloroacetic acid in the presence of
epoxidation of alkenes on a million-ton-per-year scale methyltri-n-octylammonium chloride (0.2 mol %) as
remains an important objective in the fine chemical in- PTR (Scheme 1). A maximum TON of as high as 1800
dustry.[1–5] There is an increasing need for the elabora- is reached after 4 hours with 99% selectivity. This rate
tion of selective catalytic systems involving cheap, read- of the epoxidation is compared under similar experi-
ily available, and ecologically friendly oxidants, such as mental conditions as above, to the one of Noyori et al.,
hydrogen peroxide.[6,7] In this context, polyoxotung- i.e., using sodium tungstate and (aminomethyl)phos-
states have been extensively and successfully used dur- phonic acid at 608C where a TON of 800 was reached af-
ing the past 20 years.[8–11] A key experiment was per-
formed by Venturello who selectively epoxidized 1-oc-
tene with a turnover number (TON, defined as mol
product per mol catalyst) of 20 (82% yield based on
H2O2) using a Na2WO4-H3PO4-quaternary ammonium
chloride catalyst.[12] Since then, this system has been in-
tensely studied.[13,14] Nevertheless, this catalytic biphasic Scheme 1. Epoxidation of cis-cyclooctene.
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ter 4 hours. In addition, the system with (aminomethyl)- COOH/ClCH2COONa and using the catalytic system
phosphonic acid is not very active at room temperature. 2 (see Supporting Information, Figure S1; Table S1).
On the contrary, the new catalytic system acts very effi- As the pH is increased, the catalytic activity decreases
ciently even at 208C with high conversion rates (Fig- linearly. When the weak acid CH3COOH is used and
ure 1, 3b). This condition is advantageous over that of the pH is maintained at values of 3.7 and 4.7, using
(aminomethyl)phosphonic acid, as the latter needs a CH3COOH/CH3COONa, again the catalytic activity
high temperature of 908C, as described for various sub- decreased at high pH values (see Supporting Informa-
strates[18] and ammonium hydrogen sulfate[18] rather tion, Table S1). This clearly suggests that the role of
than the typical, readily available, chlorides as PTR the organic acid is different from that of maintaining
for an efficient catalytic activity. Experiments were an acidic pH of the aqueous medium, as ClCH2COOH
performed with various combinations of the cata- (pKa ¼2.7) and CH3COOH (pKa ¼4.76) have totally
lytic mixture, i.e., Na2WO4(0.4 mol %) 1, Na2WO4 different pKa values. The low catalytic activity at higher
(0.4 mol %)þClCH2COOH (1.6 mol %) 2, Na2WO4 pH of both ClCH2COOH and CH3COOH buffers is at-
(0.2 mol %)þH2WO4
(0.2 mol %)þClCH2COOH tributed to an overall increase in the ionic strength of the
(1.6 mol %) 3, H2WO4 (0.4 mol %)þClCH2COOH aqueous medium. Therefore, other chloro-substituted
(1.6 mol %) 4, and H2WO4 (0.4 mol %) 5. The results de- acetic acids like Cl2CHCOOH and Cl3CCOOH were
picted in Figure 1 show that the system 3 is the most ef- tested for catalytic activity which have lower pKa values
ficient catalyst, even at ambient temperature. Further- than ClCH2COOH, but they resulted in lower conver-
more, no incubation period is observed, while it is re- sions compared to the above ones. Also when benzoic
quired for pure H2WO4, due to the poor solubility of acid was used, a lower activity compared to chloroacetic
tungstic acid in aqueous H2O2 at ambient tempera- acid was achieved (Figure 2).
ture.[22] From a very recent report,[22] it is clear that the
I f NaWO4 (0.4 mol %) is used alone, only 6% cyclo-
2
residual acidity of the aqueous medium acts eventually octene oxide is detected after 4 hours and only 23%
upon the rate enhancement and/or in activating the W- of cyclooctene is epoxidized when 1 equivalent of
based biphasic catalytic species. Thus, it is expected ClCH2COOH (0.4 mol %) per W atom is added to this
that the rate enhancement of cyclooctene epoxidation, reaction mixture, illustrating the importance of the
in combination with the tungsten precursors, Na2WO4 chloroacetic acid to form the active species. This system
and H2WO4, is due to the presence ofa sourceof protons, can be used repeatedly by adding new substrate and hy-
from ClCH2COOH. To confirm that the rate enhance- drogen peroxide with 94% conversion after 4 h
ment is due to the acidity of the aqueous phase, the pH (selectivity>99%) after separating the organic phase.[23]
of the aqueous medium has been followed and varied To the best of our knowledge, this catalytic system is
from 1.8 to 3.2 by using different ratios of ClCH2- more efficient and simple than all others reported so
far for the epoxidation of alkenes with a tungsten-based
catalyst at room temperature. ClCH2COOH without
tungsten also results in lack of activity. When the catalyt-
ic system (2) is used in the presence of peracetic acid (di-
luted in acetic acid) the epoxidation rate is very high in
Figure 1. Epoxidation of cyclooctene using different catalytic
systems, Na2WO4 (0.4 mol %)þClCH2COOH (1.6 mol %) 2,
Na2WO4 (0.2 mol %)þH2WO4 (0.2 mol %)þClCH2COOH
(1.6 mol %) 3a, system 3 at 208C 3b, H2WO4 (0.4 mol %)þ
ClCH2COOH (1.6 mol %) 4, H2WO4 (0.4 mol%) 5 at room
temperature. Experiment 3a has been performed with system
3 at 608C for comparison. System 1 is not included in the
graph, because of the very low conversion.
Figure 2. Epoxidation of cyclooctene using different organic
acids (1.6 mol %) at room temperature with 0.2 mol % of
Na2WO4 and 0.2 mol % of H2WO4.
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Alkene Epoxidation, using Dihydrogen Peroxide as Oxidant
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the presence of tungsten at 608C and reaching 90% con-
version after 15 minutes whereas without tungsten only
10% conversion is obtained after the same period of
time (see Supporting Information; Figure S2). As the
rate of epoxidation is higher than the analogous system
with hydrogen peroxide, the peracid itself and not only
the smaller amount of hydrogen peroxide present in
the peracetic acid solution, is activated by the W species.
This would suggest that the epoxidation may involve the
in situ formation of peracids catalyzed by tungsten pre-
cursors and/or tungsten-peroxo species in the presence
of an organic acid and hydrogen peroxide. Certain iron
complexes have indeed been reported to show enhanced
epoxidation and selectivity on the addition of more
equivalents of acetic acid over diol formation in the
case of cyclooctene and 1-decene.[24,25]
Negative ion ESI-MS experiments with the catalytic
mixture (3) (Na2WO4 – 2 mmol; H2WO4 – 2 mmol;
ClCH2COOH – 16 mmol; 50% H2O2 – 40 mmole is stir-
red in 10 mL of H2O, pH 2.2, at room temperature, and
diluted as described in the experimental part for electro-
spray experiments) yielded two major peaks at m/z¼
735 and 408, respectively, which were assigned to a
dinuclear peroxo-tungsten species with ClCH2COOH,
[W2(ClCH2COO)2(O2)4(O)3] and
peroxo-tungsten species with
a
mononuclear
ClCH2COOH,
[W(ClCH2COO)(O2)2(O)2] (Figure 3). The obtained
isotopic pattern is very similar to the theoretical pattern
for the above proposed mono/di-peroxo species (see
Supporting Information; Figure S3). A series of prod-
ucts was also observed resulting from the loss of peroxo,
oxo and ClCH2COOH from the dinuclear tungsten spe-
cies (see Supporting Information; Figure S4). At this
point, it has to be mentioned that no peaks correspond-
ing to the formation of isopolytungstates, and peroxo-
tungstates of high nuclearity were observed, supporting
a low-nuclearity tungsten-containing active species for
the catalytic activity.[26,27] The epoxidation of cyclooc-
tene performed in the presence of benzoquinone, a
known radical trap, did not lead to an inhibition of the
epoxidation. Consequently, the mechanism involved is
expected to involve direct oxygen transfer rather than
be radical-based.
Thus, system 3 which contains equivalent amounts of
Na2WO4 and H2WO4 in the presence of chloroacetic
acid efficiently enhances the rate of the epoxidation of
various alkenes as illustrated in Table 1. Cyclooctene
and cyclopentene are converted to the corresponding
epoxides within 30 minutes with high selectivity (en-
try 2) at 608C. An even more interesting observation is
the activity of system 3 towards 1-octene (entry 3), an al-
kene known to be difficult to epoxidize. After 4 hours, a
TON (based on tungsten species) of 135 is reached
(selectivity>99%) with 0.4 mol % of the tungsten cata-
lyst (0.2 mol % Na2WO4 and 0.2 mol % H2WO4); this is
a significant rate enhancement compared to the ones re-
ported in the literature for this substrate.[28,29] Another
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Figure 3. The ESImass spectra of the catalytic mixture at 20 8C (3). (A) Pattern observed for a dinuclear tungsten peroxo spe-
cies with two ClCH2COOH [W2(ClCH2COO)2(O2)4(O)3]. (B) Pattern observed for a mononuclear tungsten peroxo species
with one ClCH2COOH [W(ClCH2COO)(O2)2(O)2].
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Alkene Epoxidation, using Dihydrogen Peroxide as Oxidant
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Experimental Section
All reactants and solvents were used as received without any
further purification. The oxidations were carried out in air in
a 100-mL, three-necked round-bottom flask equipped with a
magnetic stirrer and reflux condenser.
Typically, 0.132 g(0.2 mol %) of Na2WO4 ·2 H2O and 0.112 g
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methyltri-n-octylammonium chloride (Aliquat 336, PTR).
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monitor the oxidation by GC (1,4-dibromobenzene; 1 mg·
mLꢀ1 in CH2Cl2; GC internal standard) for analyses. The prod-
ucts of the reaction were determined by comparison with the
commercially available compounds.
ESI-MS experiments were carried out using a Finnigan Aqa
Mass Spectrometer equipped with an electrospray ionization
source. The sample solution was prepared using Na2WO4 –
2 mmol; H2WO4 – 2 mmol; ClCH2COOH – 16 mmol; 50%
H2O2 – 40 mmole in 10 mL of H2O, pH 2.2, at room tempera-
ture. The solution was diluted to 1 mg/mL, in H2O and was in-
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an acetonitrile/water (50/50) mixture as eluent running at
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m/z¼1600 to 100. Approximately 25 scans were summed to
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Acknowledgements
Support from the NRSC Catalysis (a Research School Combi-
nation of HRSMC and NIOK) is kindly acknowledged. The re-
search is financially supported by the Dutch Economy, Ecology,
Technology (EET) programme, a joint programme of the Min-
istry of Economic Affairs, the Ministry of Education, Culture
and Science, and the Ministry of Housing, Spatial Planning
and the Environment. Dr. Paul Alsters (DSM, Pharmaceuticals)
is gratefully acknowledged for fruitful discussions.
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