Temperature-dependent immobilization of a tungsten peroxo complex that catalyzes the hydroxymethoxylation of olefins

Article abstract of DOI:10.1002/cplu.201402456

Abstract A tungsten peroxo complex stabilized by the bidentate picolinato ligand has been synthesized and then immobilized successfully on imidazole-functionalized silica. The immobilized tungsten-based catalyst was employed as an efficient catalyst for the one-pot synthesis of β-alkoxy alcohols from olefins and methanol with H₂O₂. Regarding the catalyst evaluation and the results of characterization by the various methods, it was demonstrated that the immobilization of tungsten peroxo complex was highly temperature-dependent. The tungsten peroxo complex can dissociate and diffuse into the liquid phase at reaction temperature, resulting in a homogeneous reaction. Nevertheless, the catalytically active species was anchored on the imidazole-functionalized silica by hydrogen bonding as the temperature was lowered to 0°C after the reaction, which thus offered a highly effective approach for recycling the catalyst for consecutive cycles. In addition, various olefins can be converted to the corresponding β-alkoxy alcohols with good conversion and selectivity under relative mild conditions by H₂O₂. Running hot and cold: A tungsten peroxo complex (see picture) can dissociate and diffuse into the liquid phase at the reaction temperature, resulting in a homogeneous reaction. After reaction, the catalytically active species was anchored on the functionalized silica by hydrogen-bonding as the temperature was lowered to 0°C. This offers an effective approach for catalyst recovery and recycling.

Full text of DOI:10.1002/cplu.201402456

DOI: 10.1002/cplu.201402456
Full Papers
Temperature-Dependent Immobilization of a Tungsten
Peroxo Complex That Catalyzes the Hydroxymethoxylation
of Olefins
Jizhong Chen, Li Hua, Chen Chen, Li Guo, Ran Zhang, Angjun Chen, Yuhe Xiu, Xuerui Liu,
and Zhenshan Hou*[a]
A tungsten peroxo complex stabilized by the bidentate picoli-
nato ligand has been synthesized and then immobilized suc-
cessfully on imidazole-functionalized silica. The immobilized
tungsten-based catalyst was employed as an efficient catalyst
for the one-pot synthesis of b-alkoxy alcohols from olefins and
methanol with H2O2. Regarding the catalyst evaluation and the
results of characterization by the various methods, it was dem-
onstrated that the immobilization of tungsten peroxo complex
was highly temperature-dependent. The tungsten peroxo com-
plex can dissociate and diffuse into the liquid phase at reaction
temperature, resulting in a homogeneous reaction. Neverthe-
less, the catalytically active species was anchored on the imida-
zole-functionalized silica by hydrogen bonding as the tempera-
ture was lowered to 08C after the reaction, which thus offered
a highly effective approach for recycling the catalyst for con-
secutive cycles. In addition, various olefins can be converted to
the corresponding b-alkoxy alcohols with good conversion and
selectivity under relative mild conditions by H2O2.
Introduction
Oxido-peroxido transition metal compounds have played an
important role in the oxidation of various substrates, including
olefins, alcohols, sulfides, and alkanes.[1–6] The catalytic activity
of peroxide metal complexes is influenced by the type of
metal atom and the number of peroxide ligands in the coordi-
nation sphere. Among these complexes, tungsten peroxo com-
plex based oxidation systems with H2O2 have attracted much
attention owing to their the high activity and inherent poor
activity for the decomposition of H2O2.[7–9] The first example of
transitition metal (Mo and W) peroxo complexes which were
stabilized by the bidentate picolinato ligand was reported by
Mares et al. in 1978.[10] They found that secondary alcohols can
be transformed into ketones by the peroxo complexes in polar
media under stoichiometric conditions or with H2O2 and with
a moderate conversion. Then, Furia et al. reported the related
oxido-bisperoxido complexes containing bidentate ligands and
employed them in the oxidation of alcohols with a good cata-
lytic activity.[11] Although various peroxo complexes have been
used for different oxidation reactions, the immobilization and
recycling of these catalytically active species still remain
a huge challenge.
chemicals, and pharmaceutical intermediates, the main proto-
col for the synthesis of b-alkoxy alcohols is the epoxidation of
olefins and sequential alcoholysis of epoxides under basic or
acidic conditions.[12–14] Obviously, the direct catalytic transfor-
mation of olefins into b-alkoxy alcohols offers a huge advant-
age over the two-step synthesis. Catalysts required for this
transformation should be efficient in the olefin epoxidation
and, as well, in the epoxide-ring-opening reaction by the alco-
hol. Nevertheless, only a few studies had reported the forma-
tion of the final b-alkoxy alcohols in one step, namely hydroxy-
methoxylation of olefin, starting from the olefin by using an
appropriate oxidant and the alcohol. The reaction was first re-
ported in 1957 and, up to now, this transformation has been
performed by several catalysts under homogeneous and heter-
ogeneous conditions.[15–17] However, these conventional sys-
tems have some drawbacks, like the limited substrate scope,
poor recyclability of catalyst, and the harsh operating condi-
tions. Therefore, the search for a highly active and recoverable
catalytic system for hydroxymethoxylation under mild reaction
conditions continues.
Compared with homogeneous catalysts, heterogeneous cat-
alysts can be recovered from the reaction medium convenient-
ly. Therefore, the immobilization of active oxido-peroxido tran-
sition metal compound on solid supports has attracted atten-
tion widely in the past decade. The usual approaches include
immobilization on polymers, silica, and magnetic particles.[18–20]
However, owing to the limited phase-transfer between solid
catalyst and substrates, heterogeneous catalysts afford relative-
ly poor yield compared with the homogeneous analogues.
Thus, in order to combine the advantages of homogenous cat-
alysts (activity and selectivity) with the facility of heterogene-
ous systems (recycling), some “smart catalysts” sensitive to
In view of the importance of b-alkoxy alcohols, which are
used as valuable organic solvents, versatile synthons, fine
[a] J. Chen, Dr. L. Hua, C. Chen, L. Guo, R. Zhang, A. Chen, Y. Xiu, X. Liu,
Prof. Z. Hou
Key Laboratory for Advanced Materials
Research Institute of Industrial Catalysis
East China University of Science and Technology
Shanghai 200237 (P. R. China)
E-mail: houzhenshan@ecust.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402456.
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temperature, CO2, light, and pH. have been developed as
well.[21–23] However, because the reaction is carried out normal-
ly at elevated temperatures and thus catalyst was typically fil-
tered off at lower temperatures, the thermoregulated catalysts
have attracted more attention recently. It has been reported
that hydrogen bonding could be the ideal type of chemical
bonding for reversible immobilization; hydrogen bonds form
at low temperature but break at elevated temperature.[24,25]
Thus, immobilization of catalytic species is possible through
hydrogen-bonding interactions. In this context, a fiber-support-
ed ionic liquid (IL) “release and catch” catalyst has been em-
ployed exclusively for a facile synthesis of bis(indolyl)methanes
by Friedel–Crafts alkylation in water at room temperature.[26]
Besides, a reversible catalyst immobilization via hydrogen-
bonding-mediated self-assembly has also been reported for
the atom transfer radical polymerization of methyl methacry-
late (MMA). The catalyst was tethered on a diaminopyridine
unit. A triple hydrogen-bond array formed between maleimide
or thymine and diaminopyridine at room temperature but
broke at elevated temperatures.[27]
Scheme 1. Preparation route of SiO2-IM and HWPy.
Regarding the above discussion and our previous research
on the epoxidation of olefins,[28–31] in the present work we at-
tempted to develop a highly efficient catalytic system for the
hydroxymethoxylation of olefins, where the tungsten peroxo
complex catalyzed the hydroxymethoxylation reaction in the
homogeneous phase, but it can be also easily separated and
recycled in a heterogeneous approach. Herein, we synthesized
a tungsten peroxo complex stabilized by the bidentate picoli-
nato ligand and then anchored it to imidazole-functionalized
silica. It was found that the tungsten peroxo complex dissoci-
ated into the reaction medium from imidazole-functionalized
silica and thus acted as “quasi-homogeneous catalyst” under
reaction conditions, but anchored robustly to the functional-
ized silica when the temperature was lowered to 08C. The cur-
rent procedure combines the advantages of both homogene-
ous catalysis with high activity and heterogeneous catalysis
with easy recovery, and allows the direct transformation of var-
ious alkenes to b-methoxy alcohols using H2O2 as the oxidant,
without the requirement of any other additives.
Figure 1. 29Si CP-MAS NMR spectra of a) SiO2; b) SiO2-IM.
tionalized SiO2 (Figure 1b), two new resonances at d=À67
and À58 ppm were observed, which were assigned to the T3
and T2 structural units, respectively. The appearance of T3 and
T2 structural units confirmed that the functional group had at-
tached to the surface of fumed silica successfully. And on the
basis of N2 adsorption–desorption isotherms, the BET surface
area of SiO2, SiO2-IM, and the recovered SiO2-IM-HWPy catalyst
was 280.3, 239.1, and 185.2 m2 gÀ1, respectively. This indicated
that after grafting with imidazole and HWPy, the functionalized
silica material had a slightly decreased BET surface area. The re-
duction in surface area may result from the presence of the or-
ganic group and the polyoxometalate on the surface of silica.
All these observation implied that imidazole ring was anchored
covalently onto the surface of SiO2 and also HWPy was loaded
on SiO2-IM.
Results and discussion
Catalyst preparation and characterization
For the preparation of an imidazole-modified silica, 3-iodopro-
pyltrimethoxysilane was first reacted with imidazole in the
presence of NaH in THF under refluxing conditions (Scheme 1).
Then the resulting N-(3-propyltrimethoxysilyl)imidazole was co-
valently anchored to silica by a convenient approach. The
tungsten peroxo complex (HWPy) was synthesized according
to the previous report.[10]
Then, X-ray diffraction patterns (XRD) of SiO2, SiO2-IM, and
the recovered SiO2-IM-HWPy catalyst were recorded (Figure 2).
The broad peak around 2q=20–308 is assigned to the amor-
phous silica. The XRD pattern of the SiO2-IM did not differ from
that of pure SiO2. In addition, there were no characteristic
peaks of HWPy in the XRD pattern of the spent SiO2-IM-HWPy,
which indicated that HWPy could be well dispersed on the sur-
face of SiO2-IM. Furthermore, from the SEM images shown in
Figure 3, it can be seen that the functionalized silica particles
and the recovered SiO2-IM-HWPy showed more rough surface
(Figure 3b,c) than the parent silica (Figure 3a), indicating that
The successful grafting of N-(3-propyltrimethoxysilyl)imida-
zole onto silica was proved by 29Si MAS NMR spectroscopy. As
shown in Figure 1a, the pure silica had two signals at d=À110
and À102 ppm, which were assigned to Si bonded to four si-
loxane units [(ÀOSi)4] and Si bonded to three siloxane units
and one silanol group [(-SiO)3SiOH], respectively. For the func-
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W=O, OÀO, W(O2)(asym), and W(O2)(sym), respectively. Interest-
ingly, the band at 1519 cmÀ1 for SiO2-IM is assigned to the imi-
dazole ring stretch, which was blue-shifted to 1542 cmÀ1 in the
recovered SiO2-IM-HWPy. This change implied that the environ-
ment of the nitrogen atoms in the imidazole ring is very differ-
ent due to the anchoring of HWPy to the imidazole; this obser-
vation coincides with previous research on the immobilization
of phosphotungstic acid (PTA) onto imidazole-functionalized
fumed silica.[32] In addition to the shift of the imidazole ring
stretching, another strong piece of evidence that HWPy is
anchored on the SiO2-IM was the characteristic FTIR peak at
977 cmÀ1 assigned to the W=O in HWPy, which was shifted to
951 cmÀ1 in SiO2-IM-HWPy.[33] Moreover, as shown in Figure 4c,
the peak at 884 cmÀ1 assigned to the stretching of the peroxy
group (ÀOÀOÀ) was also observed, indicating that the catalyti-
cally active HWPy had been anchored on the surface of func-
tional silicon successfully.
Figure 2. XRD patterns of a) HWPy; b) SiO2; c) SiO2-IM; d) SiO2-IM-HWPy (re-
covered).
Because only a small amount of the organic fraction (about
0.80 mmolgÀ1) was loaded onto silica gel, the resonance signal
1
in the solid-state H NMR spectrum was weak. In additional ex-
1
periments with solution-phase H NMR spectroscopy butylimi-
dazole (BIm) was used instead of SiO2-IM to simulate the cur-
rent SiO2-IM-HWPy system. As shown in Figure 2S, the chemical
shifts of protons (a, b, c) on the imidazole ring of BIm were d=
7.6, 7.1, and 6.9 ppm, respectively. Nevertheless, upon the ad-
dition of HWPy to BIm in CD3OD the signals of protons (c’, b’,
a’) on imidazole ring were shifted significantly to d=8.9, 7.6,
and 7.5 ppm, respectively. Meantime, it can be also observed
that the chemical shifts of protons (d, e, f) in the alkyl chain of
butylimidazole were moved downfield with the addition of
HWPy. These 1H NMR characterizations clearly indicated that
the imidazole group can act as an H-bond acceptor and the
tungsten peroxo complex (HWPy) as an H-bond donor.
Figure 3. The SEM images of a) SiO2; b) SiO2-IM; c) the recovered SiO2-IM-
HWPy, d) EDS of the recovered SiO2-IM-HWPy catalyst.
the surface morphology changed due to the immobilization of
tungsten peroxo complex after the first run.
Diffuse reflectance UV-visible spectra of SiO2-IM, HWPy, and
the recovered SiO2-IM-HWPy catalysts are shown in Figure 3S.
SiO2-IM showed an absorption band at 224 nm which was at-
tributed to imidazole, while HWPy showed an absorption band
at 281 nm which was attributed to oxygen–tungsten charge
transfer. However, for the recovered SiO2-IM-HWPy catalyst this
characteristic absorption band became sharper and was shifted
to 271 nm.[34] This result suggests that a strong interaction
exists between HWPy and SiO2-IM, which is in accordance with
the FTIR characterization.
FTIR spectra of HWPy, SiO2-IM, and the recovered SiO2-IM-
HWPy are shown in Figure 4. In the spectrum of SiO2-IM, the
characteristic peaks at 803 and 1000–1200 cmÀ1 were ascribed
to the vibrations of SiÀOÀSi (sym) and SiÀOÀSi (asym), respec-
tively; these peaks were also present in the spectrum of the re-
covered SiO2-IM-HWPy. Additionally, the spectrum of HWPy
(Figure 4a) showed characteristic peaks at 1663, 977, 883, 558,
and 538 cmÀ1, which were ascribed to the vibrations of C=O,
Catalytic performance
First, the hydroxymethoxylation of styrene to the correspond-
ing b-alkoxy alcohols at 608C for 12 h over different catalysts
was performed and the reaction results are shown in Table 1. It
can be seen that HWPy gave an excellent conversion and se-
lectivity in CH3OH, but it could not be recovered and reused
after reaction due to the solubility of HWPy in CH3OH even at
08C (Table 1, entry 1). Although HWPy was not as soluble in
the mixed methanol/toluene (2:1) solvent system, the recycla-
bility of HWPy was still very poor (Table 1, entries 2 and 3)
owing to considerable leaching of W. Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis indi-
Figure 4. FTIR spectra of a) HWPy; b) SiO2-IM; c) SiO2-IM-HWPy (recovered).
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result showed that the styrene
oxide was converted to the 2-
methoxy-2-phenylethanol within
5 min with 96% conversion and
86% selectivity.
Table 1. Hydroxymethoxylation of styrene over different catalysts.[a]
Entry Catalyst
Solv.
Conv.
[%]
Sel. [%]
Leaching of W
[ppm]
H2O2 eff.
[%][g]
Ether[b] BA BO Diol
1
2
3
4
HWPy
HWPy
HWPy
HWPy+SiO2-
IM
HWPy+SiO2-
IM
HWPy+SiO2-
IM
HWPy+SiO2-
IM
CH3OH
99.0
90.0
82.1
84.3
6.8 2.0 1.2
9.5 5.5 3.0
8.3 2.1 5.7
miscible
7000
–
87.4
69.5
71.4
87.1
This indicated that the alco-
holysis of the epoxide is much
faster than the epoxidation of
the olefin. In other words, the
epoxidation is the rate-limiting
step of the overall reaction and
the epoxide was shown as an
unstable intermediate under
these reaction conditions. In ad-
dition, the effect of temperature
on the reaction was also exam-
ined. As shown in Figure 5b, the
conversion of styrene increased
with reaction temperature and
remained unchanged at temper-
[c]
CH3OH+PhCH3
CH3OH+PhCH3
CH3OH
98.5
80.4
90.3
[d]
83.0 12.5 2.5 1.0
80.2 14.6 3.2 2.2
55.5 39.5 4.5 1.0
530
[c]
[e]
[f]
5
6
7
CH3OH+PhCH3
CH3OH+PhCH3
CH3OH+PhCH3
88.6
53.4
95.6
5.6
4.5
70.0
70.5
72.1
84.1
9.8 4.1 2.1
200
[a] Reaction condition: 0.05 g (0.12 mmol) HWPy, 0.15 g (0.12 mmol imidazole group) SiO2-IM, 1 mmol styrene,
2 mmol 30% H2O2, 2 mL solvent, 608C, 12 h. [b] The main product was 2-methoxy-2-phenylethanol. [c] Vmethanol
/
V
toluene =2:1. [d] Run 2 of entry 2, reaction time was 18 h. [e] Vmethanol/Vtoluene =1:1. [f] Vmethanol/Vtoluene =3:1. BA:
benzaldehyde, BO: styrene oxide. [g] Remaining H2O2 was determined by potential difference titration of Ce3+
/Ce4+ (0.1m of aqueous Ce(SO4)2·4H2O). H2O2 effciency (%)=products (mol)/consumed H2O2 (mol)ꢁ100.
cated that 7000 ppm of W was leached into decantate after
the first run.
In order to immobilize HWPy more effectively, SiO2-IM was
added into the reaction mixture. Although the conversion and
selectivity were slightly lower in methanol, in comparison with
that in the absence of SiO2-IM (Table 1, entries 1 vs. 4), the
leaching of W was decreased to 530 ppm. However, when
mixed methanol/toluene (2:1) was used as the reaction
medium, the subsequent analysis of the decantate demonstrat-
ed that the leaching of W-based active species was approxi-
mately 5.6 ppm by ICP-AES (Table 1, entry 5), indicating that
Figure 5. Time profile for the hydroxymethoxylation of styrene and the
effect of temperature on the reaction. a) Reaction conditions: 0.05 g
(0.12 mmol) HWPy, 0.15 g (0.12 mmol imidazole group) SiO2-IM, 1 mmol sty-
rene, 2 mmol 30% H2O2, 608C, 2 mL solvent (CH3OH/PhCH3 =2:1); b) Reac-
tion time was 12 h. (ether/2-methoxy-2-phenylethnol, BA: benzaldehyde).
the leaching of W was negligible in the mixed solvents after re-
action. In this case, the catalytic activity and selectivity were
almost same as that in methanol media (Table 1, entries 4 and
5). Different ratios of methanol to toluene were also examined.
Higher concentrations of toluene resulted in lower catalytic
performance (Table 1, entry 6), while the introduction of more
methanol afforded higher activity (Table 1, entry 7), but led to
a considerable leaching of catalytically active species (about
200 ppm W was found in decantate). In addition, it was ob-
served that most of the H2O2 was consumed for styrene oxida-
tion. The introduction of toluene decreased slightly the H2O2
efficiency (Table 1, entry 4 vs. 5), while SiO2-IM had almost no
effect on H2O2 efficiency (Table 1, entry 2 vs. 5).
aturs above 608C. Meantime, the selectivity to benzaldehyde
was favored at lower reaction temperatures (<608C). When
the reaction was carried out at low temperatures, the inter-
mediate product styrene epoxide could not react with metha-
nol quickly to afford 2-methoxy-2-phenylethanol, but rather
underwent nucleophilic attack of H2O2 to yield the by-product
benzaldehyde.[35] However, when the reaction was carried out
at much higher temperatures (ꢀ708C), direct oxidative cleav-
age of the C=C bond of styrene might happen and benzalde-
hyde was produced,[35] resulting in a slight increase of benzal-
dehyde selectivity (Figure 5b). The results demonstrated that
the reaction temperature affected both the conversion and the
selectivity of the overall reaction, and therefore a suitable reac-
tion temperature was crucial for the hydroxymethoxylation re-
action.
In the next step, the effect of the reaction parameters on
the hydroxymethoxylation of styrene was investigated. As
shown in Figure 5a, as the reaction proceeded, the conversion
of styrene increased continuously and full conversion was ach-
ieved after 18 h. On the other hand, the selectivity to the b-
alkoxy alcohols remained constant and intermediate products
such as styrene oxide were not detected in the course of the
reaction. This implied that the rate of alcoholysis of the epox-
ide is so fast that epoxide is transformed to b-alkoxy alcohols
as soon as it is generated. In order to further investigate the
rate-limiting step of the reaction, styrene oxide was employed
as the substrate under the optimized reaction conditions. The
Substrate generality and reusability of the catalyst
The optimized reaction conditions were then used to explore
the substrate scope. Firstly, we mainly focused on the catalytic
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activities of styrene and its derivatives. As shown in Table 2,
styrene, 2-methylstyrene, and 4-methylstyrene were converted
into the corresponding b-alkoxy alcohols with approximately
90% conversion and 75–80% selectivity when the reaction
was carried out for 12 h (Table 2, entries 1–3). However, the
above substrates exhibited different reactivity in the following
order: 4-methylstyrene>2-methylstyrene>styrene when the
reaction time was shortened to 6 h. Compared with the above
three substrates, 4-chlorostyrene had a relatively low reaction
rate with 87.6% of conversion and 78.3% selectivity after 24 h.
That was because the synthesis of b-alkoxy alcohols proceeded
through two steps: epoxidation of the olefin and alcoholysis of
the epoxide to b-alkoxy alcohols, and the rate-limiting step
seems to be the epoxidation of the olefin as a trace amount of
epoxide was detected in the reaction. Since the epoxidation is
an electrophilic addition reaction, the higher electron density
at the double bond favored the epoxidation reaction. There-
fore, the presence of the electron-donating group (-CH3) at the
para position or ortho position of styrene increased the reactiv-
ity of styrene, while the electron-withdrawing group (ÀCl) at
the para position of styrene decreased the reaction rate.
For cyclohexene, the conversion and selectivity were similar
to that of styrene under the same reaction conditions (Table 2,
entry 5). Terpinene and norbornene were also converted into
the corresponding b-alkoxy alcohols with excellent conversion
in short reaction time (Table 2, entries 6 and 7). The highly re-
gioselective hydroxymethoxylation of the C=C bond in terpi-
nene might result from steric hindrance. The existence of the
isopropyl group was not favorable for the nucleophiles to
attack the carbon atoms in the epoxide. It was worth noting
that terminal olefins also could
be converted into the corre-
sponding b-alkoxy alcohols. For
Table 2. Hydroxymethoxylation of various olefins over SiO2-IM-HWPy catalyst.[a]
examples, 1-hexene and 1-
octene were converted into the
corresponding products with
81.2%, 77.4% conversion and
78.2%, 83.1% selectivity, respec-
tively. Owing to the more substi-
tuted carbon in the intermediate
adduct generated, there was
slightly more primary alcohol
product than secondary alcohol
(Table 2, entries 8 and 9).
Entries
1
Substrates
Products
t [h]
Conv. [%]
Sel. [%][b]
6
12
54.1
88.6
75.3
80.2
6
12
67.3
89.5
74.4
78.3
2
3
4
5
6
12
81.2
91.4
76.2
80.2
Next, the recyclability of the
SiO2-IM-HWPy catalyst was inves-
tigated by choosing styrene as
a model substrate under the op-
timized reaction condition. As
shown in Figure 6, the catalyst
could be reused at least five
times only with a slight decrease
of the catalytic activity due to
the physical loss during the
treatment. Actually, the leaching
12
24
44.8
87.6
68.4
78.3
18
91.9
78.3
6
7
2
5
98.0
93.1
55.2
65.9c
of
W into solvent decreased
gradually and can be negligible
during the consecutive recycles.
In order to understand whether
HWPy was dissolved in the liquid
phase under the reaction condi-
tions, a fast hot filtration experi-
ment was carried out at the re-
action temperature. As shown in
Figure 7, after reaction for 6 h,
the reaction mixture was filtered.
It was observed that the styrene
in the filtrate continued to be
converted to the corresponding
b-alkoxy alcohols only with
78.2
8
9
24
24
81.2
77.4
(56:44)[d]
83.1
(53:47)[d]
[a] Reaction conditions: 0.05 g (0.12 mmol) HWPy, 0.15 g (0.12 mmol imidazole group) SiO2-IM, 1 mmol sub-
strate, 2 mmol 30% H2O2, 2 mL solvent (CH3OH/PhCH3 =2:1), 608C, the products were all trans structures.
[b] The main by-products were diols and aldehydes. [c] The ratio between exo- and endo-methoxy isomers was
about 1:1. [d] The numbers in the parenthesis referred to the ratio of two isomers. The conversion and selectiv-
ity were determined by GC analysis.
a
slight decrease of reaction
rate. The subsequent ICP-AES
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species plays an important role in catalyzing the hydroxyme-
thoxylation reaction. This revealed that that active species
could dissociate from SiO2-IM under the reaction temperature.
However, after the reaction, when the reaction mixture was
cooled to 08C, the amound of dissolved W species was found
to be negligible by ICP-AES analysis. This indicated that the W
species was anchored effectively to the surface of the function-
alized SiO2 as the solution was cooled to 08C, resulting in
almost no leaching of W-based active species.
Proposed mechanism for the catalytic system
As already described above, the one-pot synthesis of b-alkoxy
alcohols from olefin requires two catalytic centers to complete
the two different parts of reaction. In the present catalytic
system, the tungsten-based peroxo complex (HPyW) was disso-
ciated into the liquid phase under reaction conditions, and
thus catalyzed the epoxidation process. Simultaneously, the
dissociated proton further catalyzed the ring opening of the
epoxide. After reaction, the tungsten-based peroxo complex
was anchored effectively onto the surface of the modified
silica through hydrogen-bond interactions. A detailed descrip-
tion of the reaction process is shown in Scheme 2. First, in the
presence of H2O2, the anion of the tungsten peroxo complex
was transformed into the active peroxo species, which epoxi-
dized styrene to the styrene oxide, and this was the rate-limit-
ing step of the whole reaction. Then, the epoxide could be
protonated by the HWPy. Subsequently, methanol would
attack the most stable benzyl carbocation from the backside,
giving rise to the trans b-alkoxy alcohols by an SN1 mechanism.
The main by-product was benzaldehyde, which resulted from
the nucleophilic attack of H2O2 to styrene oxide followed by
cleavage of the intermediate hydroperoxystyrene, and the
trace diols were produced from hydrolysis of styrene oxide.
Figure 6. Recyclability of catalyst for hydroxymethoxylation of styrene. Reac-
tion condition: 0.05 g (0.12 mmol) HWPy, 0.15 g (0.12 mmol imidazole
group) SiO2-IM, 1 mmol styrene, 2 mmol 30% H2O2, 2 mL solvent (CH3OH/
PhCH3 =2:1), 608C, 18 h. Ether (2-methoxy-2- phenylethnol) as a main prod-
uct.
Figure 7. Time profile of reaction and fast hot filtration test of styrene cata-
lyzed with SiO2-IM-HWPy. Reaction condition: 0.05 g (0.12 mmol) HWPy,
0.15 g (0.12 mmol imidazole group) SiO2-IM, 1 mmol styrene, 2 mmol 30%
H2O2, 2 mL solvent (CH3OH/PhCH3 =2:1), 608C. Solid solid represented the
consecutive reaction without filtration, while solid circle reaction in the fil-
trate after filtration at 6 h.
Conclusion
In summary, we synthesized a tungsten peroxo complex-based
stabilized by the bidentate picolinato ligand. Detailed charac-
terization by elemental analysis, AES-ICP analysis, BET surface
analysis showed that about
46.6% of the total W was dis-
solved in the liquid phase under
reaction conditions. Thus it can
be easily determined that the
turnover frequency (TOF) of the
catalyst was about 0.48 hÀ1 with-
out filtration (filled circles in
Figure 7). Nevertheless, the TOF
of the filtrate was 0.85 hÀ1 after
filtration (filled squares in
Figure 7), which was almost
twice that of the immobilized
catalyst. Considering that almost
a half of the total W was dis-
solved in the liquid phase, we
can prove that the soluble active Scheme 2. Proposed reaction mechanism for the hydroxymethoxylation of styrene in the mixed media.
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1
tector. For H NMR and MS analysis, the crude products were puri-
fied by flash chromatography on a silica gel column using a mixture
of petrol ether/ethyl acetate (5:1, v/v) as eluant.
area measurements, FTIR spectroscopy, NMR spectroscopy, and
SEM confirmed that the tungsten peroxo complex was immo-
bilized successfully onto the surface of silica. The immobilized
tungsten peroxo complex was used as catalyst for the one-pot
synthesis of b-alkoxy alcohols from olefin and methanol with
H2O2. The catalyst can convert different kinds of olefins into
the corresponding b-alkoxy alcohols with a good conversions
and selectivities, and it was reused five times without a notable
decrease in catalytic activity. Further investigation indicated
that the catalytically active tungsten peroxo complex can dis-
sociate from silica under the reaction temperature, resulting in
a homogeneous reaction. However, the soluble W-based spe-
cies was anchored robustly on silica as the temperature was
decreased to to 08C. The proposed mechanism describes the
simple temperature-dependent immobilization in detail. The
mild reaction conditions and the simple procedure combined
with the easy reuse of the catalyst and solvent make this
method environmentally friendly and economical for the syn-
thesis of b-alkoxy alcohols. Further work to expand and apply
the catalyst is ongoing. This newly developed temperature-de-
pendent immobilization provides a new strategy for the
design and development of greener, more efficient, and eco-
nomical chemical processes, and we believe that the current
approach for immobilization of catalytically active species can
inspire future research on homogeneous catalysis.
Catalyst preparation
Synthesis of 3-iodopropyltrimethoxysilane
3-iodopropyltrimethoxysilane was prepared according to the re-
ported procedures:[36] Under argon atmosphere, 36.9 g (0.246 mol)
of sodium iodide were dissolved in 150 mL of dry acetone and
then 48.9 g (0.246 mol) of 3-chloropropyltrimethoxysilane were
added dropwisely under stirring. The mixture was heated to reflux
under stirring overnight. Afterwards, the formed precipitate
(sodium chloride) was filtered off and the solvent was removed
under vacuum. The crude residue was washed with ether for three
times and dried under vaccum. A yellowish liquid was obtained.
1
Yield: 64 g (90%). H NMR (400 MHz): d=0.74 (t, 2H, Si-CH2), 1.91
(q, 2H, I-CH2-CH2), 3.21 (t, 2H, I-CH2), 3.57 ppm (s, 9H, Si-O-CH3).
Synthesis of N-(3-propyltrimethoxysilane) imidazole
Under argon atmosphere, 4.8 g (0.12 mol) of sodium hydride was
suspended in 150 mL of dry THF, and then the mixture was cooled
to 48C with an ice bath and 8.2 g (0,12 mol) of imidazole were
added over a period of 30 min. The suspension was stirred for 2 h
until no release of hydrogen were observed. Then 26.12 g
(0.09 mol) of 3-iodopropyltrimethoxysilane were added and the
mixture was heated to reflux overnight. The orange suspension
was filtered and the solvent removed under vaccum. Afterwards,
150 mL dry dichloromethane was added and the formed precipi-
tate was filtered off. A transparent liquid was obtained. Yield:
12.5 g (60%). 1H NMR (400 MHz): d=0.57 (t, 2H, Si-CH2), 1.86 (q,
2H, N-CH2-CH2), 3.51 (s, 9H, Si-O-CH3), 3.93 (t, 2H, N-CH2), 6.91 (s,
1H, N-CH-CH-N), 7.06 (s, 1H, N-CH-CH-N), 7.48 ppm (s, 1H, N-CH-
CH-N).
Experimental Section
General remarks
All manipulations involving air-sensitive materials were carried out
using standard Schlenk line techniques under an atmosphere of ni-
trogen. All solvents (A.R. grade) were dried with the standard
methods. Silica gel was supplied by Qingdao Haiyang Chemical Re-
agents Co., Ltd, China. Commercially available H2O2 (30% in water),
dichloromethane, acetone, toluene, methanol, ether was purchase
from Sinopharm Chemical Reagent Co. Ltds. All reagents were
commercially available and were used without further purification,
unless otherwise stated. All 1H NMR spectra were recorded on
Synthesis of imidazole-functionalized silica (SiO2-IM)
SiO2-IM was prepared by addition of N-(3-propyltrimethoxysilane)
imidazole (2 g, 8.6 mmol) to a suspension of SiO2 (4 g) in toluene
(80 mL). The mixture was refluxed and stirred for 24 h. The solid
materials were filtered off and washed with acetone and ether, and
then dried under vacuum. Quantitative determination of the or-
ganic functional group covalently anchored onto the surface in the
compound was performed using thermogravimetric analysis (TGA).
Typically a loading at ca. 0.86 mmolgÀ1 for the organic functional
group is obtained by TGA analysis (Figure 1S). Additionally, the ele-
mental analysis of C, H, N for the functionalized silica also indicated
0.80 mmolgÀ1 organic functional group, which was consistent with
that obtained by TGA analysis.
1
a Bruker Avance III 400 instrument (400 MHz for H) by using CDCl3
as solvent and Tetramethyl silane (TMS) as reference. Chemical
shifts (d) are given in parts per million and coupling constants (J)
in hertz. Solid-state 29Si/MAS NMR spectra were obtained on
a Varian VAMRS 400 spectrometer. The XRD analysis was performed
in D/MAX2550 VB/PC using CuKa radiation (l=1.5406 ꢂ) operated
at 40 kV and 200 mA, scan rate 6 8minÀ1, scan area 10–808. The ele-
mental analysis of C, H, N was performed on an Elementar Vario EI
III Elementa and ICP-AES analysis of W on Vanan 710 instrument,
respectively. FT-IR spectra were recorded at room temperature on
a Niclet Fourier transform infrared spectrometer (Magna 550). Dif-
fuse reflectance UV-visible measurements were recorded at room
temperature with BaSO4 as a reference on a Varian Cary 500 Spec-
trometer. A perkin Elmer Pyris Diamond was used in the current
study for the thermogravimetric analysis (TGA) measurements. A
constant heating rate of 108CminÀ1 was used in the flow of N2.
Scanning electron microscope (SEM) images were obtained on
a JSM-6360LV microscope. BET surface areas were measured at the
temperature of liquid nitrogen using a NOVA 4200e Analyzer. The
products were analyzed by Shimadzu GC-2014 and GC-MS
equipped with a HP-5 column (30 m, 0.25 mm i.d.) and an FID de-
Synthesis of tungsten peroxo complex H
[W(O)(O2)2(C5H4NCO2)]À·H2O (HWPy)
Hydrogen oxodiperoxo(pyridine-2-carboxylato) tungsten (VI)
(HWPy) was prepared according to the literature.[10] A solution of
pyridine-2-carboxylic acid (7.4 g, 60 mmol) in 10 mL of water was
added to a solution of tungstic acid (15 g, 60 mmol) dissolved in
75 mL of 30% H2O2 chilled to 08C. The resulting solution was
stirred overnight at room temperature. The volume of the solution
was then reduced to 30 mL by evaporation under vacuum. Then,
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Found: C, 15.6; H, 1.94; N, 2.93; W, 42.0%. C6H7NO8W requires C,
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Typical procedure for the transformation of olefins into b-
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Received: December 29, 2014
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J. Chen, L. Hua, C. Chen, L. Guo, R. Zhang,
A. Chen, Y. Xiu, X. Liu, Z. Hou*
&& – &&
Temperature-Dependent
Immobilization of a Tungsten Peroxo
Complex That Catalyzes the
Hydroxymethoxylation of Olefins
Running hot and cold: A tungsten
ored on the functionalized silica by hy-
drogen-bonding as the temperature
was lowered to 0oC. This offers an effec-
tive approach for catalyst recovery and
recycling.
peroxo complex (see picture) can disso-
ciate and diffuse into the liquid phase
at the reaction temperature, resulting in
a homogeneous reaction. After reaction,
the catalytically active species was anch-
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