Microwave-assisted tandem allylation-isomerization reaction catalyzed by a mesostructured bifunctional catalyst in aqueous media

Article abstract of DOI:10.1039/b821993a

A mesoporous silica-supported bifunctional Ti-Ru-SBA-15 catalyst with an ordered two-dimensional hexagonal mesostructure was prepared by postmodifying organometallic complexes RuCl₂(PPh₃)₃ and Ti(OⁱPr)₄

Full text of DOI:10.1039/b821993a

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www.rsc.org/greenchem | Green Chemistry
Microwave-assisted tandem allylation-isomerization reaction catalyzed
by a mesostructured bifunctional catalyst in aqueous media
Guohua Liu,* Yunqiang Sun, Jianyao Wang, Chuanshou Sun, Fang Zhang and Hexing Li*
Received 9th December 2008, Accepted 24th June 2009
First published as an Advance Article on the web 14th July 2009
DOI: 10.1039/b821993a
A mesoporous silica-supported bifunctional Ti-Ru-SBA-15 catalyst with an ordered
two-dimensional hexagonal mesostructure was prepared by postmodifying organometallic
i
complexes RuCl
2
(PPh
3
)
3
and Ti(O Pr)
4
onto PPh -SBA-15. During the tandem
2
addition-isomerization reaction of benzaldehyde under microwave irradiation in aqueous media,
the mesoporous silica-supported Ti-Ru-SBA-15 catalyst exhibited high catalytic activity (more
than 97%) and selectivity (up to 96%). Such a catalyst could be recovered easily and used
repetitively five times without significantly affecting its catalytic activity and selectivity.
Introduction
to be environmentally friendly in these fields. A number of
8–10
successful examples have appeared in the literature.
Among
The tandem reaction has a significant impact on the manufacture
of fine chemicals and pharmaceutical intermediates due to atom
economy and minimum workup. A number of tandem reactions
employing homogeneous bimetal complexes as catalysts have
these catalysts, titanium complexes are highly efficient allylation
9
catalysts while ruthenium complexes are extensively employed
10
in catalytic isomerization of allylic alcohols. Although fruitful
achievements have been obtained in each research field, however,
the tandem Ti/Ru-catalyzed allylation-isomerization reaction
employing immobilization strategy in aqueous medium, espe-
cially introduction of microwave-assisted catalytic technology,
is not available.
1
been reported in the literature. However, apart from compli-
cated catalyst compatibility with residual materials (solvent,
additives and other catalysts) in solution, these homogeneous
bimetal catalysts are often difficult to recycle. To overcome
these problems, a practical strategy is to immobilize them onto
Recently, we have reported a series of mesoporous catalysts
2
supports. An immobilization strategy does not only eliminate
11
and their applications in green catalytic processes. Especially,
complicated interactions, but also solves recovery and reuse of
catalyst. More importantly, this kind of strategy can provide
more clean product due to less product contamination caused
by metal leaching. Mesoporous materials as a kind of ideal
support for immobilization of catalysts have showed some
microwave-assisted Ti-catalyzed allyation of aldehydes has
showed highly catalytic activity in solid media. As an extension
of our previous studies, we herein develop a facile preparation
9e
of mesoporous silica-supported bifunctional catalyst 1 by a
11b
postmodification method based on PPh -SBA-15, and apply
2
3
salient features. These mesoporous silica-supported catalysts
it to the microwave-assisted tandem Ti/Ru-catalyzed allylation-
isomerization reaction of benzaldehyde in aqueous media. The
research focuses on the following: (1) construction of a highly
ordered mesoporous silica-supported bifunctional Ti-Ru-SBA-
possess a high surface area and ordered mesopore channels,
which are beneficial to enhancement of loading amounts and
dispersion of catalytically active sites. Furthermore, they also
have remarkable thermal and mechanical stability, and are easy
and reliable to reuse via simple nanofiltration. Obviously, the
design of mesoporous silica-supported bifunctional catalysts
represents a rapidly growing field that is on the verge of being
applied in industry.
1
5 catalyst; (2) investigation of its catalytic performance in the
tandem allylation-isomerization reaction in aqueous media.
4
Catalytic allylation of carbonyl compounds and isomeriza-
Experimental
5
tion of homoallylic alcohols are two important methods to
General
prepare alcohols, which serve as important fundamental build-
ing blocks in many biologically active compounds. Exploration
of their industrial applications has led to great development
in nontraditional technologies and methodologies for cleaner
and more benign chemical processes based on environmental
The mesoporous silica-supported catalyst, abbreviated as
Ti-Ru-SBA-15 (1), was prepared by a postmodification
method. As shown in Scheme 1, Ru-SBA-15 was prepared
by the reaction of PPh
the reported method.
2
11b
-SBA-15 with RuCl
2
(PPh
was then anchored onto
Ru-SBA-15 followed by protection of the silicon-hydroxyl
3
)
3
following
6
demands. Microwave-assisted catalytic technology and aqueous
i
Ti(O Pr)
4
7
methodology for organic reactions have been well-known
12
groups using HMDS as reagents to afford the mesoporous
silica-supported bifunctional catalyst Ti-Ru-SBA-15 (1) as a
light yellow powder. For comparison, Ti-SBA-15 (2) was also
synthesized according to a similar method.
Department of Chemistry, Shanghai Normal University, Shanghai,
2
00234, China. E-mail: ghliu@shnu.edu.cn, HeXing-Li@shnu.edu.cn;
Fax: 86-21-64322272; Tel: 86-21-64321819
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1
3
(
6
d = -93 ppm); C CP/MAS (100.6 MHz): 16.6, 25.1, 58.9 and
8.1 ppm.
Characterization
Ru and Ti content was analyzed using an inductively coupled
plasma optical emission spectrometer (ICP, Varian VISTA-
MPX). X-ray powder diffraction (XRD) experiments were
carried out on a Rigaku D/Max-RB diffractometer with Cu
Ka radiation. Transmission electron microscopy (TEM) studies
were performed on a JEOL JEM2010 electron microscope,
operated at an acceleration voltage of 200 kV. Fourier transform
infrared (FTIR) spectra were collected on a Nicolet Magna 550
spectrometer using KBr method. Nitrogen adsorption isotherms
were measured at 77 K after being outgassed at 423 K overnight
on a Quantachrome Nova 4000 analyzer. Pore size distributions
and specific surface areas (SBET) were calculated using BJH
Scheme 1 Synthesis of 1 and 2.
Catalyst preparation
Preparation of Ti-Ru-SBA-15 (1). Ru-SBA-15 [(poresize of
◦
5
4
(
.5), 1.0 g] was dehydrated at 125 C under 0.01 Torr for
2
9
model and BET method, respectively. Solid-state Si MAS
h before addition of the fresh titanium(IV) isopropoxide
0.50 mL, 1.45 mmol) in dry toluene (20 mL). The resulting
mixture was stirred and refluxed for 24 h under an argon
atmosphere, during which time titanium(IV) isopropoxide
was grafted onto the supports. After being cooled, filtrated,
1
3
NMR and C CP MAS NMR spectra were recorded on a Bruker
AV-400 spectrometer.
Activity test
and washed thoroughly with toluene and CH
were transferred to a fresh Schlenk. Then a solution HMDS
(CH Si) N] (5.0 mL, 25.0 mmol) in 25 mL dry toluene was
slowly added to this Schlenk. The resulting suspension was
stirred overnight. Then the residues were filtrated and washed
twice with dry toluene. After Soxlet extraction in toluene
solvent to remove unreacted starting materials, the solid was
dried under reduced pressure overnight to afford 1 (1.06 g,
2
Cl
2
, the solids
Catalytic isomerization of 1-phenyl-3-buten-l-ol under mi-
crowave irradiation in aqueous media. 1-phenyl-3-buten-l-ol
(0.025 mL, 0.25 mmol) was added to a suspension of Ti-Ru-
SBA-15 (1) (42.6 mg, 0.014 mmol, based on Ru from ICP;
0.050 mmol, based on Ti from ICP) in 2 mL of water at
room temperature in a thick walled Pyrex tube. When the
addition was complete, the tube was positioned in a MAS-2
single mode cavity microwave with a water-cooled condenser
from Sineo Microwave Chemistry Technology (China) Co.
[
3
)
3
2
1
4.5% relative to titanium(IV) isopropoxide) in the form of
-
1
◦
a light yellow powder. IR (KBr) cm : 3441 (s), 3076 (w),
LTD, adjusting the reaction temperature button at 100 C and
2
1
4
958 (w), 2869 (w), 1650 (m), 1625 (w), 1433 (w), 1320 (w),
086 (s), 952 (w), 848 (m), 804 (w), 756 (w), 694 (w), 565 (w),
61 (m) cm ; Ru contents: 33.2 mg/g (0.33 mmol/g); Ti con-
producing continuous irradiation at 2.45 GHz, and the mixture
was irradiated with 700 W for 10 minutes. Then 2 mL of ethyl
acetate was added. After being filtrated, the organic layer was
dried over Na SO and concentrated. The residue was further
purified by flash column chromatography on silica gel (eluent:
ethyl acetate/hexane = 1:4) to afford 4-phenyl-3-buten-2-ol as a
colorless liquid.
-
1
tents: 56.4 mg/g (1.18 mmol/g); Anal. Found: C 30.07, H 3.22;
S
2
4
2
3
29
BET: 347 m /g, Vpore: 0.47 cm /g, dpore: 3.4 nm; Si MAS/NMR
4
3
3
(
-
79.5 MHz): Q (d = -114 ppm), Q (d = -103 ppm) and T (d =
1
3
72 ppm); C CP/MAS (100.6 MHz): 16.8, 26.3, 61.2, 69.7 and
1
31.4 ppm.
Catalytic tandem allylation-isomerization reaction under mi-
crowave irradiation in aqueous media. Tetraallyltin (0.015 mL,
0.065 mmol) and benzaldehyde (0.025 mL, 0.25 mmol)
was added to a suspension of Ti-Ru-SBA-15 (1) (42.6 mg,
0.014 mmol, based on Ru from ICP; 0.050 mmol, based on Ti
from ICP) in 2 mL of water at room temperature in a thick walled
Pyrex tube. When the addition was complete, the tube was posi-
tioned in a MAS-2 single mode cavity microwave with a water-
cooled condenser from Sineo Microwave Chemistry Technology
(China) Co., LTD, adjusting the reaction temperature button at
Preparation of Ti-SBA-15 (2). SBA-15 [(poresize of 7.6),
◦
1.0 g] was dehydrated at 125 C under 0.01 Torr for 4 h
before the addition of the fresh titanium(IV) isopropoxide
0.50 mL, 1.45 mmol) in dry toluene (20 mL). The resulting
(
mixture was stirred and refluxed for 24 h under an argon
atmosphere, during which time titanium(IV) isopropoxide was
grafted onto the supports. Then the residues were filtrated and
washed twice with dry toluene. After Soxlet extraction in toluene
solvent to remove unreacted start materials, the solid was dried
under reduced pressure overnight to afford 2 (1.07 g, 17.0%
relative to titanium(IV) isopropoxide) in the form of a white
◦
100 C and producing continuous irradiation at 2.45 GHz, and
the mixture was irradiated with 700 W for 10 minutes. Then
2 mL of ethyl acetate was added. After being filtrated, the solid
was washed several times with ethyl acetate, and finally dried at
-
1
powder. IR (KBr) cm : 3428 (s), 2986 (w), 2913 (w), 2851 (w),
◦
1
8
6
4
634 (m), 1512 (w), 1447 (w), 1343 (w), 1076 (s), 951 (w),
85 C under vacuum for recycling-experiments. The the organic
-
1
48 (m), 804 (w), 753 (w), 694 (w), 565 (w) cm ; Ti contents:
1.5 mg/g (1.28 mmol/g); Anal. Found: C 21.04, H 1.57; SBET
31 m /g, Vpore: 0.51 cm /g, dpore: 5.1 nm; Si MAS/NMR
layer was dried over Na
2
SO and concentrated. The residue was
4
:
further purified by flash column chromatography on silica gel
(eluent: ethyl acetate/hexane = 1:4) to afford 4-phenyl-3-buten-
2-ol as a colorless liquid.
2
3
29
4
3
2
(
79.5 MHz): Q (d = -111 ppm), Q (d = -103 ppm) and Q
1
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Results and discussion
{Si(OSi)
4
} and {(HO)Si(OSi)
3
} and the formation of RSi(OSi) }
3
(
R = organometallic complexes) as a part of the wall in the
13
FTIR spectra of 1–2 are shown in Fig. 1. Besides the general
characteristic peaks of pure SBA-15, the catalyst 1 displays
similar peaks when compared to Ru-SBA-15 observed in
mesoporous structure. The C CP/MAS NMR spectrum of 1
displays a peak at 131.4 ppm for benzene rings, and at 16.8, 26.3,
6
1.2 and 69.7 ppm for the others, which are marked in Fig. 2b.
11b
the literature. The main differences are that the catalyst 1
13
The C CP/MAS NMR spectrum of 2 further confirms these
chemical shift values (16.6, 25.1, 58.9 and 68.1). For comparison,
it is easily found that there are two groups of aliphatic
-
1
displays an abrupt decrease of band intensity around 3441 cm ,
suggesting substitution of a large fraction of surface OH groups
2
9
by methyl groups. The Si CP/MAS NMR measurements
i
carbon atoms of isopropoxide, corresponding to Ti-O- Pr and
4
(
(
Fig. 2) show clearly that 1 presents two strong Q peaks [Q
-114 ppm), Q (-103 ppm)] and a medium T peak (-72 ppm)
i
Si-O- Pr groups, as marked in Fig 2b. From the intensity of
3
3
peaks in 1, the peaks at 16.8 ppm and 61.2 ppm corresponding
4
3
while 2 only gives Q peaks [Q (-111 ppm), Q (-103 ppm)
i
to Ti-O- Pr groups are obviously enhanced. The enhanced peaks
2
and Q (-93 ppm)]. As compared with typical isomer shift
indicate that carbon atoms of the -PCH
2
CH
groups are overlapped, in which the shift values around
6.8 ppm are ascribed to the carbon atoms of the -CH Si- and
groups, and around 61.2 ppm are attributed to the carbon
atom of the -PCH - groups. All these observations confirm the
2
Si- moiety and
3
2
1
values of -48.5, -58.5 and -67.5 ppm for T /T /T /signals
-
SiMe
3
3
2
1
(
T {RSi(OSi)
3
}, T {R(HO)Si(OSi)
2
} and T {R(HO)
2
2
SiOSi}),
1
2
4
3
and of -91.5, -101.5, -110 ppm for Q /Q /Q signals
-
SiMe
3
4
3
2
13
(
Q {Si(OSi)
4
}, Q {(HO)Si(OSi)
3
} and Q {(HO)
2
Si(OSi)
2
}),
2
4
3
3
the strong Q , Q peaks and the relatively weak T peak
successful postmodificaton of organometallic complexes onto
the mesoporous support.
in 1 suggest that 1 possesses mainly a network structure of
Fig. 3 shows XRD patterns of 1 and 2. Similar to PPh -
2
11b
SBA-15 and Ru-SBA-15, 1 and 2 also exhibit one intense
100 diffraction and two weak peaks indicative of d110 and d200
diffractions, implying that the ordered dimensional-hexagonal
d
14
mesostructure (p6mm) could be well preserved. The TEM
morphologies further confirm that 1 and 2 have well-ordered
mesostructures with the dimensional-hexagonal arrangement
as shown in Fig. 4. Nitrogen adsorption-desorption isotherms
of 1 and 2 in Fig. 5 exhibit typical IV type N
2
adsorption-
desorption isotherms with a H hysteresis loop and a visible step
1
at P/P
0
= 0.40–0.80, corresponding to capillary condensation
i
of nitrogen in mesopores. The incorporation of Ti(O Pr)
4
onto
SBA-15 or Ru-SBA-15 causes a slight decrease in mesopore
size, surface area, and pore volume when compared to data in
the literature, obviously due to coverage of pore surface with
the organometallic complexes, leading to an increase of the wall
Fig. 1 IR spectra of 1 and 2.
11b
thickness.
Fig. 3 The powder XRD patterns of 1 and 2.
With the mesoporous silica-supported catalyst 1 in hand, we
firstly examined its catalytic isomerization of 1-phenyl-3-buten-
l-ol under microwave irradiation in aqueous media. These results
are collected in Table 1. Because such a reaction has a marked
dependence on the molar ratio of the Ru(II)/substrate, a few
reaction conditions are screened. From entries 3–5, one can see
29
15
Fig. 2 The solid-state NMR spectra of 1 and 2. (a) Si MAS NMR
and (b) C CP MAS NMR.
13
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Table 1 Ma icrowave-assisted catalytic isomerization of 1-phenyl-3-
buten-l-ol
Ru content
(%)
b
Entry Catalyst
Time (min) Conv. (%) Select. (%)
c
1
2
3
4
5
6
RuCl(PPh
Ru-SBA-15 5.6
1
1
1
1
3
)
3
5.6
1200(10)
1200(10)
10
10(180)
10(10)
10
79(99.9)
77(99.8)
92.7
95 (97.7)
c
94 (97.5)
2.4
4.0
5.6
7.2
88.9
92.3(95)
d
93.4(87)
e
96.3(99.4) 97.4(73.8)
99.4 73.8
a
Reaction conditions: MAS-2 single mode cavity microwave with a
water-cooled condenser, catalyst (42.6 mg, 0.014 mmol, based on
Ru from ICP), 1-phenyl-3-buten-l-ol (0.25 mmol), reaction time (10
Fig. 5 Nitrogen adsorption-desorption isotherms of 1 and 2.
◦
b
c
min), and reaction temperature 100 C. HPLC yields. Data were
1
1b
d
be completed at a much shorter reaction time (10 minutes).
Such a reaction rate is much faster than the corresponding
Ru-SBA-15 and the parent catalyst without microwave irra-
diation, suggesting the microwave radiation can obviously
accelerate the catalytic isomerization reaction. In order to test
the role of water in the catalytic process, the isomerization of
obtained from the literature without microwave irradiation.
Data
15 e
were obtained from the literature without microwave irradiation. Data
were obtained from the literature under microwave irradiation in solid
media.
1
-phenyl-3-buten-l-ol under microwave irradiation in solid
media is also carried out. It is found that 99.4% conversion
and 73.8% selectivity are obtained (entry 5 shown in brackets).
Although the conversion increases slightly, the selectivity is low
when compared to that in aqueous media (entry 5 versus entry
5
in brackets), which is due to the increase of the by-product
3c. Such a result suggests that the oxidation derived from the
locally high temperature under microwave irradiation in solid
media might be responsible for this phenomenon.
On the basis of the optimized molar ratio of Ru(II)/substrate,
we then tested the tandem reaction, catalytic allylation of ben-
zaldehyde followed by isomerization, under microwave irradia-
tion in aqueous media. In order to afford almost pure allylated
product and to prevent pollution from large amounts of hy-
drolyzed inorganic tin compounds during work-up stage, a small
11c,16
excess of tetraallyltin as nucleophile reagent is employed.
can be seen from Table 2, benzaldehyde is cleanly reacted with
.28 equiv. of tetraallyltin to provide the product 3b in 98.8%
As
0
Fig. 4 The TEM images of 1 and 2 viewed along the [100] and [001]
conversion and 95.5% selectivity (entry 1) with a small amount
of by-product 3a and 3c, which was obviously better than those
obtained using Ti-Ru-SBA-15 in organic solvent and in water
without microwave irradiation under the similar reaction condi-
tions (entries 2 and 3). Such a tandem allylation-isomerization
reaction could also be completed with the same result on a
large scale using 10.0 mmol of benzaldehyde as substrate. In
order to explore the nature of the catalytic performance, two
control experiments were also carried out using Ti-SBA-15 and
directions.
easily that the isomerization reactions employing 2.4%, 4.0% and
5
9
.6% Ru(II) metal (based on the data from ICP) afford 88.9%,
2.3% and 97.4% selectivity, respectively. Apparently, increasing
the amounts of catalyst result in increasing selectivity in product
formation. However, further increased amounts of catalyst to
7
.2% lead a low selectivity (entry 6). Although the selectivity of
catalyst 1 is slightly low when compared to the parent catalyst
employing 4.0% Ru(II) metal reported in the literature (entry 4
PPh
2
-SBA-15 plus RuCl
2
(PPh
3
) as catalysts under the similar
3
15
versus entry 4 shown in bracket), the selectivity employing 5.6%
Ru(II) metal is better than that obtained using the corresponding
Ru-SBA-15 catalyst without microwave irradiation (entry 5
reaction conditions. It is found that no product 3b is observed
in the former (main by-product 3a), while 90.3% selectivity is
obtained in the latter (entries 4 and 5). The former suggests
that the isomerization reaction is triggered from the ruthenium
complexes, while the latter indicates that the randomly oriented
11b
versus entries 1–2), and is nearly the same as that obtained
using the corresponding Ru-SBA-15 catalyst and the parent
homogeneous catalyst with microwave irradiation (entry 5 versus
entries 1 and 2 shown in brackets). More importantly, the
catalytic isomerization reaction under microwave irradiation can
RuCl
2
(PPh
3
3
) onto materials results in catalytic performance
that is further proved using Ru-SBA-15 as a catalyst with 92.1%
selectivity (entry 6). Comparing the latter with the mesoporous
1
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Table 2 Microwave-assisted tandem allylation-isomerizaion reaction
Development Fund (071005119 and 06JC14060), and
the Shanghai Municipal Education Commission (08YZ71,
7dz22303 and S30406) for financial support.
a
of benzaldehyde
0
References
b
Entry Catalyst
Run Conv. (%) Select. (%)
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1
2
3
4
5
6
7
8
9
0
1
1
1
1
1
1
1
2
3
4
5
98.8
87.5
76.4
95.5
56.5
38.1
9.7
90.3
92.1
95.4
94.3
91.5
91.0
c
d
e
1
1
2
>99
PPh
Ru-SBA-15
1
1
1
1
2
-SBA-15 + RuCl
2
(PPh
3
)
3
99.5
98.7
98.4
99.2
99.2
97.4
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f
f
1
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(
b) S. E. Denmark and J. Fu, Chem. Rev., 2003, 103, 2763.
a
Reaction conditions: MAS-2 single mode cavity microwave with a
water-cooled condenser, catalyst (42.6 mg, 0.014 mmol, based on
Ru from ICP; 0.050 mmol, based on Ti from ICP), tetraallyltin
0.065 mmol), benzaldehyde (0.25 mmol), reaction time (10 min), and
reaction temperature 100 C. HPLC yields. Data were obtained
using isopropanol as solvent without microwave irradiation; reaction
time (20 h). Data were obtained using water as solvent without
microwave irradiation; reaction time (20 h). Data were obtained using
(39.1 mg (0.10 mmol), based on Ti from ICP) under similar conditions.
5
6
N. Ku z´ nik and S. Krompiec, Coordin. Chem. Rev., 2007, 251, 222.
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(
6
b) B. A. Roberts and C. R. Strauss, Acc. Chem. Res., 2005, 38,
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(
◦
b
c
7
8
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d
e
2
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2
f
Recovered catalyst was used.
2
(
silica-supported Ti-Ru-SBA-15 catalyst, the higher selectivity
of Ti-Ru-SBA-15 and Ru-SBA-15 than the latter are due to the
regularly dispersive arrangement of the catalytic species onto
a highly ordered mesoporous materials. Such an arrangement
does not only restrict aggregation of the catalytic species but
also is beneficial to recognition of the substrate, resulting in a
high efficiency of the catalytic reaction.
More importantly, the design of the mesoporous silica-
supported bifunctional catalyst Ti-Ru-SBA-15 is easy and
reliable to separate via simple filtration. For example, upon
completion of the reaction, the catalyst 1 is quantitatively
recovered via filtration. The recycling catalyst can be reused
five times without obviously affecting its catalytic activity and
selectivity, which is due to the slight loss of metal confirmed by
ICP analysis after five recycles (entries 7–10).
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(
c) J. M. Brunel, Chem. Rev., 2005, 105, 857; (d) A. J. Wooten, J. G.
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184.
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In summary, we have developed an efficient method for the
tandem allylation-isomerizaion reaction of benzaldehyde under
microwave irradiation in aqueous media. The mesostructured
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five times without loss of its catalytic activity and selectivity.
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We are grateful to the China National Natural Science
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