Hydrosilylation catalyzed with Rh(PPh3)3Cl/ionic- liquid-functionalized SiO2

Article abstract of DOI:10.1080/10426500902823958

Ionic liquid-modified silica has been prepared by a one-pot reaction of activated silica, 3-chloropropyltriethoxysilane, and alkylimidazole or pyridine. It was found that the catalytic activity and β-adduct selectivity of the supported catalyst Rh(PPh₃)3Cl/ionic-liquid-modified-SiO₂ for the hydrosilylation reaction of alkenes with triethoxysilane was significantly improved. Furthermore, the catalyst system could be recovered easily.

Full text of DOI:10.1080/10426500902823958

Phosphorus, Sulfur, and Silicon, 185:484–490, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902823958
HYDROSILYLATION CATALYZED WITH
Rh(PPh3)3Cl/IONIC-LIQUID–FUNCTIONALIZED SiO2
Jiayun Li,1,2 Jiajian Peng,1 Ying Bai,1 Yingqian Hu,1
Huayu Qiu,1 Guoqiao Lai,1 and Xiaonian Li2
1Key Laboratory of Organosilicon Chemistry and Material Technology,
Ministry of Education, Hangzhou Normal University, Hangzhou, PR China
2Resources & Environment Catalysis Institute of Zhejiang University of
Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis
Technology, Hangzhou, PR China
Ionic liquid–modified silica has been prepared by a “one-pot” reaction of activated sil-
ica, 3-chloropropyltriethoxysilane, and alkylimidazole or pyridine. It was found that the
catalytic activity and β-adduct selectivity of the supported catalyst Rh(PPh3)3Cl/ionic-
liquid–modified-SiO2 for the hydrosilylation reaction of alkenes with triethoxysilane was
significantly improved. Furthermore, the catalyst system could be recovered easily.
Keywords Hydrosilylation; ionic-liquid–functionalized SiO2; recyclable; Rh(PPh3)3Cl
INTRODUCTION
Room temperature ionic liquids (ILs) have attracted considerable attention in recent
years due to their special physical and chemical properties. Ionic liquids can give rise
to a number of particular advantages over their molecular counterparts.1,2 Although the
application of ionic liquids in diverse areas has been well developed over the last two
decades, some problems such as cost must be tackled, and better ways to recycle ionic
liquids need to be found. Since 2001, many authors3–5 have found that immobilization on
solid materials such as SiO2 can minimize the consumption of ionic liquids and facilitate
the reuse of catalysts. Pyridium and imidazolium cation–based ionic liquid functionalized
silica has been applied to several organic reactions, e.g., Kharasch addition,6 Knovenagel
condensation,7 and hydroformylation,8 as well as a new stationary phase for anion-exchange
chromatography.9
Hydrosilylation is one of the most important reactions for Si C bond formation
in organosilicon chemistry.10,11 Transition metal complexes have usually been applied as
Received 3 July 2008; accepted 13 February 2009.
We thank the Distinguished Professor’s Fund of Zhejiang province (200508) and the Foundation of Zhejiang
Educational Committee (20070466) for financial support.
Address correspondence to Guoqiao Lai, Key Laboratory of Organosilicon Chemistry and Material
Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China. E-mail:
gqlai@hznu.edu.cn and Xiaonian Li, Resources & Environment Catalysis Institute of Zhejiang University of
Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou 310032,
P.R. China. E-mail: xnli@zjut.edu.cn
484

CATALYZED HYDROSILYLATION
485
catalysts for hydrosilylation reactions. However, the types of hydrosilylation catalysts avail-
able are not beneficial for product separation and are expensive.12,13 Very recently, Weyer-
shausen et al.14 reported that hydrosilylation reactions catalyzed with platinum complexes
were remarkably improved by using ionic liquids as media. Also, hydrosilylation reactions
catalyzed with Rh(PPh3)3Cl in ionic liquids were studied in our laboratory.15,16 Along with
research to minimize the amount of ionic liquid and noble metal catalyst in hydrosilylation
processes, we report in this article that IL-functionalized SiO2 can be prepared by a one-step
procedure. The Rh(PPh3)3Cl was then supported on the IL-functionalized SiO2 to obtain the
Rh(PPh3)3Cl/IL-functionalized SiO2 catalyst. Subsequently, the hydrosilylation of styrene
and α-olefins with triethoxysilane catalyzed with this Rh(PPh3)3Cl/IL-functionalized SiO2
catalyst was investigated (Scheme 1).
RCH2CH2Si(OEt)3
β
RCHSi(OEt)3CH3
+
(EtO)3SiH
R
α
R
R= Ph, n-C4H9, n-C5H11, n-C6H13, n-C9H19
Scheme 1 Hydrosilylation of olefins with triethoxysilane.
RESULTS AND DISCUSSION
Infrared spectroscopy is a useful tool to confirm the chemical modifications pro-
posed. Diffuse reflection infrared Fourier-transform spectra are shown in Figure 1 for
silica before (A) and after the modification with N,Nꢁ-dialkylimidazolium chloride (B),
N,Nꢁ-dialkylimidazolium hexafluorophosphate (C), N-alkylpyridinium chloride (D), and
N-alkylpyridinium hexafluorophosphate (E).
Two new bands in the spectra of B and C at 1565 and 1160 cm−1 are assigned to the
presence of the imidazolium ring. The new band in the spectra of D and E at 1490 cm−1
is assigned to the presence of the pyridinium ring. The new band in the spectra of C
and E at 840 cm−1 is assigned to the presence of the hexafluorophosphate. These results
demonstrate that the ILs were linked to the silica. Moreover, elemental analysis data and
maximal theoretical values for IL-functionalized silica are in close agreement (Table I). The
Table I Elemental analysis data (%) of IL over silica supports
C
H
N
IL/SiO2 (wt.%)
B
C
D
E
5.35
4.11
6.67
4.04
0.66
0.61
0.53
0.41
1.69
1.05
0.85
0.60
9.60
9.60
9.55
10.84

486
J. LI ET AL.
E
D
C
B
A
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers(cm-1)
Figure 1 IR spectra of (A) unmodified silica; (B, C) methylimidazolium-functionalized silica; and (D, E)
pyridinium-functionalized silica.
results prove that IL-functionalized silica material can be obtained by a one-pot reaction of
silica, 3-chloropropyltriethoxysilane, and pyridine or N-methylimidazole.
Results on the hydrosilylation reaction of the alkenes with triethoxysilane cat-
alyzed with Rh(PPh3)3Cl/SiO2 are listed in Table II. These results indicate that the
Rh(PPh3)3Cl/unmodified-SiO2 catalyst alone exhibits only a low selectivity for the hy-
drosilylation of styrene. However, the Rh(PPh3)3Cl/modified-SiO2 catalyst exhibits high
catalytic activity and selectivity in alkene hydrosilylation reactions. Moreover, higher se-
lectivity was obtained for hexafluorophosphate IL-functionalized silica. On the basis of
these data, we propose the reaction mechanism shown in Scheme 2.
Scheme 2 The reaction mechanism.

CATALYZED HYDROSILYLATION
487
Table II Effect of unmodified silica and modified silica on the hydrosilylation reaction of styrene with
triethoxysilane catalyzed with Rh(PPh3)3Cl
Selectivity (%)
Amount of
Entry
catalyst (g)
Conv. (%)
β
α
Alkane
1
2
3
4
5
6
7
8
9∗
10∗
11∗
12∗
13∗
14∗
15∗
16∗
17∗
18∗
Rh(PPh3)3Cl/A, 0.1
Rh(PPh3)3Cl/B, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.05
Rh(PPh3)3Cl/C, 0.025
Rh(PPh3)3Cl/C, 0.01
Rh(PPh3)3Cl/D, 0.1
Rh(PPh3)3Cl/E, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
Rh(PPh3)3Cl/C, 0.1
100
100
100
92.4
81.0
60.8
100
100
100
100
100
100
100
100
100
100
99.9
99.5
72.8
86.8
87.8
87.6
87.6
87.6
87.6
91.3
87.7
87.7
87.8
87.8
87.7
87.7
87.7
87.7
87.7
87.7
15.3
2.4
0.3
0.3
0.2
0.2
3.1
0.5
0.4
0.4
0.4
0.3
0.3
0.4
0.3
0.4
0.4
0.4
11.9
10.8
11.9
12.1
12.2
12.2
9.3
8.2
11.9
11.9
11.8
11.9
12.0
11.9
12.0
11.9
11.9
11.9
Reaction conditions: styrene, 2.5 mmol; triethoxysilane, 3.0 mmol; 90◦C,
Rh(PPh3)3Cl/SiO2.
∗Recyclable of C catalytic system.
2
h; Catalyst: 0.24%
An IL membrane forms on the surface of SiO2 after IL treatment. A possible reaction
between this membrane and the central metal Rh could weaken the complexing ability of
other ligands such as chloride, which would help to activate the alkene, and the electriferous
IL membrane could stabilize the intermediate states. It can also be speculated that steric
hindrance of the substituents attached to the cation membrane has some effect on the
reaction pathway, and the selectivity of the formation of the β-adduct is increased. At the
same time, some ethylbenzene was produced. The experimental details will be described
and discussed in a subsequent article. The hydrosilylation reactions were examined in the
presence of different amounts of rhodium loading (entries 3–6, Table II). It was found that
the conversion of styrene increases with increasing amounts of rhodium loading, while the
selectivity is almost constant.
Table III Effect of modified silica on the hydrosilylation reaction of alkenes with triethoxysilane catalyzed with
Rh(PPh3)3Cl
Selectivity (%)
Amount of
Entry
catalyst (g)
Alkene
Conv. (%)
β
α
Alkane
1
2
3
4
Rh(PPh3)3Cl/B, 0.01
Rh(PPh3)3Cl/B, 0.01
Rh(PPh3)3Cl/B, 0.01
Rh(PPh3)3Cl/B, 0.01
Hexene
Heptene
Octene
100
100
100
100
93.6
93.3
93.3
93.0
—
—
—
—
6.4
6.7
6.7
7.0
Undecene
Reaction conditions: alkene, 2.5 mmol; triethoxysilane, 3.0 mmol; 90◦C,
Rh(PPh3)3Cl/SiO2.
2 h; Catalyst: 0.24%

488
J. LI ET AL.
When aliphatic alkenes such as 1-hexene, 1-heptene, 1-octene, or 1-undecene instead
of styrene are use as substrates, excellent conversions and selectivities are obtained with
different Rh(PPh3)3Cl/B also (Table III).
The use of Rh(PPh3)3Cl/modified-SiO2 as a catalyst is advantageous for the product
separation at the end of the reaction. In general, Rh(PPh3)3Cl/modified SiO2 showed
excellent stability for the hydrosilylation of styrene and triethoxysilane. For example,
the Rh(PPh3)3Cl/SiO2 (CH2CH2CH2)MimPF6 catalyst system could be recovered easily
and reused more than 10 times without any notable loss of catalytic activity or selectivity
(Table II).
CONCLUSION
The catalytic activity and the β-adduct selectivity of supported catalysts
Rh(PPh3)3Cl/ionic-liquid–modified-SiO2 for the hydrosilylation reaction of alkenes
with triethoxysilane is significantly improved. The Rh(PPh3)3Cl/modified-SiO2 catalyst
system can be recovered easily and reused more than 10 times without any notable loss of
catalytic activity or selectivity.
EXPERIMENTAL
Pretreatment of Silica
SiO2 (200–300 mesh) was dried at 150◦C in a vacuum to constant weight.
Preparation of IL-Functionalized Silica According to Scheme 3
SiO2 (5 g, 200–300 mesh), 3-chloropropyltriethoxysilane (0.95 g), and N-
methylimidazole (0.31 g) or pyridine (0.32 g) were dissolved in acetone, then stirred
for 2 h and evaporated under reduced pressure. The sample was placed in a 10-mL
stainless steel autoclave, which was evacuated, sealed, and then heated to 110◦C for 24 h.
The residue was washed with ethyl acetate/diethyl ether and dried at 80◦C in a vacuum
for 24 h to afford B or D. B or D and NH4PF6 in H2O were stirred overnight at room
temperature. The residue was washed with H2O and dried at 90◦C in a vacuum for 24 h to
afford C or E.
Preparation of the Rh(PPh3)3Cl-Modified-SiO2 Catalyst
IL-functionalized silica (2 g) was mixed with Rh(PPh3)3Cl (0.0048 g) in 10 mL
of CHCl3. The mixture was stirred overnight under nitrogen. The solvent was evapo-
rated under reduced pressure and the residue dried at 90◦C in a vacuum to afford the
Rh(PPh3)3Cl/modified-SiO2 catalyst.
Hydrosilylation of Alkenes with Triethoxysilane
A given amount of catalyst was charged into a 10-mL round-bottomed flask equipped
with a magnetic stirrer, and the alkene and silane were added under ambient conditions. This
mixture was heated to the desired temperature, and the hydrosilylation reaction proceeded
with constant stirring for 2 h. At the end of the reaction, the product phase was separated

CATALYZED HYDROSILYLATION
489
OEt
Cl
O
OH
OH
SiCH2CH2CH3
SiO2
N
N
SiO2
+ Cl(CH2)3Si(OEt)3+
N
N
O
A
B
OEt
PF6
N
OEt
Cl
O
O
O
O
N
SiO2
SiCH2CH2CH3
C
SiO2
N
N
SiCH2CH2CH3
B
OEt
O
Cl
N
OH
SiCH2CH2CH2
+ Cl(CH2)3Si(OEt)3+
N
SiO2
SiO2
OH
O
A
D
OEt
OEt
Cl
PF6
O
O
O
O
SiO2
SiCH2CH2CH2
E
SiCH2CH2CH2
D
N
SiO2
N
Scheme 3 Preparation of IL-functionalized silica.
from the catalyst by decantation, and the alkene conversion and selectivity were determined
by GC-MS [Trace DSQ GC-MS system using a DB-5 column (30 m × 2.5 mm, 0.25 µm)].
The catalyst was recharged with fresh alkene and silane for the next catalytic run.
Characterization of IL-Functionalized Silica
Elemental analyses were performed on a VARIO EL-3 elemental analyzer and IR
spectra were recorded on a Nicolet 5700 instrument.
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