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Use of carboxylated polyethylene glycol as promoter for platinum-catalyzed hydrosilylation of alkenes.

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Communication
Received: 24 July 2010
Revised: 25 November 2010
Accepted: 25 November 2010
Published online in Wiley Online Library: 4 April 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1776
Use of carboxylated polyethylene glycol
as promoter for platinum-catalyzed
hydrosilylation of alkenes
Ying Bai, Jiajian Peng∗, Jiayun Li and Guoqiao Lai∗
Several carboxylated polyethylene glycols as promoters were applied in the platinum-catalyzed hydrosilylation of alkenes,
and polyethylene glycol maleic acid monoester as a promoter for hydrosilylation was investigated. It was found that an
improvement of the selectivity was achieved in the presence of carboxylated polyethylene glycol, and the β-adduct as major
product was obtained. Additionally, the effect of alkenes and silanes employed on the selectivity was investigated; better
c 2011 John Wiley & Sons, Ltd.
selectivity could be achieved when (EtO)3 SiH was used as the hydride than ClMe2 SiH. Copyright Keywords: hydrosilylation; platinum catalyst; carboxylated polyethylene glycol; promoter
Introduction
400
Transition metal-catalyzed hydrosilylation of carbon-carbon double bonds is one of the most important commercial reactions.
This process is usually used to synthesize functionalized silane,
polysilane and other organosilicon compounds.[1 – 4] Since the
hydrosilylation reaction was identified, various catalysts for its
accomplishment have been successively researched and developed. In 1947, Speier’s catalyst (H2 PtCl6 ·6H2 O/isopropanol)
was first found to be effective in the hydrosilylation of
alkynes and alkenes with silanes.[5 – 11] Thereafter, Karstedt’s
catalyst (Pt complex with 1,3-divinyltetramethyldisiloxane) gradually replaced Speier’s catalyst by virtue of its higher
activity.[12 – 14] However, low selectivities of the product were
observed with some particular terminal alkenes. In particular,
catalysis hydrosilylation of styrene with hydrosilane, a model
reaction often used to check the catalytic performance of
catalysts, has widely been reported,[15 – 21] and platinum catalysts always showed high activity; however, selectivity was
unsatisfactory.[22 – 25]
Hence, the development of new catalysts is an ongoing
challenge and several other active platinum catalysts have been
reported.[23,26 – 31] For example, platinum supported on silica
modified with polyethylene glycol has shown high and stable
activity for both vapor- and liquid-phase hydrosilylation; however,
the selectivity of the adduct was still not satisfactory.[32,33] On
the other hand, it was found that organic fatty acids could
be used as promoters in hydrosilylation reactions.[34,35] Very
recently, our group demonstrated that an improvement of
the activity and the selectivity of platinum catalysts could be
achieved using aminoaromatic acids as promoters.[36] On this
basis and following interest in the use of polyethylene glycol
and derivatives as promoters in organic synthesis, we wish to
report here that carboxylated polyethylene glycol (PEGCOOH)
can also efficiently improve both the activity and the selectivity
of platinum catalyst for the hydrosilylation of various alkenes
(Scheme 1).
Appl. Organometal. Chem. 2011, 25, 400–405
Experimental
1H
NMR spectra were recorded on a Bruker Advance 400 MHz
spectrometer using TMS as an internal standard and CDCl3 as a
solvent. GC-MS analyses of the hydrosilylation reaction products
were obtained with Agilent 26890N/59731 equipped with a DB-5
column (30 m × 2.5 mm × 0.25 µm).
Synthesis of Carboxylated Polyethylene Glycol (PEGCOOH)
Polyethylene glycol (PEG, Mn = 600; 12.0 g) was dissolved in
toluene (100 ml) in a three-necked flask and the stirred solution
was dried by azeotropic distillation. After cooling, maleic anhydride
(4.7 g, 0.048 mol) was then added and the reaction mixture was
stirred for 6 h at 80 ◦ C under N2 atmosphere. The toluene was
then removed by distillation under reduced pressure; the crude
product was extracted three times with diethyl ether to remove
unreacted maleic anhydride, and a yellowish-brown product was
obtained. Several carboxylated polyethylene glycol was prepared
using a similar method.
Selected 1 H, 13 C and IR data of carboxylated polyethylene glycol
were as follows.
Maleic anhydride modified PEG(1000) (yield 87%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 3.52-3.30 (m, 90 H, -CH2 CH2 O-), 4.04
(s, 4 H, CH2 OCO), 6.00 (d, 2 H, J = 12.1 Hz, CHCHCOO), 6.06 (d, 2
H, J = 12.1 Hz, CHCHCOO). 13 C NMR (100 MHz, CDCl3 ), δ (ppm):
64.13, 68.38, 70.14, 128.66, 132.13, 165.30, 166.51. IR (KBr): 2872,
1958, 1713, 1643, 1455, 1350, 1250, 1105, 951, 846 cm−1 .
Maleic anhydride modified PEG(600) (yield 91%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 3.75-3.60 (m, 50 H, -CH2 CH2 O-), 4.12
(s, 4 H, CH2 OCO), 6.25 (d, 2 H, J = 12.3 Hz, CHCHCOO), 6.38 (d, 2
∗
Correspondence to: Jiajian Peng and Guoqiao Lai, Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou
Normal University, Hangzhou 310012, China. E-mail: gqlai@hznu.edu.cn
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry
of Education, Hangzhou Normal University, Hangzhou 310012, China
c 2011 John Wiley & Sons, Ltd.
Copyright Platinum-catalyzed hydrosilylation of alkenes
R2
R3SiH
R1
R2
[Pt]/PEGCOOH
∆
R1
R2
SiR3
R1
β-adduct (2)
1
R1 =Ph, R2 =H, 1a;
H
CH3
SiR3
α-adduct (3)
R1 = t-Bu, R2 =H, 1e;
PEGCOOH
R1 =p-Me-Ph, R2 =H, 1b;
R1 =p-Cl-Ph, R2 =H, 1c;
1f;
H3C
O
CH3
O
O
O
HO
O
O
n
OH
1g
R1 =Ph, R2 =Me, 1d;
CH2
Scheme 1. Regioselective hydrosilylation of alkenes catalyzed with Pt-PEGCOOH.
H, J = 12.2 Hz, CHCHCOO). 13 C NMR (100 MHz, CDCl3 ), δ (ppm):
64.51, 68.49, 70.30, 132.26, 132.95, 166.26, 167.10. IR (KBr): 2874,
1966, 1729, 1639, 1456, 1350, 1252, 1106, 951, 830 cm−1 .
Maleic anhydride modified PEG(400) (yield 92%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 3.69-3.33 (m, 34 H, -CH2 CH2 O-),
4.21 (s, 4 H, CH2 OCO),6.14 (d, 2 H, J = 12.2 Hz, CHCHCOO),
6.23 (d, 2 H, J = 12.2 Hz, CHCHCOO). 13 C NMR (100 MHz,
CDCl3 ), δ (ppm): 64.53, 68.47, 70.30, 128.66, 132.29, 165.66,
166.29. IR (KBr): 2908, 1958, 1731, 1642, 1411, 1213, 1104, 951,
824 cm−1 .
Maleic anhydride modified PEG(200) (yield 90%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 3.71-3.58 (m, 16H, -CH2 CH2 O-), 4.27
(s, 4 H, CH2 OCO), 6.24 (d, 2 H, J = 11.2 Hz, CHCHCOO),
6.30 (d, 2 H, J = 12.4 Hz, CHCHCOO). 13 C NMR (100 MHz,
CDCl3 ), δ (ppm): 64.70, 68.59, 70.24, 129.01, 132.39, 165.96,
167.00. IR (KBr): 2916, 1952, 1728, 1636, 1414, 1215, 1101, 977,
822 cm−1 .
Phthalic anhydride modified PEG(600) (yield 83%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 3.53-3.04 (m, 50H, -CH2 CH2 O-), 4.08 (s,
4 H, CH2 OCO), 7.42-7.19 (m, 8H. Ph). 13 C NMR (100 MHz, CDCl3 ), δ
(ppm): 60.99, 70.14, 72.24, 128.39, 128.77, 128.88, 130.68, 130.73,
130.87, 130.87, 131.94, 132.18, 132.33, 168.75, 168.10, 168.75. IR
(KBr): 2872, 1958, 1723, 1453, 1351, 1255, 1108, 951, 846 cm−1 .
Butanedioic anhydride modified PEG(600) (yield 88%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 2.43(m, 8H, OOCCH2 CH2 COO), 3.49-3.24
(m, 52H, -CH2 CH2 O-), 4.05 (s, 4 H, CH2 OCO). 13 C NMR (100 MHz,
CDCl3 ), δ (ppm): 28.65, 28.93, 63.62, 68.83, 70.33, 172.19. IR (KBr):
2873, 1734, 1455, 1350, 1250, 1106, 952, 844 cm−1 .
Glutaric anhydride modified PEG(600) (yield 85%): 1 H NMR
(400 MHz, CDCl3 ), δ (ppm): 1.73(s, 4H, CH2 CH2 CH2 ), 2.22 (m, 8H,
CH2 CH2 CH2 ), 3.45-3.28 (m, 52H, -CH2 CH2 O-), 4.03 (s, 4 H, CH2 OCO).
13
C NMR (100 MHz, CDCl3 ), δ (ppm):19.74, 32.92, 63.38, 68.86,
70.27, 172.89, 176.64. IR (KBr): 2912, 1732, 1455, 1352, 1250, 1105,
952, 846 cm−1 .
Preparation of [Pt]/PEGCOOH Catalyst System
Appl. Organometal. Chem. 2011, 25, 400–405
All operations were performed without protection from air. The
requisite amounts of catalyst and alkene (4.0 mmol) were added
to a 10 ml dried flat-bottomed tube and the reaction mixture was
stirred for 5 min. Thereafter, the silane (4.4 mmol) was added to the
mixture. The resulting mixture was heated and stirred for certain
time. After cooling to room temperature, the conversion of alkene
and the selectivity were determined by GC-MS and NMR.
Selected 1 H, 13 C and 29 Si NMR data of adducts were as follows.
2a.[37] 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 1.00 (t, J = 9.0 Hz, 2H,
Si–CH2 ), 1.24 (t, J = 7.0 Hz, 9H, CH3 ), 2.74 (t, J = 8.6 Hz, 2H, CH2 ),
3.84 (q, J = 7.0 Hz, 6H, O–CH2 ), 7.16–7.27 (m, 5H, Ph).
2b. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 1.02 (t, J = 8.5 Hz, 2H,
Si–CH2 ), 1.28 (t, J = 7.0 Hz, 9H, CH3 ), 2.31 (s, 3H, CH3 ), 2.70 (t,
J = 8.6 Hz, 2H, CH2 ), 3.87 (q, J1 = 7.0 Hz, 6H, O–CH2 ), 7.07–7.11
(m, 4H, Ph). 13 C NMR (100 MHz, CDCl3 ) δ (ppm): 12.08, 18.12, 20.96,
28.53, 58.80, 127.82, 129.02, 134.97, 141.66. 29 Si NMR (80 MHz,
CDCl3 ) δ (ppm): −45.96.
2c. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.97 (t, J = 8.6 Hz, 2H,
Si–CH2 ), 1.25 (t, J = 7.0 Hz, 9H, CH3 ), 2.70 (m, 2H, CH2 ), 3.87 (q,
J = 7.0 Hz, 6H, O–CH2 ), 7.12–7.24 (m, 4H, Ph). 13 C NMR (100 MHz,
CDCl3 ) δ (ppm): 12.54, 18.26, 28.36, 58.38, 128.29, 129.18, 131.27,
143.01. 29 Si NMR (80 MHz, CDCl3 ) δ (ppm): −46.61.
2e.1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.59 (t, J = 8.5 Hz, 2H,
Si–CH2 ), 0.86 (s, 9H CH3 ), 1.23 (t, J = 7.0 Hz, 9H, CH3 ), 1.29–1.31
(m, 2H, CH2 ), 3.81 (q, J = 7.0 Hz, 6H, O–CH2 ). 13 C NMR (100 MHz,
CDCl3 ) δ (ppm): 4.64, 18.13, 28.51, 30.78, 36.35, 58.13. 29 Si NMR
(80 MHz, CDCl3 ) δ (ppm): −43.91.
2f. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.72 (t, J = 8.2 Hz,
1H, Si–CH), 1.22 (t, J = 7.0 Hz, 9H, CH3 ), 1.13–2.33 (m, 10H,
bicyclo[2.2.1]heptane), 3.82 (q, J = 7.0 Hz, 6H, O–CH2 ). 13 C NMR
(100 MHz, CDCl3 ) δ (ppm): 18.19, 24.80, 28.95, 31.50, 33.58, 36.58,
37.40, 37.68, 58.30. 29 Si NMR (80 MHz, CDCl3 ) δ (ppm): −49.78
(endo 99.6%), −48.40 (exo 0.4%).
2g. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.57–0.68 (m, 2H,
Si–CH2 ), 0.82–2.22 (m, 15H, 6,6-dimethylbicyclo[3.1.1]heptane),
1.26 (q, J = 7.0 Hz, 9H, CH3 ), 3.80 (q, J = 7.0 Hz, 6H, O–CH2 ). 13 C
NMR (100 MHz, CDCl3 ) δ (ppm): 18.17, 19.88, 22.78, 23.02, 24.66,
25.08, 26.79, 29.88, 39.39, 40.64, 48.48, 58.08. 29 Si NMR (80 MHz,
CDCl3 ) δ (ppm): −45.60 (endo 94.6%), −45.41 (exo 5.4%).
2h. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.39 (s, 6H, SiCH3 ), 1.19
(t, J = 8.6 Hz, 2H, Si–CH2 ), 2.75 (m, 2H, CH2 ), 7.17–7.29 (m, 5H,
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
401
A certain amount of a solution of H2 PtCl6 in THF (tetrohydrofuran)
(1.95 × 10−5 mol ml−1 ), according to the required loading level,
was mixed with PEGCOOH (1.0 g) in a round-bottomed flask
containing THF (10 ml). The mixture was stirred for 12 h. THF was
then removed by distillation under reduced pressure to leave the
platinum catalyst (denoted [Pt]/PEGCOOH).
Hydrosilylation of Alkenes Catalyzed by [Pt]/PEGCOOH
Y. Bai et al.
Table 1. Effect of anhydride on the hydrosilylation of styrene with triethoxysilane
Selectivity (%)a
Entry
b
1
2c
3d
4e
5
6
7
8
Anhydride
Conversion (%)a
β-Adduct
α-Adduct
Ethylbenzene
Dehydrogenative silylation
β/α
–
–
–
–
Maleic anhydride
Butanedioic anhydride
Glutaric anhydride
Phthalic anhydride
99.8
99.8
100
99.9
99.8
99.0
99.1
97.5
59.3
61.5
60.2
70.0
97.2
92.5
92.5
91.9
38.1
34.9
38.2
27.2
1.0
3.0
2.3
2.5
2.1
3.6
1.2
2.7
1.7
3.7
4.1
5.3
0.5
0
0.4
0.1
0.1
0.8
1.1
0.3
1.6
1.8
1.6
2.6
97
31
40
37
Reaction conditions: n(Pt)/n(COOH) = 1/500, n(Pt)/n(styrene) = 1/40 000, 90 ◦ C, 3 h.
a Determined by GC-MS.
b
Using H2 PtCl6 in THF.
c Using Speier’s catalyst.
d Using Karstedt’s catalyst.
e [Pt]/PEG.
Ph). 13 C NMR (100 MHz, CDCl3 ) δ (ppm): 1.86. 20.79, 29.32, 126.09,
128.23, 128.53, 143.89. 29 Si NMR (80 MHz, CDCl3 ) δ (ppm): 31.49.
2i. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.39 (s, 6H, SiCH3 ),
1.17 (t, J = 8.5 Hz, 2H, Si–CH2 ), 2.31 (s, 3H, CH3 ), 2.70 (m, 2H,
CH2 ), 7.08–7.11 (m, 4H, Ph). 13 C NMR (100 MHz, CDCl3 ) δ (ppm):
1.92, 20.98, 22.90, 28.94, 128.03, 129.38, 135.20, 140.89. 29 Si NMR
(80 MHz, CDCl3 ) δ (ppm): 31.50.
2j. 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.40 (s, 6H, SiCH3 ), 1.15
(t, J = 8.5 Hz, 2H, Si–CH2 ), 2.71 (m, 2H, CH2 ), 7.11–7.25 (m, 4H,
Ph). 13 C NMR (100 MHz, CDCl3 ) δ (ppm): 1.76, 20.54, 28.60, 128.57,
129.36, 131.57, 142.32. 29 Si NMR (80 MHz, CDCl3 ) δ (ppm): 31.28.
2l.[38] 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.36 (s, 6H,
SiCH3 ), 0.81 (t, J = 8.0 Hz, 1H, Si–CH), 1.15–2.31(m, 10H, bicyclo[2.2.1]heptane). 13 C NMR (100 MHz, CDCl3 ) δ (ppm): 0.57, 28.08,
31.01, 31.05, 31.36, 33.91, 36.74, 37.79.
2m.[39] 1 H NMR (400 MHz, CDCl3 ) δ (ppm): 0.41 (s, 6H,
SiCH3 ), 0.86 (m, Si–CH2 ), 0.78–2.23 (m, 15H, 6,6-dimethylbicyclo[3.1.1]heptane). 13 C NMR (100 MHz, CDCl3 ) δ (ppm): 2.94,
20.10, 23.14, 23.40, 24.69, 25.47, 27.51, 30.95, 39.50, 40.69, 49.06,
49.69.
Results and Discussion
Effect of Anhydride on the Hydrosilylation of Styrene
with (EtO)3 SiH
402
The [Pt]/PEGCOOH catalysts prepared were used in the hydrosilylation reaction of styrene with (EtO)3 SiH at 90 ◦ C, and the results
are listed in Table 1. The hydrosilylation of styrene with triethoxysilane generated mainly the linear alkylsilane with approximately
60% selectivity in favor of the β-adduct when using H2 PtCl6 (THF
solution), Speier’s catalyst or Karstedt’s catalyst (entries 1–3, Table 1). These results are consistent with the results reported in the
literature.[37,40] Then, polyethylene glycol was used as a promoter
for the same reaction, whereupon a slight increase in selectivity
was observed (entry 4, Table 1). However, the selectivity of the
β-adduct was found to be significantly improved by using carboxylated polyethylene glycol (PEGCOOH) as the additive. In all
cases (entries 5–8, Table 1), [Pt]/PEGCOOH catalyst systems show
high catalytic activity; over 97% conversions of styrene can be
wileyonlinelibrary.com/journal/aoc
obtained. Furthermore, over 90% selectivity of the β-adduct is obtained by using PEG modified with glutaric anhydride or phthalic
anhydride as promoter. Although 99% conversion was obtained
by using polyethylene glycol butanedioic acid monoester as promoter, better selectivity of the β-adduct and higher ratio of the
β-adduct to the a-adduct was obtained when maleic anhydridemodified PEG was used as promoter. It is speculated that olefinic
bond of the polyethylene glycol maleic acid ester has some influence on the platinum catalyst. In addition, the steric interval of
two carbonyls of the compound also has a slight influence on the
catalytic hydrosilylation (see entries 6 and 7).
Effect of PEG Molecular Weight on the Hydrosilylation
of Styrene with (EtO)3 SiH
The results of hydrosilylation using platinum complex combined
with various average molecular weight polyethylene glycols
modified with maleic anhydride as catalyst are listed in Table 2.
The results indicated that the average molecular weight of
polyethylene glycol played a role in the catalytic activity and
selectivity. Both the catalytic activity and selectivity of Pt-PEGCOOH
system decreased along with increasing the length of PEG
chain. It is possible that a longer chain would surround the
platinum, and sequentially inhibit the platinum catalyst touching
the substrate during the catalytic reaction. Subsequently, small
molecule alcohols and oligoethylene glycols modified with maleic
anhydride were used as promoters in the platinum-catalyzed
hydrosilyaltion; higher conversion with excellent selectivity of the
β-adduct was attained. However, it is easy to attain phenylethane
and dehydrogenative silylation as byproducts with short chains of
polyethylene glycol or other small molecular alcohol.
Effect of Ratio n(Pt)/n(COOH) on the Hydrosilylation of Styrene
with (EtO)3 SiH
The functionalized PEG with average molecular weight 600 was
selected as a promoter to investigate the influence of ratios
n(Pt)/n(COOH) and n(Pt)/n(styrene) on hydrosilylation. As can
be seen from Table 3, the molar ratio of n(Pt)/n(styrene) has
a significant effect on the conversion of styrene (entries 5–7,
Table 3). The conversion of styrene decreased with decreasing ratio
of n(Pt)/n(styrene), and the molar ratio of n(Pt)/n(COOH) mildly
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 400–405
Platinum-catalyzed hydrosilylation of alkenes
Table 2. Effect of PEG molecular weight on the hydrosilylation of styrene
Selectivity (%)a
Entry
1b
2c
3d
4
5
6
7
8
9
10
PEG (average molecular weight)
Conversion (%)a
β-Adduct
α-Adduct
Phenylethane
Dehydrogenative silylation
β/α
Aleicacidmonomethylester
(EG + maleic anhydride)
(TEG + maleic anhydride)
200
400
600
1000
2000
4000
6000
99.9
99.3
99.5
99.9
99.7
99.8
98.7
95.5
89.7
81.1
94.1
92.2
94.4
95.0
96.5
97.2
95.1
92.7
91.8
85.9
1.4
4.7
3.5
0.9
1.0
1.0
3.3
5.4
6.6
12.4
3.2
2.6
1.7
4.1
2.4
1.7
1.6
1.8
1.5
1.6
1.3
0.5
0.5
0
<0.1
<0.1
0
0
<0.1
<0.1
67
20
27
107
97
97
29
17
14
7
Reaction conditions: n(Pt)/n(COOH)= 1/500, n(Pt)/n(styrene) = 1/40 000, 90 ◦ C, 3 h.
a Determined by GC-MS.
b
Instead of PEG with MeOH,
.
O
HOOCCH=CHCOCH3
c
Instead of PEG with EG (ethylene glycol),
d
Instead of PEG with TEG (triethylene glycol),
O
O
.
HOOCCH=CHCOCH2CH2OCCH=CHCOOH
O
O
.
HOOCCH=CHCO(CH2CH2O)3CCH=CHCOOH
Table 3. Hydrosilylation of styrene with triethoxysilane catalyzed by [Pt]/PEGCOOH)
Selectivity (%)a
Catalyst
Entry
Pt content (%)
n(Pt)/n(COOH)
n(Pt)/n(styrene)
Conversion (%)a
β-Adduct
α-Adduct
Phenylethane
β/α
0.2
0.1
0.1
0.1
0.05
0.05
0.05
0.02
0.02
0.02
0.005
0.005
0.005
1/250
1/500
1/500
1/500
1/1 000
1/1 000
1/1 000
1/2 500
1/2 500
1/2 500
1/10 000
1/10 000
1/10 000
1/20 000
1/40 000
1/80 000
1/800 000
1/40 000
1/80 000
1/800 000
1/80 000
1/200 000
1/800 000
1/80 000
1/200 000
1/800 000
99.8
99.7
99.0
50.4
99.5
99.0
48.1
92.1
79.6
37.9
89.6
47.5
15.1
96.9
98.0
98.1
97.7
98.0
97.8
96.7
98.2
98.5
97.9
97.0
96.8
96.6
1.1
1.0
0.9
1.5
0.8
1.1
1.3
1.0
0.9
1.2
1.9
2.0
2.6
2.0
0.7
1.1
0.8
1.2
1.1
2.0
0.8
0.6
0.9
1.1
1.0
0.8
88
98
109
65
122
89
74
98
109
81
51
48
37
1
2
3
4
5
6
7
8
9
10
11
12
13
Reaction conditions: styrene (4 mmol), triethoxysilane (4.4 mmol), 90 ◦ C, 3 h.
a Determined by GC-MS.
Table 4. Hydrosilylation of styrene with various silanes
Selectivity (%)a
Entry Silanes
1
2
3
4
5
6
(EtO)2 SiH
Et3 SiH
PhSiH3
Ph2 ClSiH
PhCl2 SiH
Me2 ClSiH
Conversion
(%)a
β-Adduct α-Adduct Phenylethane β/α
99.8
<5.0
<2.0
0
0
96.3
97.2
97.8
–
–
–
98.1
1.0
0.9
–
–
–
1.1
1.6
1.3
–
–
–
0.7
97
108
–
–
–
86
Appl. Organometal. Chem. 2011, 25, 400–405
Hydrosilylation of Styrene with Various Silanes
Under similar conditions, different silanes were used as hydrides
(Table 4). When (EtO)3 SiH and Me2 ClSiH were used, corresponding
β-adducts with only minor amounts of α-adducts with over 96%
conversions of styrene were obtained. However, 5% conversion
was obtained when Et3 SiH was used as hydride, and no reaction
could be detected using PhSiH3 , Ph2 ClSiH or PhCl2 SiH as hydrides
under the same conditions.
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
403
Reaction conditions: n(Pt)/n(COOH) = 1/1000, n(Pt)/n(styrene) =
1/40 000, 90 ◦ C, 3 h.
a Determined by GC-MS.
affected the selectivity of the adduct. In addition, excess carboxyl
resulted in decreasing catalytic activity of catalysis system. The best
results in terms of the conversion of styrene and the selectivity of
the adduct were achieved with n(Pt)/n(styrene) = 1 : 40 000 and
n(Pt)/n(COOH) = 1 : 1000 (entry 5, Table 3).
Y. Bai et al.
Table 5. Hydrosilylation of various alkenes with (EtO)3 SiH or ClMe2 SiH
Entry
Styrene
1
Silane
T (◦ C)/h
(EtO)3 SiH
90/3
(EtO)3 SiH
90/3
(EtO)3 SiH
90/3
(EtO)3 SiH
90/8
1a
2
Cl
97.2
Si(OEt)3
95.3
2b
1c
4
Si(OEt)3
2a
1b
3
Yield (%)a
Adduct
Si(OEt)3
98.5
Cl
2c
30.7
Si(OEt)3
1d
5
2d
(EtO)3 SiH
60/3
6
(EtO)3 SiH
90/3
(EtO)3 SiH
90/8
2e
H3 C
CH3
CH2
2f
H3 C
ClMe2 SiH
44.3c
CH3
C Si(OEt)3
H2
2g
1g
8
94.2b
Si(OEt)3
1f
7
99.7
Si(OEt)3
1e
60/3
89.3
Si
1a
Cl
2h
9
ClMe2 SiH
60/3
81.9
Si
1b
Cl
2i
10
ClMe2 SiH
60/3
86.5
Si
Cl
1c
2j
11
ClMe2 SiH
Cl
60/8
22.0
Si
1d
Cl
2k
ClMe2 SiH
12
Cl
60/3
93.9
Si
13
H3 C
ClMe2 SiH
CH3
CH2
Cl
2l
1f
60/8
1g
H3 C
83.4
CH3
C Si
H2 Cl
2m
Reaction conditions: alkene (4 mmol), silane (4.4 mmol), n(Pt)/n(COOH) = 1 : 1000, n(Pt)/n(alkene) = 1 : 80 000.
a Determined by GC-MS and NMR.
b Endo : exo = 99.6 : 0.4.
c Endo : exo = 94.6 : 5.4.
404
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c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 400–405
Platinum-catalyzed hydrosilylation of alkenes
Hydrosilylation of Various Alkenes with (EtO)3 SiH or ClMe2 SiH
Various terminal alkenes were evaluated for this reaction (Table 5).
In all cases, using [Pt]/PEGCOOH as catalyst, the substituted alkenes
were smoothly converted to their corresponding β-adducts with
only minor amounts of α-adducts. Among the aromatic alkenes,
excellent conversion and selectivity were attained in all cases
except with α-methylstyrene. The highest selectivity with excellent
conversion was achieved when 4-chlorostyrene was used as the
substrate.
Unfortunately, the activity of the catalyst system and the
selectivity in favor of the β-adduct decreased significantly when
α-methylstyrene was used as the substrate; steric hindrance is
the reason as described in the literature.[41] A slight change in
conversion and β-selectivity was observed when HSiMe2 Cl in
place of (EtO)3 SiH was used as a hydride resource.
As a comparison, several kinds of alkenes were used in the
hydrosilylation reaction. It was found that even sterically hindered
alkenes such as 3,3-dimethyl-1-butylene and norbornene could
also be applied. In the case of β-pinene, superior conversion was
attained when HSiMe2 Cl was used as hydride.
Conclusions
In summary, we found that [Pt]/PEGCOOH efficiently catalyzes the
regioselective hydrosilylation of terminal alkenes with various
silanes. In the presence of PEGCOOH, the hydrosilylation of
styrene with triethoxysilane is catalyzed by platinum and affords
predominantly the β-adduct. However, a reasonable mechanism
to explain the effect of the promoter PEGCOOH on the reaction, in
particular, the selectivity in favor of the β-adduct, is not clear, and
it will be pursued in our further research.
Acknowledgments
We are grateful to the Program of Zhejiang Province (2008C14041)
and the Fund of Zhejiang Province (Y4100248) for financial support.
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