Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate: An efficient heterogeneous catalyst for silylation of alcohols and phenols with HMDS

Article abstract of DOI:10.1002/aoc.1809

Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate (³⁰P/S-Bi) effectively activates hexamethyldisilazane (HMDS) for the silylation of alcohols and phenols. By the use of this heterogeneous catalytic system, a wide range of alcohols as well as phenols are converted into their corresponding trimethylsilyl ethers in high yield under mild reaction conditions. The catalyst was reused more than 10 times without significant loss of its catalytic activity.

Full text of DOI:10.1002/aoc.1809

Full Paper
Received: 1 March 2011
Revised: 18 April 2011
Accepted: 23 April 2011
Published online in Wiley Online Library: 8 July 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1809
Cross-linked poly(4-vinylpyridine/styrene)
copolymer-supported bismuth(III) triflate: an
efficient heterogeneous catalyst for silylation
of alcohols and phenols with HMDS
Sang-Hyeup Lee and Santosh T. Kadam∗
Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate (30P/S-Bi) effectively activates hexam-
ethyldisilazane (HMDS) for the silylation of alcohols and phenols. By the use of this heterogeneous catalytic system, a wide
range of alcohols as well as phenols are converted into their corresponding trimethylsilyl ethers in high yield under mild
c
reaction conditions. The catalyst was reused more than 10 times without significant loss of its catalytic activity. Copyright ꢀ
2011 John Wiley & Sons, Ltd.
Keywords: silylation; polymer-supported Lewis acid; alcohol; phenol; HMDS
Introduction
less expensive, remarkably non-toxic and easily prepared even
on a multi-gram scale from commercially available bismuth oxide
and triflic acid.[14] In addition, immobilization of these triflates
will provide more practical, reusable and cost-effective catalytic
system.
Our group has developed cross-linked poly(4-vinylpyri-
dine/styrene) copolymer-supported ytterbium(III) triflate (30P/S-
Yb) as an efficient heterogeneous catalyst for the synthesis of
β-amino ketones.[15] In continuation of our research on supported
Lewis acids, we report efficient and reusable heterogeneous
poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III)
triflate (30P/S-Bi) for the trimethylsilylation of alcohols and phe-
nols.
Silylation is an important and useful transformation for the pro-
tection of alcohol and phenol moieties.[1] It is also used to
prepare volatile derivatives of alcohols and phenols for GC and
GC-MS analyses.[2] In general, formation of silyl ether is car-
ried out using various silylating agents, such as allylsilane,[3]
hexamethyldisiloxane[4] and chlorotrimethylsilane,[5] with a va-
riety of alcohols. 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) is an
inexpensive, commercially available alternative reagent and gives
ammonia as the only by-product for the silylation of hydrogen-
labile substrate. Activation of HMDS was done using a variety
of catalysts such as various metal salts[6] and heterogeneous
catalysts.[7] Along with various catalytic systems, silylation was
also promoted in nitromethane as a solvent.[8] Although these
catalytic methods enhanced the activity of HMDS for sily-
lation, some of the catalysts still required a long reaction
time,[6p] high reaction temperature[6h,7d] and an excess amount of
reagent.[7i]
Heterogeneous catalysis is one of the hottest issues in organic
synthesis owing to the awareness of society and industry of
the need for ‘green’ chemistry, in terms of the removal of
toxic metals from the waste stream as well as the potential to
control costs via catalyst recovery and recycling.[9] Significant
efforts have been made towards this objective in an attempt to
develop a homogeneous catalyst that is anchored to polymer
support.[10]
Experimental
All the glassware was silanized by treating with Sigmacote or
10% dichlorodimethylsilane in toluene followed by dry methanol.
FT-IR spectra were recorded using a Jasco 4100 spectrometer
equipped with a diamond single-reflection attenuated total
1
reflectance accessory. H NMR (400 MHz) spectra were recorded
with Varian INOVA-399. Chemical shifts were reported in ppm in
CDCl3 with tetramethylsilane as the internal standard. 13C NMR
data were collected on a Varian Inova-399 (100 MHz) or Varian
VNS600 (150 MHz). Mass data were obtained from the Korea Basic
Science Institute (Daegu) on a Jeol JMS 700 high-resolution mass
spectrometer.
Recently, lanthanide triflates have received considerable atten-
tion as mild, water-stable Lewis acids in a wide range of organic
transformations.[11] So far, these catalysts have been mainly uti-
lized in homogeneous reaction conditions in most applications.
Polymer-supported lanthanide triflates have been also reported
by Koyabashi and Janda’s group.[12,13] However, lanthanide tri-
flates are expensive, and thus, their use in large-scale synthesis
is limited. Therefore, cheaper and more efficient catalysts are
desirable. Compared with lanthanide triflates, bismuth triflate is
∗
Correspondence to: Santosh T. Kadam, Department of Life Chemistry, Catholic
University of Daegu, Gyeongsan 712-702, Korea.
E-mail: stk2010@cu.ac.kr
Department of Life Chemistry, Catholic University of Daegu, Gyeongsan 712-
702, Korea
c
Appl. Organometal. Chem. 2011, 25, 608–615
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate
Heterogenizaton of Bismuth Triflate on Cross-linked Poly(4-
vinylpyridine/styrene) Copolymer with 30% Pyridine Func-
tionality (30P/S-Bi)
Trimethyl(3,4,5-trimethoxybenzyloxy)silane(Table 2, entry 3)
IR (neat): 2955, 2898, 2838, 1590, 1457, 1249, 1126, 1097, 871, 837,
751 cm−1. 1H NMR (400 MHz, CDCl3): 0.00 (s, 9H, -Si(CH3)3), 3.65 (s,
6H, 2 × -OCH3), 3.68 (s, 3H, -OCH3), 4.45 (s, 2H, -CH2Ar), 6.38 (s, 2H,
Ar–H). 13CNMR(100 MHz, CDCl3):0.0[-Si(CH3)3], 56.3(-OCH3), 61.1
(-OCH3), 65.1 (-CH2Ar), 104.0 (Ar–C), 137.0 (Ar–C), 137.3 (Ar–C),
153.5(Ar–C).HRMS(FAB):m/z [M]+ calcdforC13H22O4Si:270.1287;
observed: 270.1285.
As previously reported,[12] cross-linked poly(4-vinylpyridine/
styrene) copolymer with 30% pyridine functionality was pre-
pared by radical suspension copolymerization of 4-vinylpyridine
(30 mol%), styrene (70%) and 1 mol% of the 1,4-bis(4-
vinylphenoxy)butane as a flexible cross-linker. A 500 mg aliquot
of 30P/S resin was placed in a 20 ml vial containing 150 mg of
Bi(OTf)3. A 10 ml aliquot of methanol–dichloromethane (1 : 1) co-
solvent was added to the vial. After being tightly capped, these
vials were shaken for 24 h at room temperature. The resultant
resin was collected in plastic syringe equipped with a polystyrene
frit and washed with methanol–dichloromethane (1 : 1) solvent
several times, then dried under reduced pressure to give Bi(OTf)3-
immobilized resin [578 mg, loading level: 0.2 mmol Bi(OTf)3/g
resin] as pale yellow solid.
Ph
OTMS
Trimethyl[(2-methylbiphenyl-3-yl)methoxy]silane (Table 2, entry 8)
IR (neat): 3063, 3034, 2956, 2900, 1707, 1599, 1477, 1250, 1121,
1
1066, 883, 835, 755 cm−1. H NMR (400 MHz, CDCl3): 0.03 (s, 9H,
-Si(CH3)3), 1.20 (s, 3H, -CH3), 4.54 (s, 2H, -CH2Ar), 6.96–7.23 (m, 8H,
Ar–H).13CNMR(100 MHz,CDCl3):−0.1(-Si(CH3)3),16.0(-CH3),63.6
(-CH2Ar), 125.6 (Ar–C), 126.4 (Ar–C), 127.0 (Ar–C), 128.2 (Ar–C),
129.2 (Ar–C), 129.6 (Ar–C), 133.1 (Ar–C), 139.4 (Ar–C), 142.4
(Ar–C), 142.6 (Ar–C). HRMS (FAB): m/z [M]+ calcd for C17H22OSi:
270.1440; observed: 270.1436.
General Procedure for Trimethylsilylation of Alcohols
and Phenols
30P/S-Bi (50 mg, corresponding to 0.01 mmol Bi, 1 mol%) was
added to the mixture of alcohol or phenol (1.0 mmol) and HMDS
(0.8 mmol) in CH2Cl2 (4 ml). The reaction mixture was stirred
at room temperature for the appropriate time (see Tables 2
and 3) and the progress of the reaction was monitored by
TLC. The reaction mixture was filtered through a plastic syringe
with a polyethylene frit and washed with dichloromethane.
Concentration of the organic layer under vacuum gave a crude
mass, which was purified by flash chromatography on silica gel to
give the desired product. A reusability test of 30P/S-Bi catalyst was
performed using the recovered catalyst from the previous run for
a successive 10 times after simple filtration, washing and drying
under vacuum. All the reactions in Tables 2 and 3 were performed
OTMS
CF3
Trimethyl[3-(trifluoromethyl)phenethoxy]silane(Table 2, entry 13)
IR (neat): 2956, 2899, 2864, 1597, 1323, 1492, 1323, 1251, 1162,
1
1093, 838, 798, 701 cm−1. H NMR (400 MHz, CDCl3): 0.02 [s, 9H,
1
using the recovered catalyst from the previous reaction. H and
-Si(CH3)3], 2.85 (t, J = 6.8 Hz, 2H, -CH2Ar), 3.75 [t, J = 6.8 Hz,
2H, -CH2OSi(CH3)3], 7.30–7.50 (m, 4H, Ar–H). 13C NMR (125 MHz,
CDCl3): −0.7 [-Si(CH3)3], 39.0 (-CH2Ar), 63.2 [-CH2OSi(CH3)3], 123.7
(q, 3JC−F = 3.8 Hz, Ar–C), 124.3 (q, 1JC−F = 270.6 Hz, -CF3), 125.9
(q, 3JC−F = 3.8 Hz, Ar–C), 128.6 (Ar–C), 130.6 (q, 2JC−F = 31.9 Hz,
Ar–C), 132.5 (q, 4JC−F = 1.3 Hz, Ar–C), 140.2 (Ar–C). HRMS (FAB):
m/z [M + H]+ calcd for C12H18F3OSi, 263.1079; observed, 263.1075.
13C NMR data for known products were the same as the literature
values.[6e,j,p,7c,19] Spectral data for new products are given below
(Table 2, entries 2, 3, 8, 13, 14 and Table 3, entries 5, 9, 13, 15).
Me
OTMS
Me
OTMS
S
(3,5-Dimethylbenzyloxy)trimethylsilane (Table 2, entry 2)
Trimethyl[2-(phenylthio)ethoxy]silane (Table 2, entry 14)
IR (neat): 3017, 2956, 2898, 1608, 1459, 1249, 1093, 870, 836 cm−1
.
1H NMR (400 MHz, CDCl3): 0.02 [s, 9H, -Si(CH3)3], 2.14 (s, 6H, 2 ×
-CH3), 4.45 (s, 2H, -CH2Ar), 6.72 (s, 1H, Ar–H), 6.77 (s, 2H, Ar–H).
13C NMR (100 MHz, CDCl3): 0.0 [-Si(CH3)3], 21.6 (2 × −CH3), 65.0
(-CH2Ar), 125.0 (Ar–C), 129.1 (Ar–C), 138.1 (Ar–C), 141.0 (Ar–C).
HRMS (FAB): m/z [M]+ calcd for C12H20OSi: 208.1283; observed:
208.1281.
IR (neat): 3074, 2955, 2897, 1584, 1481, 1249, 1081, 871, 837,
736 cm−1. 1H NMR (400 MHz, CDCl3): 0.02 [s, 9H, -Si(CH3)3], 2.97 (t,
J = 7.2 Hz, 2H, -SCH2Ar), 3.66 [t, J = 7.2 Hz, 2H, -CH2OSi(CH3)3],
7.04–7.24 (m, 5H, Ar–H). 13C NMR (100 MHz, CDCl3): −0.2 [-
Si(CH3)3], 36.0 (-SCH2Ar), 61.8 [-CH2OSi(CH3)3], 126.2 (Ar–C), 129.1
(Ar–C), 129.3 (Ar–C), 136.4 (Ar–C). HRMS (FAB): m/z [M]+ calcd for
C11H18OSSi, 226.0848; observed, 226.0849.
MeO
OTMS
OTMS
MeO
OMe
c
Appl. Organometal. Chem. 2011, 25, 608–615
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S.-H. Lee and S. T. Kadam
Table 1. 30P/S-Bi catalyzed silylation of benzyl alcohol with HMDSa
Catalyst
OH
OSiMe3
+
(Me3Si)2NH
NH3
+
CH2Cl2, rt
Entry
Catalyst (mg)
Time (min)
Yield (%)b
1
None
60
60
60
45
30
10
10
4
23
NR
NR
56
62
92
92
99
2
30P/S resin (30)
Pyridinec
3
4
30P/S-Bi (10)
30P/S-Bi (30)
30P/S-Bi (50)
30P/S-Bi (70)
5
6
7
8[6i]
d
Bi(OTf)3
a Reaction conditions: alcohol (1.0 mmol), HMDS (0.8 mmol) in CH2Cl2 (4 ml). b Isolated yield. c 1.0 mmol.
d 0.5 mol%.
Trimethyl(2-phenylpropan-2-yloxy)silane (Table 3, entry 5)
4-(Trimethylsilyloxy)butan-2-one (Table 3, entry 15)
IR (neat): 2958, 2898, 1715, 1358, 1250, 1169, 1098, 877, 837,
IR (neat): 3087, 3063, 2978, 1601, 1493, 1249, 1174, 1031, 835, 760,
698 cm−1. 1H NMR (400 MHz, CDCl3): 0.02 [s, 9H, -Si(CH3)3], 1.97 (s,
6H, 2 × -CH3), 7.04–7.24 (m, 5H, Ar–H). 13C NMR (100 MHz, CDCl3):
0.0 [-Si(CH3)3], 30.1 (2 × −CH3), 72.8 [-C(CH3)2OSi(CH3)3], 122.3
(Ar–C), 124.0 (Ar–C), 125.5 (Ar–C), 147.6 (Ar–C). HRMS (FAB): m/z
[M − CH3]+ calcd for C11H17OSi: 193.1049; observed: 193.1050.
1
749 cm−1. H NMR (400 MHz, CDCl3): 0.04 [s, 9H, -Si(CH3)3], 2.08
(s, 3H, CH3CO-), 2.5 (t, J = 6.4 Hz, 2H, -COCH2-), 3.7 [t, J = 6.4 Hz,
2H, -CH2OSi(CH3)3]. 13C NMR (100 MHz, CDCl3): 0.4 [-Si(CH3)3],
30.8 (CH3CO-), 46.5 (-COCH2-), 58.1 [-CH2OSi(CH3)3], 207.8 (-CO-).
HRMS(FAB):m/z [M+H]+ calcdforC7H17O2Si,161.0998;observed,
161.0996.
Br
OTMS
Results and Discussion
By considering the excellence of poly(4-vinylpyridine/styrene)
copolymer (P/S resins) as an effective supporting material for
immobilization of Lewis acid previously reported by our group,[15]
we decided to use P/S resin as a support for the heterogenization
of bismuth triflate.
The immobilization was done by the treatment of bismuth
triflatewith30P/Sresin(thesuperscript30indicatesthepercentage
pyridine content) in a mixture of methanol and dichloromethane
byapreviouslyreportedmethod.[12] Theswellingpropertyofcross-
linked resin in organic solvents is an important factor for effective
solid-phase reactions.[16] 30P/S-Bi catalyst showed outstanding
swelling in dichloromethane, hence dichloromethane was chosen
as the solvent system for this conversion. We have examined
the potential of 30P/S-Bi as a heterogeneous catalyst for silylation
of benzyl alcohol as a model substrate with HMDS at room
temperature, as shown in Table 1.
(4ꢁ-Bromobiphenyl-4-yloxy)trimethylsilane (Table 3, entry 9)
IR (neat): 3040, 2956, 1598, 1476, 1251, 1199, 1078, 914, 835,
.
752 cm−1 1H NMR (400 MHz, CDCl3): 0.02 [s, 9H, -Si(CH3)3],
6.81–6.85 (m, 4H, Ar–H), 7.38–7.47 (m, 4H, Ar–H). 13C NMR
(100 MHz, CDCl3): 0.4 [-Si(CH3)3], 116.0 (Ar–C), 121.0 (Ar–C),
128.4 (Ar–C), 131.5 (Ar–C), 132.0 (Ar–C), 133.4 (Ar–C), 140.0
(Ar–C), 155.3 (Ar–C). HRMS (FAB): m/z [M]+ calcd for C15H17BrOSi,
320.0232; observed, 320.0234.
OTMS
OH
O
In preliminary experiments, when the reaction was performed
without any catalyst (Table 1, entry 1), the reaction was very
sluggish to give only 23% silylation product after 1 h. The polymer
support (30P/S resin) itself retards the silylation reaction and gives
no silylation product even after 1 h reaction time (entry 2). By
considering this result, we assume that the presence of pyridine
functionality of the polymer support retards the reaction. This
hypothesis is supported by the same retardation of reaction that
was observed when pyridine was added to the reaction mixture
(entry 3). Various amounts of 30P/S-Bi heterogeneous catalyst
were tested for optimization of catalyst loading and reaction times
of silylation. Accordingly 50 mg of supported 30P/S-Bi catalyst
[corresponding to 1 mol% of Bi(OTf)3] at room temperature
was chosen as the best and optimum reaction condition for
silylation of alcohol with HMDS (Table 1, entry 6). The optimal
2-[3-(Trimethylsilyloxy)phenyl]acetic acid (Table 3, entry 13)
IR (neat): 3035, 2959, 2899, 1708, 1602, 1487, 1270, 1252, 1157, 983,
840 cm−1. 1H NMR (400 MHz, CDCl3): 0.02 [s, 9H, -Si(CH3)3], 3.32 (s,
2H, -CH2COOH), 6.45–6.55 (m, 2H), 6.62 (d, J = 7.2 Hz, 1H, Ar–H),
6.92 (t, J = 7.8 Hz, 1H, Ar–H), 11.2 (brs, 1H, -COOH). 13C NMR
(100 MHz, CDCl3): 0.4 [-Si(CH3)3], 41.2 (-CH2COOH), 121.4 (Ar–C),
122.6 (Ar–C), 129.8 (Ar–C), 155.5 (Ar–C), 178.2 (-COOH). HRMS
(FAB): m/z [M + H]+ calcd for C11H17O3Si, 225.0947; observed,
225.0949.
O
OTMS
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 608–615

Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate
Table 2. 30P/S-Bi catalyzed silylation of benzylic and primary aliphatic alcohols with HMDSa
30P/S-Bi
+
(Me3Si)2NH
Products
OTMS
R
OSiMe3
NH3
+
R
OH
CH2Cl2, rt
Entry
1
Time (min)
10
Yield (%)b
92
2
3
4
10
10
15
94
96
94
Me
OTMS
Me
MeO
MeO
OTMS
OTMS
OMe
O
Ph
5
6
10
10
88
90
OTMS
OPh
Cl
OTMS
Cl
7
8
10
10
89
91
O
O
OTMS
Ph
OTMS
9
40
15
90
92
OTMS
O
10
OTMS
11
12
10
10
91
93
OTMS
Me
OTMS
MeO
c
Appl. Organometal. Chem. 2011, 25, 608–615
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S.-H. Lee and S. T. Kadam
Table 2. (Continued)
30P/S-Bi
+
R
OH
(Me3Si)2NH
OTMS
R
OSiMe3
NH3
+
CH2Cl2, rt
Entry
13
Products
Time (min)
25
Yield (%)b
90
CF3
14
15
10
20
86
74
OTMS
OTMS
S
a Reaction conditions: alcohol (1.0 mmol), HMDS (0.8 mmol), 30P/S-Bi (50 mg, corresponding to 0.01 mmol Bi, 1 mol%) in CH2Cl2 (4 ml).
b Isolated yield.
molar ratio of alcohol, HMDS and 30P/S-Bi was found to be
1.0 mmol–0.8 mmol–1.0 mol%, respectively. Silylation of alcohol
with HMDS using unsupported bismuth triflate, in which 0.5 mol%
of the catalyst was typically employed, has been also reported.[6i]
To achieve similar catalytic performance, in terms of reaction
time and product yield, this heterogenized bismuth triflate was
used in 1 mol%. It is accepted that immobilized catalyst shows
somewhat lower catalytic activity compared with its soluble
counterpart, as observed in most heterogenized catalysts.[17]
However, more importantly, heterogenization of the catalyst
enables facile recovery and reuse in compensation for its lowered
catalytic activity.[18]
The catalytic activities of 30P/S-Bi for the silylation of aliphatic
and aromatic alcohols are shown in Tables 2 and 3. A wide
range of alcohols enabled production of the corresponding
trimethylsilyl ethers in excellent to good yields (Table 2). Various
benzyl alcohols substituted with electron-donating and electron-
withdrawing groups underwent smooth silylation in excellent
yields (Table 2, entries 1–8). Benzyl alcohol containing electron-
withdrawing chlorine atoms produced the silylation product in
a good yield (entry 6), and this indicates that electronic and
steric effects by the chlorine atoms have no serious effect on
reactivity. The silylation of furfuryl alcohol preceded efficiently
under this reaction condition (entry 9). The catalytic system was
equally effective for phenethyl alcohols with electron-donating
and electron-withdrawing groups on their phenyl rings (entries
10–13). In addition to benzyl and phenethyl types of alcohols, this
catalytic system is also applicable for the silylation of long-chain
aliphatic alcohols (entries 14 and 15).
ucts in 87 and 88% yield in short reaction times, respectively
(entries 3 and 4). These results indicate that the steric effect does
not play a considerable role in silylation of secondary alcohols.
However, in the case of tertiary alcohol (2-phenyl-2-propanol), a
relatively longer reaction time (180 min) was required to obtain
the product in reasonable yield (entry 5). This may be due to the
steric hindrance of the two methyl groups and the benzene ring
on the reaction site.
The catalytic activity of 30P/S-Bi was also investigated for silyla-
tion of phenols (entries 6–10). Phenols with various substituents
underwent silylation in high yield (entries 6–10). However, com-
pared with aliphatic alcohols, aromatic alcohol substrates needed
somewhat longer reaction times to obtain the silylation products
in reasonable yield. For example, 2-naphthol offered 74% yield in
55 min reaction time (entry 10). Unfortunately, the present hetero-
geneous catalytic system was unable to catalyze the silylation of
other types of hydrogen-labile substrates such as amine and thiol.
4-Methoxybenzenethiol and benzylamine gave no silylation prod-
uct at all under the present reaction conditions (entries 11 and 12).
Thelackofreactivitywasduetothehigheraffinityofsiliconatomof
HMDStowardstheoxygenofthehydroxylgroupthanthenitrogen
and sulfur of amine and thiol, respectively.[20] This heterogeneous
catalyticprotocolwasutilizedsuccessfullyintheselectivesilylation
of hydroxyl group in the presence of other functional groups in the
same molecule (entries 13–15). The present method tolerates the
presence of carboxylic acid (entry 13), amine (entry 14) and enoliz-
able carbonyl group (entry 15). We also investigated the selective
silylation of benzyl alcohols over tertiary and phenolic hydroxyl
groups (Scheme 1). When an equimolar mixture of benzyl alcohol
and 2-phenyl-2-propanol (Scheme 1a) or 2-naphthol (Scheme 1b)
was reacted with 0.8 mmol of HMDS under the same reaction
conditions, only the benzyl alcohol converted predominantly to
silylation products.
We extended this catalytic methodology to secondary and ter-
tiary alcohols and phenols using the same reaction conditions
(Table 3, entries 1–10). Various secondary alcohols gave corre-
sponding silyl ethers in high yields (entries 1–4). Sec-1-phenethyl
alcoholand1-phenyl-2-propanolgivethecorrespondingsilylation
products in high yield (entries 1 and 2). It should be noted that
secondary alcohols bearing sterically demanding substituents,
such as cyclopropyl ring (α-cyclopropylbenzyl alcohol) and phenyl
(diphenylmethanol) also gave the corresponding silylation prod-
30P/S-Bi is not only a stable, efficient and water-tolerant solid
catalyst, but it can also be recycled and reused without losing
its activity. The reusability of this catalyst was examined using
trimethylsilylationofbenzylalcoholwithHMDSasamodelreaction
for 10 consecutive times. It was found that the catalytic efficiency
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 608–615

Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate
Table 3. 30P/S-Bi catalyzed silylation of secondary, tertiary alcohols and phenols with HMDSa
30P/S-Bi
+
R
OH
(Me3Si)2NH
Products
OTMS
R
OSiMe3
NH3
+
CH2Cl2, rt
Entry
1
Time (min)
10
Yield (%)b
90
2
3
10
10
86
87
OTMS
OTMS
OTMS
OTMS
4
5
15
88
78
180
6
7
8
20
15
20
86
83
76
OTMS
Me
OTMS
OTMS
Br
Br
Cl
9
25
55
71
74
Br
OTMS
10
OTMS
11
12
180
180
NR
NR
STMS
MeO
NHTMS
c
Appl. Organometal. Chem. 2011, 25, 608–615
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S.-H. Lee and S. T. Kadam
Table 3. (Continued)
30P/S-Bi
+
R
OH
Products
OTMS
(Me3Si)2NH
R
OSiMe3
NH3
+
CH2Cl2, rt
Entry
13
Time (min)
Yield (%)b
88
20
OH
O
14
15
10
12
92
90
OTMS
NH2
O
OTMS
a Reaction conditions: alcohol or phenol (1.0 mmol), HMDS (0.8 mmol), 30P/S-Bi (50 mg, corresponding to 0.01 mmol Bi, 1 mol%) in CH2Cl2 (4 ml).
b Isolated yield.
(a)
OTMS
OH
HMDS (0.8 mmol)
30P/S-Bi (1 mol%)
OH
OH
OTMS
OTMS
CH2Cl2, rt, 10 min
0%
91%
88%
1 mmol
1 mmol
1 mmol
(b)
HMDS (0.8 mmol)
30P/S-Bi (1 mol%)
OTMS
OH
CH2Cl2, rt, 10 min
10%
1 mmol
Scheme 1. 30P/S-Bi catalyzed selective silylation of benzyl alcohol over tertiary and phenolic alcohols.
NH3
Table 4. 30P/S-Bi catalyzed silylation of alcohols in comparison with
other literature
HMDS
No.
Catalyst (mg)
(mmol) Time (min)
Reference
1
2
3
4
5
6
7
30P/S-Bi (50)
SO3H–SiO2 (19)
0.8
0.6
0.8
0.5
0.6
0.6
0.6
10–55
35–420
3–10
Present method
30P/S-Bi-NH3
30P/S-Bi
Me3Si-NH-SiMe3
[7c]
III
[7a]
[7h]
[7d]
[7j]
HClO4 –SiO2 (50)
[SnIV(TPP)(OTf)2] (10)
H-β zeolite (19)
R-O-SiMe3
1–6
5–30 h
5–30
TiCl2(OTf)–SiO2 (60)
LiClO4 –SiO2 (1000)
R-OH
[7i]
5–120
SiMe3
a
30P/S-Bi-NH
30P/S-Bi-NH2-SiMe3
All reactions were conducted at room temperature. The reaction of
entry 5 was carried out at 80 ◦C.
SiMe3
II
I
R-O-SiMe3
R-OH
remained almost unchanged in terms of reaction yield (92, 90 and
91% yield for first, fifth and tenth cycles, respectively). The catalytic
efficiency of 30P/S-Bi was compared with reported heterogeneous
catalytic systems as shown in Table 4. The yields and reaction
times of present methods were comparable with those of reported
methods.
Scheme 2. Plausible mechanism for silylation of an alcohol with HMDS.
The evolution of ammonia was confirmed by its pungent odor
and using red litmus paper, which turned blue. The evolution
c
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Appl. Organometal. Chem. 2011, 25, 608–615

Cross-linked poly(4-vinylpyridine/styrene) copolymer-supported bismuth(III) triflate
of ammonia suggests a plausible reaction path, as shown in
Scheme 2. Lewis acid–base reaction between the 30P/S-Bi and the
nitrogen of HMDS induces the polarization of the N–Si bond to
give the reactive silylating agent (I), which rapidly interacts with
alcohol to give the corresponding silyl compounds and complex
(II). This complex (II) reacts with another molecule of alcohol to
produce the corresponding silyl derivative. Finally, complex (III)
releases the ammonia and catalyst for completion of the catalytic
cycle.
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We have demonstrated the catalytic activity and reusability
of poly(4-vinylpyridine/styrene) copolymer-supported Bi(OTf)3
(
30P/S-Bi)tobehighlyefficient, non-corrosiveandenvironmentally
benign for the silylation of alcohols and phenols at room tempera-
ture.Varioustypesofalcohols,includingbenzyl,secondary,tertiary
and aromatic alcohols, are able to give silylation products in good
to excellent yield. 30P/S-Bi is also able to selectively silylate the
hydroxyl group in the presence of carboxylic acid, amine and
enolizable carbonyl groups in good yield. 30P/S-Bi is not only
an efficient stable solid catalyst but can also be reused without
considerable loss of activity.
Catal.
B
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Acknowledgments
[8] S. T. Kadam, S. S. Kim, Green Chem. 2010, 12, 94.
This Work was supported by research grants from the Catholic
University of Daegu.
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