A Facile and efficient trimethylsilylation of hydroxyl groups using silica-supported zinc chloride and alumina-supported sodium hydrogensulfate as recyclable heterogeneous catalysts

Article abstract of DOI:10.1080/10426500802446579

Silica-supported zinc chloride (SiO₂-ZnCl₂) and novel alumina-supported sodium hydrogensulfate (NaHSO₄-Al ₂O₃) as recyclable heterogeneous catalysts have been used for the mild trimethylsilylation of hydroxyl groups under ambient conditions. This procedure also allows for the selective protection of primary and secondary alcohols in the presence of tertiary alcohols.

Full text of DOI:10.1080/10426500802446579

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Phosphorus, Sulfur, and Silicon
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A Facile and Efficient
Trimethylsilylation of
Hydroxyl Groups Using Silica-
Supported Zinc Chloride and
Alumina-Supported Sodium
Hydrogensulfate as Recyclable
Heterogeneous Catalysts
a
a
Hamid Reza Shaterian , Fahimeh Khorami
,
a
a
Razieh Doostmohammadi , Azita Amirzadeh
&
a
Majid Ghashang
a
Department of Chemistry, Faculty of Sciences ,
University of Sistan and Baluchestan , Zahedan, Iran
Published online: 04 Sep 2009.
To cite this article: Hamid Reza Shaterian , Fahimeh Khorami , Razieh
Doostmohammadi , Azita Amirzadeh & Majid Ghashang (2009) A Facile and Efficient
Trimethylsilylation of Hydroxyl Groups Using Silica-Supported Zinc Chloride and
Alumina-Supported Sodium Hydrogensulfate as Recyclable Heterogeneous Catalysts,
Phosphorus, Sulfur, and Silicon and the Related Elements, 184:9, 2227-2237
To link to this article: http://dx.doi.org/10.1080/10426500802446579
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Phosphorus, Sulfur, and Silicon, 184:2227–2237, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802446579
A Facile and Efficient Trimethylsilylation of Hydroxyl
Groups Using Silica-Supported Zinc Chloride and
Alumina-Supported Sodium Hydrogensulfate as
Recyclable Heterogeneous Catalysts
Hamid Reza Shaterian, Fahimeh Khorami, Razieh
Doostmohammadi, Azita Amirzadeh, and Majid Ghashang
Department of Chemistry, Faculty of Sciences, University of Sistan
and Baluchestan, Zahedan, Iran
Silica-supported zinc chloride (SiO2-ZnCl2) and novel alumina-supported sodium
hydrogensulfate (NaHSO -Al2O3) as recyclable heterogeneous catalysts have been
4
used for the mild trimethylsilylation of hydroxyl groups under ambient conditions.
This procedure also allows for the selective protection of primary and secondary
alcohols in the presence of tertiary alcohols.
Keywords Heterogeneous catalyst; NaHSO4-Al2O3; SiO2-ZnCl2 (silzic); O-
trimethylsilylation
INTRODUCTION
There is current and general interest in heterogeneous systems be-
cause of their importance in industry and in the development of new
1
technologies. Heterogeneous reagent systems have many advantages,
such as simple experimental procedures, mild reaction conditions, and
minimum chemical wastes, when compared to analogous liquid-phase
2
reactions. Solid acid catalysts are easier to handle because they hold
the acidity internally and are easily separated from products by simple
filtration. Moreover, constraining a reaction to the surface of a solid ha-
bitually allows one to apply milder conditions. So far, several number
Received 20 May 2008; accepted 2 September 2008.
We are thankful to the Sistan and Baluchestan University Research Council for the
partial support of this research.
Address correspondence to Hamid Reza Shaterian, Department of Chemistry, Faculty
of Sciences, University of Sistan and Baluchestan, PO Box 98135-674, Zahedan, Iran.
E-mail: hrshaterian@hamoon.usb.ac.ir
2227

2228
H. R. Shaterian et al.
3
4
of silica- and alumina-supported reactions have been reported in the
literature for chemical transformations.
Hexamethyldisilazane (HMDS) is frequently used for the
trimethylsilylation of hydroxyl groups. The low silylation power of
5
–24
HMDS is the main drawback to its application.
We now show that
SiO2-ZnCl2 (silzic) and NaHSO4-Al2O3 as recyclable heterogeneous
catalysts can be used to activate this reagent at room temperature
(
Scheme 1).
SCHEME 1
These heterogeneous catalysts are safe, inexpensive, recyclable, easy
to handle, environmentally benign, present fewer disposal problems,
and stable in reaction media.
RESULTS AND DISCUSSION
Silzic was prepared according to the reported procedure, and was char-
acterized by comparison of XRD, TGA, DSC, and BET with authen-
2
5
tic sample. NaHSO4-Al2O3 was reported for the first time. The idea
for synthesis of alumina-supported sodium hydrogensulfate (NaHSO4-
Al2O3) is based on the preparation of silica-supported sodium hydro-
2
6
gensulfate, which was first synthesized by Breton.
To optimize the reaction conditions, initially we converted benzyl
alcohol (1 mmol) to its corresponding benzyl trimethylsilyl ether with
the amount of SiO2-ZnCl2 (0.025 g) as catalyst and HMDS (0.75 mmol)
in the presence of various solvents and also solvent-free conditions at
room temperature (Table I). The results in Table I show that among
these solvents, acetonitrile was the solvent of choice in terms of time.
To find out the optimum quantity of the catalyst, the reaction of
benzyl alcohol (1 mmol) with HMDS (0.75 mmol) was carried out using
different quantities of SiO2-ZnCl2 under ambient conditions in acetoni-
trile as solvent (Table II). As can be seen from Table II, the best results
were obtained using 0.025 g of the catalyst.
This reaction was also performed to optimize the organic solvent
THF, CH2Cl2, MeCN, EtOAc, n-hexane, CHCl3, diethyl ether) or
(
solvent-free, and also the amount of the NaHSO4-Al2O3 as catalyst

Trimethylsilylation of Hydroxyl Groups
2229
TABLE I Preparation of Benzyl Trimethylsilyl Ether Using
2 2
SiO -ZnCl (0.025 g) as Catalyst Under Solvent and Solvent-Free
Conditions at Room Temperature
Entry
Solvent
Time (min)
GC yield (%)
Yield ( %)a
1
2
3
4
5
6
Dichloromethane
Chloroform
Ethyl acetate
n-Hexane
Acetonitrile
Solvent-free
365
660
425
360
5
100
100
100
100
100
100
87
85
81
89
93
87
95
aIsolated yield and the product gave satisfactory IR and NMR spectra.
(
0.0125, 0.025, 0.05 g). The best results were obtained using 0.05 g of
the catalyst in acetonitrile (2 mL) as solvent.
Thus, we prepared a range of silylethers under the optimized reac-
tion conditions: hydroxyl compound (1 eq), HMDS (0.75 eq), SiO2-ZnCl2
(
(
0.025 g) or NaHSO4-Al2O3 (0.05 g) as catalysts, and acetonitrile (2 mL)
Table II).
A wide range of structurally diverse and functionalized phenols,
alcohols, and oximes underwent silylation by this procedure to pro-
vide the corresponding TMS ethers in good to excellent isolated yields
(
Table III). Primary alcohols mostly reacted faster than secondary and
tertiary alcohols. Generally, in the all cases of benzyl, primary, sec-
ondary, and tertiary alcohols, the reactions were completed within
less than 25 min at ambient conditions accompanied by evolution of
NH3 gas from the reaction mixture. The amine functional group in
4
-aminophenol (Table III, Entry 23) remained unaffected under the
reaction conditions. In order to examine the functional group compati-
bility, we test some more alcohols having other functional groups such
as carbonyl group, amino group, alkene, and ethers. Alcohols were suc-
cessfully converted to the corresponding silyl ethers, whereas, other
functional groups were intact (Table III, Entries 2, 5, 23, and 24).
TABLE II Optimization Amount of SiO
2 2
-ZnCl Catalyst
in the Synthesis of Benzyl Trimethylsilyl Ether
Entry
Catalyst (g)
Time (min)
Isolated yield (%)
1
2
3
0.05
0.025
0.01
3
5
17
87
93
90

2230
H. R. Shaterian et al.
TABLE III Silylation of Alcohols, Phenols, Naphthols, and Oximes
with HMDS in the Presence of SiO -ZnCl (0.025 g) and NaHSO -Al
0.05 g) as Catalysts in Acetonitrile as Solvent
2
2
4
2 3
O
(
SiO2-ZnCl2
NaHSO4-Al2O3
Time Yield
Time
(min)
Yield
(%)
a
a
Entry
Substrate
(min)
(%)
1
2
3
4
5
6
7
8
9
Benzyl alcohol
5
8
11
13
12
8
10
16
17
13
15
93–95b
93
96
95
82
81
83
95
85
2
6
13
6
7
5
6
9
15
8
15
95–97b
94
95
94
90
85
87
92
90
4-Methoxybenzyl alcohol
2-Phenyl ethanol
Diphenyl methanol
Allyl alcohol
1-Butanol
3-Methyl-1-butanol
Cyclohexanol
tert-Butanol
1
1
0
1
Tetrahydro-2-furylmethanol
5-Methyl-2-(1-ethylmethyl)
cyclohexanol
93
96
90
95
12
13
14
15
16
17
18
19
20
21
22
23
24
1,4-Butandiol
10
21
11
25
7
87c
98
96
94
96
96
89
85
97
96
91
90
81
8
16
10
12
4
5
6
8
15
14
9
6
4
90c
97
95
94
96
93
90
90
98
94
90
87
80
Tricyclo[3,3,1,13,7] doc-1-ol
1-Octanol
2-Octanol
Phenol
4-Methylphenol
3-Methylphenol
2-Methylphenol
1-Naphthol
8
10
12
20
17
8
2-Naphthol
Benzaldehyde oxime
4-Aminophenol
Benzoin
9
25
a
The pure isolated products were characterized by the comparison of their physical
5–24
data with those of known compounds.
b
Isolated yields after five times recovery of the catalyst.
The amount of HMDS was chosen (1.5 mmol).
c
We also investigated selective silylation of different binary mixtures
of alcohols (Table IV). This method was shown to be highly selective
for primary alcohols, such as benzyl alcohols, compared to secondary
and tertiary alcohols (Table III, Entries 1–4). The primary alcohols
were completely converted to the corresponding silylether, while the
secondary and tertiary alcohols were converted to the corresponding
silylated product with 2–10% yield in the presence of SiO2-ZnCl2 and
NaHSO4-Al2O3 as catalyst.

Trimethylsilylation of Hydroxyl Groups
2231
TABLE IV Selective Silylation of Different Binary Mixtures of
Alcohols
Binary mixture
Molar Ratio Substrate 1/
Time GC yield
Entry Substrate
1
Product
Substrate 2 /HMDS/Catalyst (g) (min)
(%)
100
1
/1/0.75/0.025a
/1/0.75/0.025a
10
10
10
2
3
100
1
10
100
1
/1/0.75/0.025a
10
10
5
4
100
1
/1/0.75/0.05b
2
a
Catalyst: SiO2-ZnCl2.
Catalyst: NaHSO4-Al2O3.
b
The reusability of the catalysts is an important benefit and makes
them useful for commercial applications. Thus, the recovery and
reusability of SiO2-ZnCl2 and NaHSO4-Al2O3 were investigated. The
recyclability of the catalyst in the reaction of benzyl alcohol and HMDS
in the presence of these catalysts was checked (Table III, Entry 1). The
separated catalyst can be reused after washing with n-hexane and dry-
◦
ing at 100 C. The catalyst was recovered in excellent yields: after it was
used in the mentioned reaction five times, it showed the same activity
as fresh catalyst without any loss of its activity.
EXPERIMENTAL
All reagents were purchased from Merck and Aldrich and used with-
out further purification. SiO2-ZnCl2 was prepared according to the

2232
H. R. Shaterian et al.
reported procedure.25 All yields refer to isolated products after pu-
rification. Products were characterized by comparison with authentic
1
samples and by spectroscopy data (IR, H NMR spectra). The NMR
spectra were recorded on a Bruker Avance DPX 300 and 500 MHz in-
strument. The spectra were measured in CDCl3 relative to TMS (0.00
ppm). GC analysis was run with Shimadzu GC-14A. IR spectra were
recorded on a JASCO FT-IR 460plus spectrophotometer. TLC was per-
formed on silica-gel polygram SIL G/UV 254 plates.
Preparation of Alumina-Supported Sodium Hydrogensulfate
(
NaHSO -Al O )
4 2 3
NaHSO4-Al2O3 was reported for the first time. The idea for synthesis of
alumina-supported sodium hydrogensulfate (NaHSO4-Al2O3) is based
on the preparation of silica-supported sodium hydrogensulfate, as it
was first synthesized by Breton,26 by the following procedure.
To a solution of 4.14 g (0.03 mol) of NaHSO4.H2O in 20 mL of water
in a 100-mL beaker containing a stir bar, 10 g of Al2O3 was added.
The mixture was stirred for 15 min and then gently heated on a hot
plate, with intermittent swirling, until a free-flowing white solid was
obtained. The catalyst was further dried by placing the beaker in an
◦
oven maintained at 120 C for at least 48 h prior to use.
+
The amount of H in the NaHSO4-Al2O3 was determined by acid-
base titration according to the following reaction (Equation 1):
ꢀ
⊕
NaHSO4-Al2O3 + H2O → NaHSO4-Al2 O3 + H3 O
(1)
+
The librated H3O was titrated by standard NaOH, and the amount
of H in NaHSO4-Al2O3 was calculated (1 g of NaHSO4-Al2O3 equal to
+
+
0
.085 mmol H ).
X-Ray Diffraction (XRD) of NaHSO -Al O
3
4
2
Powder X-ray diffraction (XRD) measurements were performed using
D8 Advance diffract meter made by a Bruker AXS company in Ger-
many. Scans were taken with a 2θ step size of 0.02 and a counting time
of 1.0s using a CuKα radiation source generated at 40 KV and 30 mA.
Specimens for XRD were prepared by compaction into a glass-backed
◦
aluminum sample holder. Data was collected over a 2θ range from 4
◦
to 70 , and phases were identified by matching experimental patterns
plus
to entries in the Diffract
version 6.0 indexing software. The catalyst
containing alumina-supported sodium hydrogensulfate was character-
ized by XRD, and the pattern is presented in Figure 1. As is shown in

Trimethylsilylation of Hydroxyl Groups
2233
FIGURE 1 XRD pattern of alumina-supported sodium hydrogensulfate.
this figure, the actual phases that were identified for this catalyst un-
der the specified preparation conditions were NaAl(SO4).2H2O (cubic),
2NaAl2O3.3H2O (cubic), and Al2O3 (tetragonal), and it is considered
that the high catalyst performance for this catalyst is attributed to
these phases.
Thermal Gravimetric Analysis (TGA) and Differential Scanning
Calorimetry (DSC) of NaHSO -Al O
4
2
3
The weight change of the catalyst was measured using a TGA/DSC
simultaneous thermal analyzer apparatus from Rheometric Scientific
Company (STA 1500+ Model) under a flow of dry air. The temperature
◦
was raised from room temperature to 500 C using a linear programmer
◦
at a heating rate of 5 C per min. The TGA and DSC curves for the cata-
lyst are illustrated in Figure 2. The weight losses found from TGA mea-
surements agree fairly well with those expected for the decomposition
of alumina-supported sodium hydrogensulfate to alumina and Na2SO4
and other stable phases. For the this catalyst, the thermogravimetric
curve seems to indicate tree-stage decomposition, which is considered
to be due to the removal of physical absorbed water and crystalline
◦
water (70–140 C), and the second stage is due to the decomposition
◦
of sodium hydrogensulfate (150–230 C), respectively. The peak around
80–350 C is due to the decomposition of sodium hydrogensulfate to
◦
2
sulfate and sodium sulfate. The TGA curve is involved with a total
overall weight loss of ca. 23 wt%. DSC measurement was performed
in order to provide further evidence for the presence of the various
species and evaluates their thermal behavior. As shown in Figure 2,
the endothermic peak at lower temperature represents the removal

2234
H. R. Shaterian et al.
FIGURE 2 Thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) of alumina-supported sodium hydrogensulfate.
of the physically adsorbed water and crystalline water from the ma-
terial, while the endothermic peak at higher temperature represents
solely the decomposition of the sodium hydrogensulfate to sulfate and
sodium sulfate.
Specific Surface Area, Total Pore Volume, and Average Pore
Diameter of NaHSO -Al O
4
2
3
N2 adsorption measurements of the catalyst were carried out using
micrometrics adsorption equipment (Quantachrome instrument, model
Nova 2000, USA). N2 (99.99%) as the analysis gas and the catalyst sam-
◦
ples were slowly heated to 120 C for 3 h under nitrogen atmospheric.
The specific surface area measurements were evacuated by BET, BJH,
and DH methods and also total pore volume and average pore diameter
were calculated from the nitrogen adsorption/desorption curve using
◦
the BJH and DH methods at –196 C for 300 minutes. Characterization
of the catalyst was carried out using N2 adsorption/desorption measure-
ments, and the results are given in Table V. These results show that the
catalyst have high specific surface area and high pore volume, and this

Trimethylsilylation of Hydroxyl Groups
2235
TABLE V Specific Surface Area, Total Pore Volume, and Average
Pore Diameter of Pure Alumina and Alumina-Supported Sodium
Hydrogensulfate
Specific surface
Pore volume
(× 102 cc/g)
2
area (m /g)
Pore diameter (Å)
Sample
BET
BJH
DH
BJH
DH
BJH
DH
Al2O3
NaHSO4-Al2O3
182.3
167.7
181.1
169.3
179.9
170.8
17.12
18.77
17.12
18.77
9.03
8.32
8.98
8.01
is the reason for the high catalytic performance of alumina-supported
sodium hydrogensulfate in the experimental conditions.
General Procedure for Trimethylsilylation of Hydroxyl
Compounds Using SiO -ZnCl and NaHSO -Al O as Catalyst
2
2
4
2
3
To a stirred solution of alcohols or oximes (1 mmol), HMDS (0.75 mmol)
and acetonitrile as solvent (2 mL), and SiO2-ZnCl2 (0.025 g) or NaHSO4-
Al2O3 (0.05 g) were added at room temperature, and the mixture was
stirred for the appropriate time (Table III). The reaction was followed
by TLC (n-hexane–EtOAc, 9:1). After completion of the reaction, the
catalyst was filtered. The resulting mixture was passed through a short
pad of silica gel. Then, the pad column was washed with n-hexane (2 ×
1
0 mL). Evaporation of the solvent under reduced pressure gave pure
product(s) (Table III). The desired pure products were characterized by
comparison of their physical data with those of known compounds.
5
–24
CONCLUSION
In summary, SiO2-ZnCl2 and NaHSO4-Al2O3 are effective catalysts for
O-trimethylsilylation of primary and secondary unhindered alcohols,
and also for secondary hindered and tertiary ones, oximes and phenols.
These excellent heterogeneous catalysts allow the reactions to be per-
formed in ambient conditions with good to excellent yields. The degree
of selectivity of primary over secondary and tertiary is good to excel-
lent. NaHSO4-Al2O3 was reported and characterized for the first time.
Simple work-up procedure and reusability of these catalysts are other
advantages of these methods.

2236
H. R. Shaterian et al.
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