Alumina sulfuric acid as an efficient and recyclable heterogeneous catalyst for the O-silylation of alcohols, phenols, and oximes

Article abstract of DOI:10.1080/10426500801967955

Alumina sulfuric acid as a recyclable catalyst conducts the transformation of various types of alcohol, phenols, and oximes with hexamethyldisilazane (HMDS) to the corresponding O-trimethylsilylated compounds in good to excellent yields under mild and ambient conditions with short reaction times. The method is highly selective for the conversion of primary alcohols in the presence of secondary and tertiary alcohols. Additionally, the catalyst can be easily recovered and reused at least eight times without detectable loss of reactivity.

Full text of DOI:10.1080/10426500801967955

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Alumina Sulfuric Acid as an
Efficient and Recyclable
Heterogeneous Catalyst for
the O-Silylation of Alcohols,
Phenols, and Oximes
Hamid Reza Shaterian a , Fahimeh Khorami a , Azita
Amirzadeh a , Majid Ghashang a & Asghar Hosseinian
a
a Department of Chemistry, Faculty of Sciences,
University of Sistan and Baluchestan, Zahedan, Iran
Published online: 26 Sep 2008.
To cite this article: Hamid Reza Shaterian , Fahimeh Khorami , Azita Amirzadeh ,
Majid Ghashang & Asghar Hosseinian (2008) Alumina Sulfuric Acid as an Efficient and
Recyclable Heterogeneous Catalyst for the O-Silylation of Alcohols, Phenols, and
Oximes, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:10, 2584-2595,
DOI: 10.1080/10426500801967955
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Phosphorus, Sulfur, and Silicon, 183:2584–2595, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500801967955
Alumina Sulfuric Acid as an Efficient and Recyclable
Heterogeneous Catalyst for the O-Silylation of Alcohols,
Phenols, and Oximes
Hamid Reza Shaterian, Fahimeh Khorami,
Azita Amirzadeh, Majid Ghashang, and
Asghar Hosseinian
Department of Chemistry, Faculty of Sciences,
University of Sistan and Baluchestan, Zahedan, Iran
Alumina sulfuric acid as a recyclable catalyst conducts the transformation of var-
ious types of alcohol, phenols, and oximes with hexamethyldisilazane (HMDS) to
the corresponding O-trimethylsilylated compounds in good to excellent yields un-
der mild and ambient conditions with short reaction times. The method is highly
selective for the conversion of primary alcohols in the presence of secondary and
tertiary alcohols. Additionally, the catalyst can be easily recovered and reused at
least eight times without detectable loss of reactivity.
Keywords Alcohols; alumina sulfuric acid; hexamethyldisilazane; oximes; phenols;
trimethylsilylation
INTRODUCTION
Trimethylsilylation of hydroxyl functional groups is frequently used
as a protection method in several chemical transformations involving
the multi-step synthesis of natural and synthetic products, and phar-
maceuticals and is often used in analytical chemistry to prepare silyl
ethers as volatiles derivatives of hydroxyl groups.1 Silyl ethers are re-
sistant to oxidation; have good heat stability; have low viscosity; and
allow their parent compounds to be recovered easily.2 Silyl ethers have
a general stability for most non-acidic reagents, and a high solubility
in non-polar solvents.3
Received 2 December 2007; accepted 19 January 2008.
We are thankful to the Sistan and Baluchestan University Research Council for the
partial support of this work.
Address correspondence to Hamid Reza Shaterian, Department of Chemistry, Faculty
of Sciences, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran.
E-mail: hrshaterian@hamoon.usb.ac.ir
2584

Heterogeneous Catalyst for O-Silylation
2585
The preparation of silyl ethers can be carried out by treatment
of alcohols with silyl chlorides or silyl triflates in the presence of a
organic base.7−14 However, some of these methods frequently suffered
from drawbacks such as lack of reactivity or the difficulty in removal
of amine salts.4 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) is a stable,
commercially available, and cost-effective reagent for trimethylsilyla-
tion of hydrogen-labile substrates, giving NH3 as the only byproduct.1−4
Even though the handling of this reagent is easy, the low silylat-
ing ability of HMDS is one of the main drawbacks to its continued
application; therefore, there are a variety of catalysts for activating
of this reagent, such as (CH3)3SiCl,15 K-10 montmorillionite,16 sul-
fonic acids,17 zirconium sulfophenyl phosphonate,18 ZnCl2,19 Envirocat
EPZGO,20 tungstophosphoric acid,21 iodine,22 lithium perchlorate,23
cupric sulfate pentahydrate,24 H–β Zeolite,25 MgBr2,26 lithium per-
chlorate suported on silica gel,27 Al(HSO4)3,28 Al(OTF)3,29 magnesium
33
triflate,30 copper triflate,31 ZrCl43,2 silica-HClO and silica chloride.34
4,
However, in most of these cases, a long reaction time, drastic reac-
tion conditions, or tedious workup is needed. In addition, many of these
reagents are moisture sensitive or expensive. The lack of a facile and
effective synthetic methodology for the silylation of hydroxyl groups
prompted us to develop a convenient and practical procedure for the
protection of hydroxyl groups in the presence of heterogeneous cata-
lyst under ambient conditions. The use of heterogeneous catalysts is
becoming very popular as it has many advantages: reduced pollution,
reusability, high selectivity, low cost, and simplicity in process and in
handling. These factors are especially important in industry. In the
present research for functional group transformation, we wish to de-
scribe a new protocol for the mild and rapid trimethylsilylation of a
wide variety of hydroxyl groups using HMDS and a catalytic amount
of alumina sulfuric acid as a recyclable solid Brønsted acid catalyst at
room temperature (Scheme 1).
Alumina sulfuric acid (cat)
ROH
HMDS
+
ROTMS
neat,
time: 2 -150 min
R: Aryl, primary, secondary, tertiary alkyl, and oxime
(-NH3)
r.t,
SCHEME 1
This environmentally benign and inexpensive catalyst was prepared
from the reaction of alumina with chlorosulfonic acid (Scheme 2).35

2586
H. R. Shaterian et al.
SCHEME 2
RESULTS AND DISCUSSION
To optimize the reaction conditions, initially, we tried to convert ben-
zyl alcohol (1 mmol) to trimethylbenzyloxy silane with alumina sul-
furic acid (0.025 g, 0.075 mmol of H+)35,36 and HMDS (0.75 mmol) in
the presence of various solvents and also under solvent-free conditions
(Table I).
As shown in Table I, the yield of the product in acetonitrile as a sol-
vent is higher and the reaction time is shorter. Therefore, we employed
the acetonitrile for the conversion of the various hydroxyl groups to the
corresponding trimethylsilyl ethers (Scheme 1 and Table II).
A wide range of structurally diverse and functionalized alcohols, phe-
nols and naphthols underwent silylation by this procedure provide the
corresponding TMS ethers in good to excellent isolated yields (Table II,
entries 1−23). Inspection of the data in Table II clearly shows that dif-
ferent types of hindered secondary and tertiary alcohols were success-
fully converted to the corresponding silyl ethers. Timethylsilylation of
aldoxime and ketoxime also produce the corresponding trimethylsilyl
ether at the same experimental conditions (Table II, Entries 24−26).
In the all cases, the desired trimethylsilyl ethers were prepared accom-
panied by evolution of NH3 gas from the reaction mixture.
TABLE I Trimethyl Silylation of Benzyl Alcohols
(1 mmol) with HMDS (0.75) in the Presence of Alumina
Sulfuric Acid as Catalyst under Ambient Conditions
Entry
Solvent
Time (min) GC Yield (%) Yield (%)a
1
2
3
4
5
6
7
Dichloromethane
Chloroform
Ethyl acetate
n-Hexane
Diethyl ether
Acetonitrile
Solvent-Free
25
20
70
60
40
2
100
100
100
100
100
100
100
89
91
91
93
89
98
93
75
aYields refer to pure isolated trimethylbenzyloxy silane.

Heterogeneous Catalyst for O-Silylation
2587
TABLE II Silylation of Alcohols, Phenols, Naphthols, and Oximes
(1 mmol) with HMDS (0.75 mmol) in the Presence of Alumina Sulfuric
Acid as Catalyst (0.025 g, 7.5 mol% of H+) in Acetonitrile under
Ambient Conditions
Sub/ HMDS Time Yielda
Entry
1
Substrate
Product
ratio
(min)
(%)
1/0.75
4
96–98b
2
3
1/0.75
1/1.4
4
5
96
91
4
1/0.75
3
96
5
6
1/0.75
1/1.5
4
3
95
95
7
8
1/0.75
1/0.75
2
2
98
95
(Continued on next page)

2588
H. R. Shaterian et al.
TABLE II Silylation of Alcohols, Phenols, Naphthols, and Oximes (1
mmol) with HMDS (0.75 mmol) in the Presence of Alumina Sulfuric
Acid as Catalyst (0.025 g, 7.5 mol% of H+) in Acetonitrile under
Ambient Conditions (Continued)
Sub/ HMDS Time Yielda
Entry
9
Substrate
Product
ratio
(min) (%)
1/0.75
12
7
97
89
10
11
1/0.75
1/0.75
8
91
12
13
1/0.75
1/0.75
10
8
85
87
14
1/0.75
15
91
15
1/0.75
13
95
16
17
1/1.35
1/0.75
8
90
95
150
18
19
1/0.75
1/0.75
7
95
93
12
20
1/0.75
6
97
(Continues on next page)

Heterogeneous Catalyst for O-Silylation
2589
TABLE II Silylation of Alcohols, Phenols, Naphthols, and Oximes (1
mmol) with HMDS (0.75 mmol) in the Presence of Alumina Sulfuric
Acid as Catalyst (0.025 g, 7.5 mol% of H+) in Acetonitrile under
Ambient Conditions (Continued)
Sub/ HMDS Time Yielda
Entry
21
Substrate
Product
ratio
(min)
(%)
1/0.75
15
89
22
23
1/0.75
1/0.75
65
7
86
94
24
1/0.75
7
90
25
26
1/0.75
1/0.75
8
8
89
92
aYields refer to the pure isolated products. All known products have been reported
previously in the literature and were characterized by comparison of IR and NMR
spectra with authentic samples.15−34
bThe yields of eight consecutive catalytic reactions using the recovered catalyst.
We also investigated selective silylation of different binary mixture
of alcohols in Table III. This method was shown highly selectivity
for primary alcohols such as benzyl alcohols (Table III, entries 1−3).
The primary alcohols were completely converted to the corresponding
silylethers, while only 15−21% conversion was observed for the sec-
ondary alcohols (Table III, entries 1, 2). Excellent selectivity was

2590

2591

2592
H. R. Shaterian et al.
also observed for the conversion of primary and secondary alcohols
in the presence of tertiary alcohols such as adamantanol (Table III,
entries 3, 4).
The suggested mechanism of the alumina sulfuric acid catalyzed
transformation is shown in Scheme 2. We suggested this mechanism
according to the evolution of NH3 under the reaction conditions. We
propose that an acid-base interaction between H+ in alumina sulfuric
acid as catalyst and nitrogen in HMDS polarizes N-Si bond of HMDS
to produce a reactive silylating agent (I). A rapid reaction with alco-
hol then ensues, leading to the ammonium silylating species (II) with
concomitant release of the corresponding silyl ether. Irreversible cleav-
age of (II) with alcohol leading to ROTMS and also formation of NH+4 .
Release of H+ from intermediate (III), leading to the fast evolution of
NH3 and re-enters catalytic cycle (Scheme 3).
Al2O3-SO3H
+
(Me3Si)2NH
(Me3Si)2NH2
ROH
NH3
(I)
(Me3Si)2NH
ROSiMe3
Me3SiNH3
NH4
(II)
(III)
ROSiMe3
ROH
SCHEME 3
To show the efficiency of the non toxic alumina sulfuric acid in com-
parison with reported results in the literature, Table IV compares some
of our results with iodine,22 lithium perchlorate,23 and cupric sulfate
pentahydrate24 respect to reaction times, easy work up and yields of
the obtained products.
The catalyst could be separated by filtering it from the reaction mix-
ture and then washing with ethyl acetate followed by draying in an
oven at 100◦C for 30 min. The reusability of the recovered catalyst
was checked by the reaction of benzyl alcohol and HMDS in the pres-
ence of the 7.5 mol% of Al2O3-SO3H at ambient conditions (Table II,

Heterogeneous Catalyst for O-Silylation
2593
TABLE IV Comparison Result of Alumina Sulfuric Acid with Iodine22
Lithium Perchlorate,23and Cupric Sulfate Pentahydrate24 in the
Synthesis of Trimethylsilyl Ethers Using HMDS
Subs/HMDS/
Catalyst
Yielde
(%)
Entry
1
Substrate
Product
Condition
a
a) 1/0.8/0.01
b) 1/0.7/0.5
CH2Cl2 as solvent; r.t.
–
Solvent-free; r.t.;
time= 20 min
96b
c) 1/0.7/0.01
CH3CN as solvent;
r.t.; time = 1.5 h
95c
d) 1/0.75/0.075 CH3CN as solvent; r.t. 97d
; time = 4 min
CH2Cl2 as solvent; r.t.
a
2
3
a) 1/0.8/0.01
b) 1/0.7/0.5
–
Solvent-free; r.t.;
time= 20 min
80b
c) 1/0.7/0. 1
CH3CN as solvent;
r.t.; time = 38 h
50c
96d
d) 1/0.75/0.075 CH3CN as solvent;
r.t.; time = 4 min
a) 1/0.8/0.01
b) 1/0.7/0.5
CH2Cl2 as solvent;
r.t.; time = 2 min
Solvent-free; r.t.;
time= 25 min
98a
99b
98c
98d
c) 1/0.7/0. 01 CH3CN as solvent;
r.t.; time = 12 min
d) 1/0.75/0.075 CH3CN as solvent;
r.t.; time = 2 min
Catalyst: aiodine; blithium perchlorate; ccupric sulfate pentahydrate; dalumina
sulfuric acid; and eisolated yield.
entry 1). The results indicate that the recovered catalyst can be used
at least eight consecutive times without any loss of its activity.
In conclusion, we have demonstrated that alumina sulfuric acid is
a new, efficient and heterogeneous catalyst for trimethylsilylation of a
variety of hydroxyl groups using HMDS under ambient conditions. The
reactions were carried out at room temperature with short reaction
time and produce the corresponding timethylsilyl ethers in good to
excellent yields. Reusability of the recovered catalyst, simple work-up
procedure, including filtering the mixture through a short pad of silica
gel column followed by evaporation of the solvent, is another advantage
of this method.

2594
H. R. Shaterian et al.
EXPERIMENTAL
All reagents were purchased from Merck and Aldrich and used without
further purification. Alumina sulfuric acid was prepared according to
the reported procedure.35,36 All yields refer to isolated products after
purification. Products were characterized by comparison with authen-
tic samples and by spectroscopy data (IR, 1H NMR spectra). The NMR
spectra were recorded on a Bruker Avance DPX 300 and 500 MHz
instrument. The spectra were measured in CDCl3 relative to TMS
(0.00 ppm). GC analysis was run with Shimadzu GC-14A. IR spec-
tra were recorded on a Perkin-Elmer 781 spectrophotometer. TLC was
performed on Silica-gel polygram SIL G/UV 254 plates.
General Procedure for Silylation of Alcohols Using HMDS
Catalyzed with Al2O3-SO3H
To a stirred solution of alcohol (1 mmol) and HMDS (0.75 mmol) was
added alumina sulfuric acid (0.025 g, 0.075 mmol of H+) and the mix-
ture was stirred at room temperature for the time specified in Table II.
The reaction was followed by TLC (n-Hexane-EtOAc, 9:1). After com-
pletion of the reaction, the heterogeneous catalyst was filtered and the
resulting mixture was passed through a short pad of silica gel. Then,
the pad column was washed with n-hexane (2 × 10 mL). Evaporation
of the solvent under reduced pressure gave pure product(s) (Table II).
The desired pure product(s) was characterized by comparison of their
physical data with those of known compounds.15−34
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