A novel efficient method for the silylation of alcohols using hexamethyldisilazane in an ionic liquid

Article abstract of DOI:10.1080/10426500500326941

Benzylic and primary alcohols were protected by hexamethyldisilazane in good to excellent yields at r.t. in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) ionic liquid. In addition, the ionic liquid was recovered quantitatively and reused efficiently in the next reactions. The protocol was successfully applied to the protection of phenols. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500500326941

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A Novel Efficient Method for
the Silylation of Alcohols Using
Hexamethyldisilazane in an
Ionic Liquid
Mohammad M. Mojtahedi a , Hassan Abbasi a & M.
Saeed Abaee a
a Chemistry and Chemical Engineering Research
Center of Iran, Tehran, Iran
Published online: 01 Feb 2007.
To cite this article: Mohammad M. Mojtahedi , Hassan Abbasi & M. Saeed
Abaee (2006): A Novel Efficient Method for the Silylation of Alcohols Using
Hexamethyldisilazane in an Ionic Liquid, Phosphorus, Sulfur, and Silicon and the
Related Elements, 181:7, 1541-1544
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Phosphorus, Sulfur, and Silicon, 181:1541–1544, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500500326941
A Novel Efficient Method for the Silylation of Alcohols
Using Hexamethyldisilazane in an Ionic Liquid
Mohammad M. Mojtahedi
Hassan Abbasi
M. Saeed Abaee
Chemistry and Chemical Engineering Research Center of Iran, Tehran,
Iran
Benzylic and primary alcohols were protected by hexamethyldisilazane in
good to excellent yields at r.t. in 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim][BF ]) ionic liquid. In addition, the ionic liquid was recovered quantita-
4
tively and reused efficiently in the next reactions. The protocol was successfully
applied to the protection of phenols.
Keywords Silyl ethers; HMDS; ionic liquid; alcohols
Protected hydroxyl groups are very important for their organic1 and an-
alytical chemistry2 applications. Many organic transformations require
at least one step of alcohol group protection.3,4 One of the most popular
methods for this purpose is to convert alcohols into their corresponding
silyl ethers.5,6 However, some of the reported procedures suffer from
drawbacks, such as a limited reactivity and a cumbersome removal
of byproducts. Hexamethyldisilazane (HMDS) is a cheap and commer-
cially available reagent frequently used for the trimethylsilylation of
hydroxyl groups, giving ammonia as the only byproduct.7 However, poor
silylation power is a main limitation for the application of HMDS.8 Var-
ious catalytic systems have been developed to ease these processes.9−20
Although these methods improve reaction conditions and shorten the
reaction time, in many cases, drastic conditions still are necessary for
the completion of the process.
Received May 12, 2005; accepted August 2, 2005.
Partial financial support by the Ministry of Science, Research, and Technology of Iran
is greatly appreciated. M. Mirzaee is also acknowledged for conducting GC experiments.
Address correspondence to Mohammad M. Mojtahedi, Chemistry and Chemical En-
gineering Research Center of Iran, PO Box 14335-186, Tehran, Iran. E-mail: mojtahedi@
ccerci.ac.ir
1541

1542
M. M. Mojtahedi et al.
Ionic liquids have been studied extensively as a clean solvent for a
wide range of chemical transformations.21 In view of the emerging im-
portance of ionic liquids as novel reaction media and in continuation of
our previous experiences on the protection of hydroxyl groups,11 here we
report a mild and highly efficient method for the protection of alcohols
using HMDS in [bmim][BF4] (Scheme 1).
SCHEME 1
The treatment of benzyl alcohol with HMDS in [bmim][BF4] at r.t. af-
forded the corresponding trimethylsilyl derivative of the alcohol within
30 minutes in a 96% yield (Table I, entry 1). The product was extracted
by diethyl ether from the reaction medium. A variety of benzylic and
primary alcohols were converted to their corresponding TMS ethers by
using the same procedure (entries 2–13). The reactions were complete
in less than 60 min for benzylic alcohols and in less than 120 min for
other primary alcohols, and yields were 75–96%. The chemoselectivity
of the protocol was demonstrated using an equimolar mixture of benzyl
alcohol, cyclohexanol, and HMDS (Scheme 2). After 30 min, benzyl al-
cohol was converted to trimethylsilylbenzylether quantitatively, and no
formation of trimethylsilylcyclohexylether was detected. This chemose-
lectivity was further confirmed using tert-butyl alcohol. The subjection
of tertiarybutylalcohol to the same conditions expectedly resulted in
the formation of <10% of the corresponding silyl ether after 4 h. The
results are summarized in Table I.
SCHEME 2
In summary, the present protocol involves mild reaction conditions at
an ambient temperature with high yields of isolated products in short
reaction times. The silyl ether products were easily separable from the
reaction mixture by a simple extraction with diethyl ether. The use
of ionic liquid as a novel reaction medium for the transformation of
alcohols to silyl ethers is another advantage of the procedure, which
eases the reaction conditions. The ionic liquid can be recovered easily

Silylation of Alcohols in an Ionic Liquid
1543
TABLE I The Silylation of Alcohols Using HMDS in [bmim][BF4]
RT in
[bmin][BF4]
Reflux in
CH2Cl2
Time Yield Time Yield
Entry
Substrate
Product
(min) (%)a,b
(h)
(%)b,c
1
2
3
4
30
60
30
45
96
90
95
95
24
24
24
24
55
42
35
50
5
6
7
60
60
92
95
75
24
24
24
43
22
10
120
8
9
10
CH3(CH2)3CH2OH CH3(CH2)3CH2OTMS
CH3(CH2)4CH2OH CH3(CH2)4CH2OTMS
CH3(CH2)8CH2OH CH3(CH2)8CH2OTMS
120
120
120
75
75
85
24
24
24
12
10
10
11
12
45
45
92
96
24
24
10
12
13
14
60
90
87
95
24
24
15
11
15
90
97
24
18
aIsolated yields.
bReactions were monitored by TLC, and products were identified by GC-Mass
(GC-Mass Analysis: A Fisons Instruments gas chromatograph 8000 equipped with
mass detector (Trio 1000) with 70 ev were used).
cGC yields.

1544
M. M. Mojtahedi et al.
after the completion of the reaction and reused in at least 10 subsequent
experiments.
GENERAL PROCEDURE FOR THE PREPARATION
OF TRIMETHYLSILYLETHERS
A mixture of alcohol (2 mmol) and hexamethyldisilazane (2 mmol) in
2 mL of [bmim][BF4] was stirred at an ambient temperature for an
appropriate length of time (Table I). The course of the reaction was
monitored by thin layer chromatography (TLC) or gas chromatography
(GC). The reaction mixture was extracted twice by 10-mL portions of
diethyl ether. Combined ethereal phases were washed with water and
dried over anhydrous calcium chloride. Products were obtained after
evaporation of the volatile portion and were purified with a bulb-to-
bulb distillation unit.
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