Trimethylsilylation of hydroxyl group with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) catalyzed by tribromomelamine (TBM)

Article abstract of DOI:10.1002/jccs.200800180

Tribromomelamine (TBM) can be used as a novel catalyst for the trimethylsilylation of alcohols and phenols with 1,1,1,3,3,3- hexamethyldisilazane (HMDS). A wide variety of hydroxyl groups were selectively protected in CH₂Cl₂/CH₃CN under mild conditions.

Full text of DOI:10.1002/jccs.200800180

1208
Journal of the Chinese Chemical Society, 2008, 55, 1208-1213
Communication
Trimethylsilylation of Hydroxyl Group with 1,1,1,3,3,3-
Hexamethyldisilazane (HMDS) Catalyzed by Tribromomelamine (TBM)
Arash Ghorbani-Choghamarani,a* Mohammad Ali Zolfigol,b Maryam Hajjamib and Shila Jafarib
aDepartment of Chemistry, Faculty of Science, Ilam University, P. O. Box 69315516, Ilam, Iran
bFaculty of Chemistry, Bu-Ali Sina University, Hamadan, Iran
Tribromomelamine (TBM) can be used as a novel catalyst for the trimethylsilylation of alcohols and
phenols with 1,1,1,3,3,3-hexamethyldisilazane (HMDS). A wide variety of hydroxyl groups were selec-
tively protected in CH2Cl2/CH3CN under mild conditions.
Keywords: Tribromomelamine (TBM); Trimethylsilylation; Hexamethyldisilazane (HMDS); Al-
cohols.
Silyl ethers are the most popular and useful protect-
ing groups of the hydroxy function in synthetic organic
chemistry and various types of silyl ethers have been devel-
oped.1 Silylation of alcohols is an important process not
only as a protecting method of alcohols, but also for the
synthesis of functional organosilicon compounds.2 Tri-
methylsilylation is used extensively for the protection and
the derivatization of most functional groups to increase
volatility for gas chromatography and mass spectrome-
try.3-6
ested to synthesis tribromomelamine (TBM) via reaction of
melamine with bromine in alkali media (Scheme I). It is in-
teresting to note that the producing of TBM is easy and
clean without a difficult work-up procedure.
Scheme I
BrHN
NaOH (aq)
N
NHBr
H2N
N
NH2
N
N
N
N
+ Br2
Although many reagents are available for the silyla-
tion of hydroxyl groups,7-29 some of these methods suffered
from serious obstacles such as low selectivity, long reac-
tion times, low yields of products, toxicity, expensive re-
agents and catalysts, delicate purification step, lack of reac-
tivity or the difficulty in removal of by-products. 1,1,1,3,3,3-
Hexamethyldisilazane (HMDS) as a cheap, stable and
commercially available reagent is one of the most widely
used reagents for the silylation of hydroxyl groups. Its han-
dling does not require special precautions, and the workup
is not time-consuming, because the by-product of the reac-
tion is ammonia, which is simple to remove from the reac-
tion media. However, the low silylating power of HMDS is
the main drawback to its application. Therefore to activate
this reagent, an appropriate catalyst should be used. In con-
tinuation of our ongoing effort to develop new methods in
organic functional group transformations,30-36 especially
for the protection of the hydroxyl groups,37-41 we are inter-
NHBr
NH2
TBM
Consequently, herein we disclose a very effective and
mild procedure for the trimethylsilyl protection of a wide
variety of OH-containing substrates, including primary,
secondary, benzylic, hindered secondary, tertiary alcohols
and some phenols using hexamethyldisilazane (HMDS) in
the presence of catalytic amounts of tribromomelamine
(TBM) under mild conditions at room temperature.
Initially, we screened different solvents for the tri-
methylsilylation of benzyl alcohol with HMDS in the pres-
ence of a catalytic amount of TBM. As can be seen from Ta-
ble 1, benzyl alcohol was silylated in acetonitrile and di-
chloromethane faster than the other solvents. But, the re-
sults showed that reaction was performed in the mixture of
acetonitrile and dichloromethane as fastest media. The sol-
vent system of dichloromethane and acetonitrile (v/v = 4:1)
* Corresponding author. Tel/Fax: +98-811-8257404; E-mail: arashghch58@yahoo.com

Trimethylsilylation of Alcohols and Phenols
J. Chin. Chem. Soc., Vol. 55, No. 6, 2008 1209
ing 1,1,1,3,3,3-hexamethyldisilazane (HMDS) I in the
presence of a catalytic amount of tribromomelamine (TBM)
II in the mixture of dichloromethane/acetonitrile (v/v =
4:1) at room temperature with good to excellent yields
(Scheme II and Table 2).
Table 1. Trimethylsilylation of benzyl alcohol using HMDS in
the presence of TBM as catalyst at room temperature in
different solventsa
Entry
Solvent
Time (Min)
Yield (%)b
1
2
3
4
5
6
Acetonitrile
Chloroform
65
240
30
100b
50b
To show the reactivity and selectivity of described
system, some hydrogen-labile substrates, in addition to al-
cohols; such as phenols, amines and thiols were subject to
trimethylsilylation with hexamethyldisilazane in the pres-
ence of catalytic amounts of TBM and found that only phe-
nols were silylated (Table 3).
Dichloromethane
n-Hexane
100b
c
180
180
10
-
c
Ethyl acetate
CH2Cl2/CH3CNd
-
98e
a Substrate/HMDS/TBM (1:0.8:0.03 mmol).
b GC yield.
c Trace conversion.
d CH2Cl2:CH3CN = 4:1 (mL).
e Isolated yield.
A reaonable mechanism of this reaction is shown in
Scheme III based on reported pathway in literatures,14,16,42
our observations, and the obtained results. Tribromomela-
mine acts as a source of Br+, which polarizes the Si-N bond
in HMDS to produce active silylating agent.
was used throughout the work due to its high reaction rate
and easy work up procedures.
In conclusion, we introduce a new silylating catalyst
for the simple and mild synthesis of trimethylsilylated
products using 1,1,1,3,3,3-hexamethyldisilazane (HMDS).
Eventually, we wish to report here the selective tri-
methylsilylation of different types of hydroxyl groups us-
Scheme II

1210 J. Chin. Chem. Soc., Vol. 55, No. 6, 2008
Ghorbani-Choghamarani et al.
Table 2. Trimethylsilylation of alcohols 1 to the corresponding trimethylsilyl alcohols 2 using HMDS
I in the presence of a catalytic amount of TBM II in the mixture of CH2Cl2/CH3CN (v/v =
4:1) at room temperature
Substrate/HMDS/TBM (mmol)
Time
Yield
(%)a
Entry Substrate Product
Ref.c
(Min)
I
II
1
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
0.8
0.8
0.8
1.6
1.2
1.2
1.2
1.2
0.8
0.8
1.2
1.2
1.2
0.8
0.9
0.8
0.8
0.8
1.2
5.5
0.9
0.03
0.03
0.03
0.10
0.04
0.06
0.06
0.06
0.04
0.04
0.06
0.06
0.06
0.03
0.04
0.03
0.03
0.03
0.06
0.14
0.04
10
75
20
95
30
30
75
10
25
25
45
45
45
30
20
10
20
30
25
1800
90
98
98
14,16,42,43,45
2
42
3
94
14,16,42
4
98
14,16,42,45
5
79
42
6
96
100b
16,42
7
1g
1h
1i
2g
2h
2i
14
8
71
14
9
88b
94b
87
48
10
11
12
13
14
15
16
17
18
19
20
21
1j
2j
48
1k
1l
2k
2l
47
94
42,43
1m
1n
1o
1p
1q
1r
1s
2m
2n
2o
2p
2q
2r
2s
97
16,42
84
42
93
14,16,42,44
42
88
97
67b
16,43,45
14,16,42,43,45
14,42
80
1t
2t
83
16,43
16
1u
2u
99
a Isolated yield.
b GC yield.
c All products are know and were identified using comparison of their physical and spectral data (IR,
NMR, or MS) with those reported in the literature.
Scheme III
isting methodologies.
Me3SiNHSiMe3
EXPERIMENTAL
N
Chemicals were purchased on Fluka, Merck and Aldrich
chemical companies. The silylated products were charac-
terized by comparison of their spectral (IR, 1H NMR, and
13C NMR) and physical data with authentic samples.43-45
Synthesis of tribromomelamine (TBM)
Br
HN
N
N
N
NH
NHBr
SiMe3
Me3Si
R
OH
A 100 mL round bottom flask was equipped with a
dropping funnel containing Br2 (6 mL, 118 mmol). The
flask was charged with 5 M NaOH (30 mL) and melamine
(5 g, 39 mmol). Then bromine was added drop wise for 10
min at room temperature. After the addition, the mixture
was stirred for 2 hours then filtered off. Residue was washed
with water (6 ´ 50 mL) then dried. Tribromomelamine was
obtained quantitatively. Tribromomelamine (TBM): yel-
low solid; Elemental analysis: calculated: C (9.9%), H
(0.8%), N (23.1%); obtained: C (9.2%), H (1.0%), N
NH3
R
OSiMe3
+
R
OSiMe3
H2
N
Me3Si
Br
R
OH
NH
N
N
This method offers the advantage of shorter reaction times,
high chemoselectivity, and easy workup. We believe that
this methodology could be an important addition to the ex-

Trimethylsilylation of Alcohols and Phenols
J. Chin. Chem. Soc., Vol. 55, No. 6, 2008 1211
Table 3. Trimethylsilylation of phenols, amines and thiols to the corresponding trimethylsilylated
products using HMDS I in the presence of a catalytic amount of TBM II in the mixture
of CH2Cl2/CH3CN (v/v = 4:1) at room temperature
Substrate/HMDS/TBM
Time Yield
Entry
Substrate
Product
Ref.c
(mmol)
(h)
3.5
(%)a
100a
I
II
3a
OH
4a
OTMS
1
2.5
0.06
16,42
3b
OH
4b
OTMS
2
2.5
0.06
3.5
33a
46
COCH3
COCH3
3c
OH
4c
OTMS
3
4
2.5
2.5
0.06
0.06
3
97a
16,42
---
OCH3
OCH3
5a
N
6a
N
4.5
--b
SH
STMS
S
S
5b
SH
6b
STMS
5
6
2.5
2.5
0.06
0.06
4.5
4.5
--b
---
---
Br
Br
7a
8a
N
H
N
--b
TMS
a GC yield.
b No reaction.
c All products are know and were identified using comparison of their physical and spectral data
(IR, NMR, or MS) with those reported in the literature.
(19.9%); MS: m/z = 126 [M+ - (3Br) + 3H+, 89%], 203 [M+ -
Some representative spectral data
(2Br)], 283 [M+ - (Br)], 362 [M+ (very week)], 368 [M + 6].
General procedure for trimethylsilylation of alcohols
using HMDS catalyzed by TBM
2a) Benzyl trimethylsilyl ether: 1H-NMR (90 MHz,
CDCl3): d = 0.19 (s, 9H), 4.73 (s, 2H), 7.34 (m, 5H) ppm;
IR (nojole): n = 682, 752, 1016, 1249, 1381, 1450, 2853
cm-1.
To a mixture of HMDS (0.7 mmol) and catalysts
(0.03-0.14 mmol) in CH2Cl2 (4 mL)/CH3CN (1 mL) an
alcohol (1 mmol) was added, and the mixture was stirred at
room temperature for the specified time (Table 2). Then re-
action was quenched with water (5 mL), and organic phase
dried over Na2SO4 (3 g) after 5 min. Evaporation of the di-
chloromethane gave the highly pure products without fur-
ther purification.
1
2b) p-Bromobenzyl trimethylsilyl ether: H-NMR
(90 MHz, CDCl3): d = 0.17 (s, 9H), 4.65 (s, 2H), 7.15-7.51
(m, 4H) ppm; IR (nojole): n = 690, 842, 874, 1011, 1085,
1250, 1487, 1593, 2957, 3026 cm-1.
1
2c) p-Chlorobenzyl trimethylsilyl ether: H-NMR
(90 MHz, CDCl3): d = 0.19 (s, 9H), 4.68 (s, 2H), 7.29 (m,
4H) ppm; IR (nojole): n = 657, 874, 1015, 1252, 1372,

1212 J. Chin. Chem. Soc., Vol. 55, No. 6, 2008
Ghorbani-Choghamarani et al.
844, 888, 1109, 1251, 1377, 1458, 1597, 1687 cm-1.
2t) 1-Phenyl-2-methyl trimethylsilyl ether: 1H-NMR
(90 MHz, CDCl3): d = 0.56 (s, 9H), 1.20 (s, 6H), 2.71 (s,
2H), 7.21 (m, 5H) ppm; IR (nojole): n = 700, 741, 838,
1044, 1249, 1364, 1381, 1454, 1494, 1604 cm-1.
1491, 2958, 3029 cm-1.
2d) p-Nitrobenzyl trimethylsilyl ether: 1H-NMR (90
MHz, CDCl3): d = 0.00 (s, 9H), 4.60 (s, 2H), 7.33-7.89 (2d,
4H) ppm; IR (nojole): n = 741, 847, 1015, 1254, 1348,
1378, 1465, 1524, 2928 cm-1.
2u) 2-Admanthyl trimethylsilyl ether: 1H-NMR (90
MHz, CDCl3): d = 0.08 (s, 9H), 1.72 (m, 14H), 3.77 (m,
1H) ppm; IR (nojole): n = 752, 839, 880, 1036, 1071, 1103,
1133, 1249, 1354, 1381, 1450, 1460 cm-1.
2e) 2,4-Dichlorobenzyl trimethylsilyl ether: 1H-NMR
(90 MHz, CDCl3): d = 0.12 (s, 9H), 4.64 (s, 2H), 7.24 (m,
3H) ppm; IR (nojole): n = 725, 843, 875, 1091, 1253, 1375,
1465, 1565, 1591, 2920 cm-1.
2f) 2-Phenyl ethyl trimethylsilyl ether: 1H-NMR (90
MHz, CDCl3): d = 0.14 (s, 9H), 2.90 (t, 2H), 3.85 (t, 2H),
7.29 (s, 5H) ppm; IR (nojole): n = 698, 748, 880, 1031,
1251, 1457, 1497, 1605, 3065, 3087 cm-1.
2g) 2-[(Trimethylsilyloxy)methyl]pyridine: 1H-NMR
(90 MHz, CDCl3): d = 0.11 (s, 9H), 4.74 (s, 2H), 7.07-7.45
(m, 3H), 8.41 (m, 1H) ppm; IR (nojole): n = 753, 877, 1083,
1114, 1251, 1377, 1462, 1593, 2854 cm-1.
1
4a) Trimethyl phenoxy silane: H-NMR (90 MHz,
CDCl3): d = 0.24 (s, 9H), 6.87-7.21 (m, 5H) ppm; IR (KBr):
n = 692, 759, 844, 1024, 1165, 1253, 1493, 1597, 2961,
3040 cm-1.
1
4c) 4-Methoxy phenoxy trimethylsilane: H-NMR
(90 MHz, CDCl3): d = 0.22 (s, 9H), 3.75 (s, 3H), 6.8-7.26
(m, 4H) ppm; IR (KBr): n = 846, 924, 1040, 1234, 1252,
1466, 1507, 2959 cm-1.
2l) 1-Octhyl trimethylsilyl ether: 1H-NMR (90 MHz,
CDCl3): d = 0.58 (s, 9H), 0.84-1.23 (m, 17H), 3.52 (t, 2H)
ppm; IR (nojole): n = 746, 840, 1099, 1250, 1386, 1468
cm-1.
ACKNOWLEDGMENT
Financial support for this work by the Center of Ex-
cellence of Development of Chemical Methods (CEDCM)
of Bu-Ali Sina University, Hamadan, Iran is gratefully
appreciated.
1
2m) Benzhydryloxytrimethylsilane: H-NMR (90
MHz, CDCl3): d = 0.39 (s, 9H), 5.84 (s, 1H), 7.32-7.97 (m,
10H) ppm; IR (nojole): n = 721, 841, 1098, 1250, 1377,
1456, 1463 cm-1.
Received July 14, 2008.
1
2n) Indenoxytrimethylsilane: H-NMR (90 MHz,
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