Highly efficient and chemoselective trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by reusable electron-deficient [TiIV(salophen)(OTf)2]

Article abstract of DOI:10.1016/j.poly.2011.09.023

In the present work, highly efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by high-valent [Ti ^IV(salophen)(OTf)₂] is reported. Under these conditions, primary, secondary and tertiary alcohols as well as phenols were silylated in short reaction times and high yields. It is noteworthy that this method can be used for chemoselective silylation of primary alcohols in the presence of secondary and tertiary alcohols and phenols. The catalyst was reused several times without loss of its catalytic activity.

Full text of DOI:10.1016/j.poly.2011.09.023

Polyhedron 31 (2012) 332–338
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Highly efficient and chemoselective trimethylsilylation of alcohols and phenols
with hexamethyldisilazane (HMDS) catalyzed by reusable electron-deficient
[TiIV(salophen)(OTf)2]
b
b
Maryam Yadegari a, Majid Moghadam b, , Shahram Tangestaninejad , Valiollah Mirkhani ,
⇑
Iraj Mohammadpoor-Baltork b
a Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
b Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-7344, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2011
Accepted 17 September 2011
Available online 24 September 2011
In the present work, highly efficient trimethylsilylation of alcohols and phenols with hexamethyldisilaz-
ane (HMDS) catalyzed by high-valent [TiIV(salophen)(OTf)2] is reported. Under these conditions, primary,
secondary and tertiary alcohols as well as phenols were silylated in short reaction times and high yields.
It is noteworthy that this method can be used for chemoselective silylation of primary alcohols in the
presence of secondary and tertiary alcohols and phenols. The catalyst was reused several times without
loss of its catalytic activity.
Keywords:
Titanium Schiff base
HMDS
Ó 2011 Elsevier Ltd. All rights reserved.
Alcohol
Phenol
TMS-ether
1. Introduction
[30] have been reported for trimethylsilylation of alcohols and
phenols. Although these procedures provide an improvement,
The protection of hydroxyl groups is an important reaction
especially in the synthesis of fine chemicals and natural products.
Several methods such as acetylation, tetrahydropyranylation,
methoxymethylation and trimethylsilylation have been reported
for protection of hydroxyl groups [1,2]. The common method for
the silylation of hydroxy functional groups is the use of hexameth-
yldisilazane (HMDS). One disadvantage of HMDS is its poor silylat-
ing power in the absence of a suitable catalyst which needs forceful
conditions and long reaction times in many cases [3]. To solve this
problem, a variety of catalysts including (CH3)3SiCl [4], ZrCl4 [5],
ZnCl2 [6], K-10 montmorillonite [7], LiClO4 [8], H3PW12O40 [9],
iodine [10], InBr3 [11], zirconium sulfophenyl phosphonate [12],
CuSO4ꢀ5H2O [13], sulfonic acid-functionalized nanoporous silica
[14], MgBr2ꢀOEt2 [15], LaCl3 [16], poly(N-bromobenzene-1,3-disulf-
onamide) and N,N,N0,N0-tetrabromobenzene-1,3-disulfonamide
[17], Fe(TFA)3 [18], Fe3O4 [19], (n-Bu4N)Br [20], trichloroisocyanu-
ric acid (TCCA) [21], HClO4–SiO2 [22], Pd [23], ZrO(OTf)2 [24],
many of these catalysts or activators need long reaction times,
drastic reaction conditions or tedious work up, or are moisture sen-
sitive or expensive. Hence, introduction of new procedures to cir-
cumvent these problems is still in demand.
Schiff base complexes of Mn, Fe, Ru, Cr, Co and V have found many
applications in organic chemistry [31]. These compounds have been
used as catalyst in the oxidation of alkenes and alkanes, sulfides,
amines and alcohols. Titanium Schiff bases have been used as cata-
lyst for polymerization of ethylene and propene [32], regio- and ster-
eoselective epoxidation of allylic alcohols [33], asymmetric ring-
opening of epoxides by dithiophosphorus acid [34], enantioselective
catalytic ring-opening of epoxides with carboxylic acids [35], effi-
cient kinetic resolution of terminal epoxides by means of catalytic
hydrolysis [36], enantioselective trimethylsilylcyanation of alde-
hydes [37,38], oxidation of sulfides to sulfoxides with hydrogen per-
oxide [39], enantioselective ring-opening of meso-epoxides with
ArSH [40], asymmetric alkynylation of aldehydes [41] and enantio-
selective Pinacol coupling of aryl aldehydes [42].
LiClO4–SiO2 [25], PhMe3N+Br3 [26], poly(4-vinylpyridinium tri-
ꢁ
bromide) [27], 1,3-dichloro-5,5-dimethylhydantoin (DCH) and/or
trichloromelamine (TCM) [28] sulfamic acid [29] and H5IO6/KI
Electron-deficient complexes of Fe, Cr, Sn and V have been used
as mild Lewis acids in organic transformations [43–60]. In this pa-
per, we report the use of high-valent titanium(IV)salophen trifluo-
romethanesulfonate, [TiIV(salophen)(OTf)2], in the rapid and highly
efficient silylation of alcohols and phenols with HMDS at room
temperature (Scheme 1).
⇑
Corresponding author. Tel.: +98 311 7932712; fax: +98 311 6689732.
E-mail address: moghadamm@sci.ui.ac.ir (M. Moghadam).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.023

M. Yadegari et al. / Polyhedron 31 (2012) 332–338
333
OTf
N
N
Ti
O
O
H
N
OTf
CH2Cl2
R
OH
R OSiMe3
+
Me3Si
SiMe3
NH3
+
Scheme 1. Trimethylsilylation of alcohols and phenols with HMDS catalyzed by [TiIV(salophen)(OTf)2].
2.1. Preparation of [TiIV(salophen)Cl2]
Table 1
Optimization of catalyst amount in the silylation of benzyl alcohol with HMDS
catalyzed by [TiIV(salophen)(OTf)2] at room temperature.a
In a typical procedure, a solution of TiCl4(THF)2 (669 mg,
2.0 mmol) in dry THF (10 mL) was added slowly to a solution of
H2Salophen (633 mg, 2.0 mmol) in THF (10 mL), resulting in a
red-brown solution. The reaction mixture was stirred under reflux
at 70 °C for 1 h, and then cooled to room temperature, and the sol-
vent was evaporated. The solid was slurried with Et2O (20 mL), fil-
tered through a fine-fritted funnel, washed with additional Et2O,
and dried under vacuum at 80 °C for 2 h [61].
Entry
Catalyst amount
(%mol)
Solvent
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
1
2
3
0
0.1
0.1
0.5
1
2
3
CH3CN
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
EtOAc
1
1
1
1
1
10
1
1
1
1
44
56
79
16
33
47
42
75
100
100
37
15
30
2.2. Preparation of [TiIV(salophen)(OTf)2]
9
10
11
12
13
To a solution of [TiIV(salophen)Cl2] (434 mg, 1 mmol) in CH2Cl2
(15 mL) was added an acetonitrile solution (15 mL) of AgOTf
(513.9 mg, 2 mmol), producing a brown precipitate. The AgCl was
filtered through a fine fritted funnel, and the filtrate was concen-
trated to dryness. The solid was extracted with CH2Cl2 and the
resulting solid was isolated [61].
2
2
2
1
1
1
n-Hexane
CHCl3
a
Reaction conditions: benzyl alcohol (1 mmol), HMDS (0.7 mmol), solvent
(1 mL).
b
GC yield.
1H NMR (400 MHz, CD3OD) d = 9.44 (s, 2H, H–C@N), 8.10 (dd,
2H, Ar, 1J = 6.2, 2J = 3.2 Hz), 7.95 (d, 2H, Ar, J = 7.6 Hz), 7.72 (m,
2H, Ar), 7.64 (dd, 2H, Ar, 1J = 6.4, 2J = 3.2 Hz), 7.18 (m, 2H, Ar),
6.96 (d, 2H, Ar, J = 8 Hz); 13C NMR (100 MHz, CD3OD) d = 116.53,
117.24 (CH), 117.58 (O–C–C), 121.24 (CF3), 123.62, 130.41,
136.37, 138.10 (CH), 141.46 (N–C), 160.56 (O–C), 161.64 (N@CH).
2. Experimental
vAll chemicals were purchased from Merck or Fluka chemical
companies. FT-IR spectra were obtained with potassium bromide
pellets in the range 400–4000 cmꢁ1 with a Nicolet Impact 400D
spectrometer. Gas chromatography experiments (GC) were per-
formed with a Shimadzu GC-16A instrument using a 2 m column
packed with silicon DC-200 or Carbowax 20 m. In the GC exper-
iments, n-decane was used as an internal standard. 1H NMR
IR (KBr): m(CC@N) 1603, m(SC@O) 1168 and m(C–F) 1280.
2.3. General procedure for trimethylsilylation with HMDS catalyzed by
[TiIV(salophen)(OTf)2]
To a mixture of alcohol or phenol (1 mmol), HMDS (0.7 mmol
per OH group) in CH2Cl2 (1 mL) was added [TiIV(Salophene)(OTf)2]
(0.01 mmol) and stirred at room temperature for appropriate time
spectra were recorded on
spectrometer.
a
Bruker-Avance AQS 400 MHz
N
N
O
MeOH
HO
OH
+
2
OH
H2N
NH2
TiCl4(THF)2
THF, Reflux
OTf
Cl
N
O
N
O
AgOTf (2 eq)
N
O
N
O
Ti
Ti
Cl
OTf
Scheme 2. The preparation route for catalyst.

334
M. Yadegari et al. / Polyhedron 31 (2012) 332–338
Table 2
Trimethylsilylation of alcohols with HMDS catalyzed by [TiIV(salophen)(OTf)2] at room temperature.a
Entry
Hydroxy compound
TMS ether
Time (min)
1
Yield (%)b
100
1
CH2OH
Me
CH2OSiMe3
Me
OH
OSiMe3
2
3
1
90
1
1c
1d
1e
100
100
96
MeO
CH2OH
MeO
CH2OSiMe3
95
4
CH2OH
OMe
4
95
CH2OSiMe3
OMe
1
1c
1d
1e
100
99
97
5
6
t-Bu
Cl
CH2OH
t-Bu
CH2OSiMe3
95
15
15c
15d
15e
90
90
89
87
Cl
CH2OSiMe3
CH2OH
CH2OH
7
8
9
6
90
CH2OSiMe3
O2N
O2N
O2N
O2N
CH2OSiMe3
15
15c
15d
15e
100
98
95
CH2OH
94
CH2OH
NO2
CH2OSiMe3
NO2
35
90
10
11
12
10
10
90
Cl
CH2OH
Cl
CH2OSiMe3
Cl
Cl
CH2OH
CH2OSiMe3
100
Br
Br
1
1c
1d
1e
100
100
94
OH
OSiMe3
N
N
94
8
90
90
13
14
CH2CH2CH2OSiMe3
OSiMe3
CH2CH2CH2OH
OH
15
15
16
15
10
95
96
OH
OSiMe3
OSiMe3
OH
OH
OSiMe3
17
10
100

M. Yadegari et al. / Polyhedron 31 (2012) 332–338
335
Table 2 (continued)
Entry
Hydroxy compound
TMS ether
Time (min)
Yield (%)b
18
19
15
15
97
98
OH
OSiMe3
OH
CH3
OSiMe3
CH3
Me
OSiMe3
Me
Me
Me
OH
20
21
25
25
96
98
OH
OSiMe3
a
Reaction conditions: alcohol (1 mmol), HMDS (0.7 mmol), catalyst (1 mol%), CH2Cl2 (1 mL).
GC yield.
Data refers to second, third and fourth runs, respectively.
b
c–e
(Table 1). The progress of the reaction was monitored by GC or TLC.
n-hexane (10 mL) was added and the catalyst was filtered off.
After the reaction was complete, the solvent was evaporated,
The filtrates were washed with brine and dried over Na2SO4 and
Table 3
Trimethylsilylation of phenols with HMDS catalyzed by [TiIV(Salophen)(OTf)2] at room temperature.a
Entry
1
Phenol
TMS ether
Time (min)
10
Yield (%)b
95
OSiMe3
OSiMe3
OH
8
8
95
95
2
3
Cl
Cl
OH
OH
OSiMe3
Cl
Cl
Me3SiO
4c
20
20
97
95
OSiMe3
OH
HO
5c
OH
OSiMe3
OSiMe3
OH
20
95
6c
OH
OSiMe3
OH
Me3SiO
Me3SiO
7c
1
100
OH
OSiMe3
OSiMe3
HO
OH
(continued on next page)

336
M. Yadegari et al. / Polyhedron 31 (2012) 332–338
Table 3 (continued)
Entry
Phenol
TMS ether
Time (min)
15
Yield (%)b
94
8
9
O2N
OH
OH
O2N
H3C
OSiMe3
OSiMe3
15
4
90
95
H3C
10
OH
OSiMe3
H3C
H3C
a
Reaction conditions: phenol (1 mmol), HMDS (0.7 mmol), catalyst (1 mol%), CH2Cl2 (1 mL).
GC yield.
Reaction was performed with 0.7 mmol of HMDS per OH group.
b
c
concentrated under reduced pressure to afford the crude product.
phen)(OTf)2], [TiIV(salophen)Cl2] and [TiIV(salophen)(OPh)2] was
investigated in the silylation of benzyl alcohol with HMDS. The re-
sults showed that the catalytic activity of these catalysts is in the
following order: [TiIV(salophen)(OTf)2] (100%) > [TiIV(salophen)Cl2]
(61%) > [TiIV(salophen)(OPh)2] (37%). The results clearly show that
introducing the OTf groups on the Ti(salophen) increases the elec-
tron-deficiency of the catalyst which in turn increases the catalytic
activity of [TiIV(salophen)(OTf)2] in the trimethylsilylation with
HMDS. The reaction parameters such as catalyst amount and kind
of solvent were also optimized in the reaction of benzyl alcohol
with HMDS. When the same reaction was carried out using differ-
The identity of the products was verified by 1H NMR and IR spectral
data.
3. Results and discussion
3.1. Trimethylsilylation of alcohols and phenols with HMDS catalyzed
by [TiIV(salophen)(OTf)2]
The preparation route for the [TiIV(salophen)(OTf)2] catalyst is
shown in Scheme 2. First, the catalytic activity of [TiIV(salo-
Table 4
Selective silylation of alcohols and phenols catalyzed by [TiIV(salophen)(OTf)2] in CH2Cl2.a
Row
ROH
Silyl ether
Time (min)
Yield (%)b
100
CH2OH
OH
CH2OSiMe3
OSiMe3
1
1
0
100
0
CH2OH
CH2OSiMe3
Me
2
3
4
1
1
1
Me
OH
Me
Me
OSiMe3
CH2OSiMe3
CH2OH
90
10
OH
OSiMe3
100
CH2OSiMe3
OSiMe3
CH2OH
OH
0
a
Reaction conditions for a binary mixture: 1 mmol of each alcohol or phenol, HMDS (0.7 mmol), catalyst (1 mol%), CH2Cl2 (1 mL).
GC yield.
b

M. Yadegari et al. / Polyhedron 31 (2012) 332–338
337
Table 5
Comparison of the results obtained for the trimethylsilylation of benzyl alcohol catalyzed by [TiIV(salophen)(OTf)2] with those obtained by the recently reported catalysts.
Catalyst
OSiMe3
OH
Entry
Catalyst
HMDS (mmol)
Catalyst (mol%)
T (°C)
Time (min)
Yield (%)
TOF (hꢁ1
)
Ref.
1
2
3
4
5
6
7
8
[TiIV(salophen)(OTf)2]
ZrCl4
H3PW12O40
Iodine
0.7
0.8
0.8
0.8
0.7
0.6
0.6
0.7
1.4
0.7
1
1
2
1
1
1
R.T.
R.T.
55–60 °C
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
1
1
100
95
90
98
98
99
98
91
90
92
98
90
98
92
6000
2850
235
2940
490
36
235
3
45
442
118
3.8
1176
11 040
This work
[5]
[9]
23
2
12
55
5
3 h
30
5
[10]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[21]
[22]
[24]
CuSO4ꢀ5H2O
Sulfonic acid@nanoporous silica
3
5
MgBr2ꢀOEt2
LaCl3
10
4
2.5
10
6
9
N,N,N0,N0-tetrabromobenzene-1,3-disulfonamide
10
11
12
13
14
Fe(TFA)3
Fe3O4
Trichloroisocyanuric acid (TCCA)
HClO4–SiO2
ZrO(OTf)2
5
4 h
2
0.8
0.8
2
2.5
0.5
R.T.
1
ent amounts of catalyst, the highest yield was obtained in the pres-
ence of 1 mol% of [TiIV(salophen)(OTf)2]. A lower amount of cata-
lyst did not improve the yield of the product even after longer
reaction time. In order to choose the reaction media, the above
mentioned reaction was performed in different solvents. Among
CH2Cl2, CH3CN, EtOAc and n-hexane and chloroform, the highest
yield was observed in CH2Cl2. Since, the catalyst has a heteroge-
neous nature in CH2Cl2, therefore, the catalyst reusability in this
solvent is simpler, and therefore dichloromethane was used as
solvent.
Under the optimized reaction conditions, a wide range of alco-
hols including primary (benzylic and linear ones), secondary and
tertiary alcohols were successfully trimethylsilylated with HMDS
in the presence of [TiIV(salophen)(OTf)2]. In the case of benzylic
alcohols, electron-withdrawing substituents require longer
reaction times for completion, while benzylic alcohols bearing
electron-donating substituents were completed in shorter reaction
times (Table 2). The results in Table 2 also show that steric hin-
drance of ortho substituents did not affect the reaction time. The
only exception is 2-nitrobenzyl alcohol. It seems that intramolecu-
lar hydrogen bonding between hydroxyl and nitro groups reduces
the acidity of alcohol and therefore the silylation require longer
reaction time.
In the case of linear, secondary and tertiary alcohols the reac-
tion times were longer in comparison with benzylic alcohols.
The trimethylsilylation of phenols was also investigated and the
corresponding TMS-ethers were produced in high yields (Table 3).
Due to the high catalytic activity of [TiIV(salophen)(OTf)2] in the
trimethylsilylation of both alcohols and phenols, therefore, we
decided to investigate the chemoselectivity of the presented
method. In this manner, a set of competitive reactions was con-
ducted between primary or secondary and tertiary alcohols or phe-
nols (Table 4). The results indicated that primary alcohols are more
reactive in the presence of secondary or tertiary alcohols and also
phenols. It is important to note that due to the lesser nucleophilic-
ity of sterically hindered alcohols and phenols in comparison with
primary benzylic alcohols, only primary benzylic alcohols were
silylated with 0.7 mmol of HMDS (Table 4, entries 1, 2 and 4). How-
ever, in the presence of 1.1 mmol of HMDS, both phenol and benzyl
alcohol were silylated completely.
(Table 5). Some of catalysts such as sulfonic acid@nanoporous sil-
ica [14], LaCl3 [16] and TCCA [21] need long reaction times (1–4 h).
H3PW12O40 needs high temperature [9]. N,N,N0,N0-tetrabromoben-
zene-1,3-disulfonamide [17] and ZrO(OTf)2 [24] were carried out
with high amounts of HMDS. ZrCl4 is degraded in the presence of
moisture [5]. SiMe3Cl is carcinogen [4]. MgBr2ꢀOEt2 [15], LaCl3
[16], Fe3O4 [19] and TCCA [21] need high catalyst amount (5–
10%). Except ZrO(OTf)2 [24], which needs 2 mmol HMDS, the
[TiIV(salophen)(OTf)2] is much efficient than the others and higher
TOF was observed. On the other hand, simple preparation of the
catalyst, short reaction times, using low catalyst amount and
HMDS, high product yields and the reusability of the catalyst are
noteworthy advantages of this catalytic system.
3.2. Catalyst reusability
Finally, the reusability of the catalyst was tested in the reaction
of 4-chlorobenzyl alcohol as a model substrate. It was observed
that the activity of catalyst did not decrease after four consecutive
times (Table 2, entry 6). For a complete study, the reusability of
catalyst was also investigated in the trimethylsilylation of different
substrates such as 4-methoxybenzyl alcohol (Table 2, entry 3), 4-
tert-butylbenzyl alcohol (Table 2, entry 5), 3-pyridylmethanol (Ta-
ble 2, entry 8) and 4-nitrobenzyl alcohol (Table 2, entry 12) with
HMDS. As can be seen, the results are as same as for 4-chlorobenzyl
alcohol.
Acknowledgement
The partial support of this work by University of Isfahan and the
Islamic Azad University is gratefully acknowledged.
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