[SnIV(TPP)(BF4)2]: An efficient and reusable catalyst for chemoselective trimethylsilylation of alcohols and phenols with hexamethyldisilazane

Article abstract of DOI:10.1016/j.poly.2009.07.008

Tin(IV)tetraphenylporphyrinato tetrafluoroborate, [Sn^IV(TPP)(BF₄)₂], was used as an efficient catalyst for trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS). High-valent [Sn^IV(TPP)(BF₄)₂] catalyzes trimethylsilylation of primary, secondary and tertiary alcohols as well as phenols, and the corresponding TMS-ethers were obtained in high yields and short reaction times at room temperature. While, under the same reaction conditions [Sn^IV(TPP)Cl₂] is less efficient to catalyze these reactions. One important feature of this catalyst is its ability in the 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.2009.07.008

Polyhedron 29 (2010) 212–219
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Polyhedron
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[SnIV(TPP)(BF4)2]: An efficient and reusable catalyst for chemoselective
trimethylsilylation of alcohols and phenols with hexamethyldisilazane
*
*
Majid Moghadam , Shahram Tangestaninejad , Valiollah Mirkhani, Iraj Mohammadpoor-Baltork,
Shadab Gharaati
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Tin(IV)tetraphenylporphyrinato tetrafluoroborate, [SnIV(TPP)(BF4)2], was used as an efficient catalyst for
trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS). High-valent
[SnIV(TPP)(BF4)2] catalyzes trimethylsilylation of primary, secondary and tertiary alcohols as well as phe-
nols, and the corresponding TMS-ethers were obtained in high yields and short reaction times at room
temperature. While, under the same reaction conditions [SnIV(TPP)Cl2] is less efficient to catalyze these
reactions. One important feature of this catalyst is its ability in the 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.
Article history:
Available online 6 August 2009
Keywords:
Alcohol
Phenol
Trimethylsilyl ether
Hexamethyldisilazane
High-valent tin(IV) porphyrin
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
InBr3 [16], zirconium sulfophenyl phosphonate [17], CuSO4ꢀ5H2O
[18], sulfonic acid-functionalized nanoporous silica [19],
The protection of functional groups is often necessary during
the course of various transformations in a synthetic sequence,
especially in the synthesis of fine chemicals and natural products
[1–3]. Hydroxy functional group, which is a versatile functional
group, can be protected by a wide variety of methods such as tet-
rahydropyranylation, acetylation, methoxymethylation and trim-
ethylsilylation. Commonly, silyl ethers are prepared by treatment
of hydroxyl compounds with silyl chlorides or silyl triflates in
the presence of bases such as imidazole [4], 4-(N,N-dimethyl-
amino)pyridine [5], N,N-diisopropylethylamine [6] and Li2S [7].
However, some of these silylation methods suffer from disadvan-
tages such as the lack of reactivity or the difficulty in removal of
amine salts derived during the silylation reaction. To overcome
these problem, 1,1,1,3,3,3-hexamethyldisilazane (HMDS), which
is a stable, commercially available and cheap reagent, is used for
preparation of silyl ethers from hydroxyl compounds. This silyla-
tion method does not need special precautions and produces
ammonia as by-product, but silylation in the absence of a suitable
catalyst needs forceful conditions and long reaction times [8]. In
order to increase the silylating power of HMDS, a variety of cata-
lysts including trichloroisocyanuric acid (TCCA) [9], silica sup-
ported perchloric acid (HClO4–SiO2) [10], ZrCl4 [11], K-10
montmorilonite [12], LiClO4 [13], H3PW12O40 [14], iodine [15],
MgBr2ꢀOEt2 [20], LaCl3 [21], poly(N-bromobenzene-1,3-disulfona-
mide) and N,N,N0,N0-tetrabromobenzene-1,3-disulfonamide [22],
Fe(TFA)3 [23], Fe3O4 [24], (n-Bu4N)Br [25] and ZrO(OTf)2 [26] have
been reported. Although these procedures provide an improve-
ment, but many of these catalysts need long reaction times, drastic
reaction conditions or tedious workups, moisture sensitive or
expensive of the catalyst. Hence, introduction of new procedures
to circumvent these problems is still in demand.
Electron-deficient metalloporphyrins have been used as mild
Lewis acids catalysts [27–32]. Recently, chromium and iron por-
phyrins have been used for organic transformations. Chromium(III)
tetraphenylporphyrinato chloride, Cr(tpp)Cl, has been applied for
regio-selective [3,3] rearrangement of aliphatic allyl vinyl ethers
and for Claisen rearrangement of simple aliphatic allyl vinyl ethers.
Iron(III)tetraphenylporphyrinato triflate, Fe(tpp)OTf, has been used
for rearrangement of
a,b-epoxy ketones into 1,2-diketones and
chromium(III)tetraphenylporphyrinato triflates, Cr(tpp)OTf, has
been reported for highly regio- and stereoselective rearrangement
of epoxides to aldehydes [33–36].
Recently, we have reported the use of tin(IV)tetraphenylpor-
phyrinato perchlorate [37,38], tin(IV)tetraphenylporphyrinato
trifluoromethanesulfonate [39,40], and tin(IV)tetraphenylpor-
phyrinato tetrafluoroborate [41,42] in organic synthesis.
In this paper, we wish to report a rapid and highly efficient
method for trimethylsilylation of alcohols and phenols with hex-
amethyldisilazane catalyzed by high-valent [SnIV(TPP)(BF4)2] at
room temperature (Scheme 1).
* Corresponding authors. Tel.: +98 311 7932712; fax: +98 311 6689732
(M. Moghadam).
E-mail addresses: moghadamm@sci.ui.ac.ir, majidmoghadamz@yahoo.com
(M. Moghadam), stanges@sci.ui.ac.ir (S. Tangestaninejad).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.07.008

M. Moghadam et al. / Polyhedron 29 (2010) 212–219
213
Table 1
BF4
Investigation of catalytic activity of different metalloporphyrin catalysts in the
N
N
N
IV
Sn
trimethylsilylation of 4-chlorobezyl alcohol with HMDS.a
N
H
N
Entry
Catalyst
Time (min)
Yield (%)b
BF4
R
OH
R OSiMe3
+
Me3Si
SiMe3
NH3
[SnIV(TPP)(BF4)2]
[SnIV(TPP)Cl2]
[FeIII(TPP)(BF4)]
[MnIII(TPP)(BF4)]
[FeIII(TPP)Cl]
1
1
1
1
1
1
1
100
21
77
43
26
17
8
+
1
2
3
4
5
6
7
CH3CN
Scheme 1. Trimethylsilylation of alcohols and phenols with HMDS catalyzed by
[SnIV(TPP)(BF4)2].
[MnIII(TPP)Cl]
[Cu(TPP)]
2. Experimental
a
Reaction conditions: 4-chlorobenzyl alcohol (1 mmol), HMDS (0.5 mmol), cat-
alyst (1 mol%), CH3CN (0.5 ml).
Chemicals were purchased from Merck chemical company. 1H
NMR spectra were recorded in CDCl3 solvent on a Bruker AM
80 MHz or a Bruker AC 400 MHz spectrometer using TMS as an
internal standard. Infrared spectra were run on a Philips PU9716
or Shimadzu IR-435 spectrophotometer. All analyses were per-
formed on a Shimadzu GC-16A instrument with a flame ionization
detector using silicon DC-200 or Carbowax 20M columns and n-
decane was used as internal standard. The tetraphenylporphyrin
was prepared and metallated according to the literature [43,44].
The [SnIV(TPP)(BF4)2] catalyst was prepared as reported previously
[40].
b
GC yield.
2.1. General procedure for the silylation of alcohols and phenols with
HMDS catalyzed by [SnIV(TPP)(BF4)2]
A mixture of alcohol or phenol (1 mmol), [SnIV(TPP)(BF4)2]
(10 mg, 0.01 mmol) and HMDS (0.5 mmol per OH group) in CH3CN
(1 mL) was prepared and stirred at room temperature for appropri-
ate time (Table 1). The progress of the reaction was monitored by
GC and TLC in 30 s intervals. After completion of the reaction, the
Table 2
Trimethylsilylation of alcohols with HMDS catalyzed by [SnIV(TPP)(BF4)2] at room temperature.a
Entry Hydroxy compound
1
TMS-ether
Time (min)
1
Yield (%)b
100
CH2OH
CH2OSiMe3
2
3
1
2
100
100
Cl
CH2OH
Cl
CH2OSiMe3
CH2OH
CH2OSiMe3
O2N
O2N
O2N
O2N
4
5
2
2
100
100
CH2OH
CH2OSiMe3
CH2OSiMe3
NO2
CH2OH
NO2
6
7
1
2
100
100
MeO
CH2OSiMe3
MeO
CH2OH
CH2OH
OMe
CH2OSiMe3
OMe
8
1
100
t-Bu
CH2OSiMe3
t-Bu
CH2OH
(continued on next page)

214
M. Moghadam et al. / Polyhedron 29 (2010) 212–219
Table 2 (continued)
Entry Hydroxy compound
9
TMS-ether
Time (min)
1
Yield (%)b
100
CH2OSiMe3
CH2OH
Br
Br
CH2OH
10
1
100
CH2OSiMe3
11
12
13
1
1
1
100
100
100
Cl
CH2OSiMe3
Cl
CH2OH
Cl
Cl
CH2OH
CH2OSiMe3
Br
Br
CH2OH
CH2OSiMe3
Me
Me
14
1
100
CH2OSiMe3
CH2OH
Me
Me
15
16
1
1
100
100
CH2OH
OH
CH2OSiMe3
OSiMe3
Me
Me
17
18
19
1
1
1
100
100
100
CH2CH2OH
CH2CH2OSiMe3
CH2CH2CH2OH
CH2CH2CH2OSiMe3
OH
OSiMe3
20
21
1
2
100
100
OSiMe3
OH
OH
OSiMe3

M. Moghadam et al. / Polyhedron 29 (2010) 212–219
215
Table 2 (continued)
Entry
22
Hydroxy compound
TMS-ether
Me
Time (min)
1
Yield (%)b
100
Me
OH
OSiMe3
23
1.5
100
OSiMe3
Me
OH
Me
Me
Me
24
1
100
OH
OH
OSiMe3
25
1
100
OSiMe3
26
27
2
2
95
OSiMe3
OSiMe3
OH
100
OH
28
29
5
2
99
97
OSiMe3
OH
C
OH
C
OSiMe3
30
1
97
CH3
C
CH3
C
H3C
CH3
H3C
CH3
OSiMe3
OH
a
Reaction conditions: alcohol (1 mmol), HMDS (0.5 mmol), catalyst (1 mol%), CH3CN (1 mL).
GC yield.
b

216
M. Moghadam et al. / Polyhedron 29 (2010) 212–219
Table 3
Trimethylsilylation of phenols with HMDS catalyzed by [SnIV(TPP)(BF4)2] at room temperature.a
Entry
1
Phenol
TMS-ether
Time (min)
1.5
Yield (%)b
100
OSiMe3
OH
OH
2
3
2
2
100
100
OSiMe3
Cl
Cl
OH
OSiMe3
OSiMe3
Cl
Cl
4
2
100
OH
Cl
Cl
5c
2
2
100
100
Me3SiO
OSiMe3
HO
OH
6c
OH
OSiMe3
OH
OSiMe3
7c
2
2
100
100
OSiMe3
OH
Me3SiO
HO
8c
OH
OSiMe3
OSiMe3
OSiMe3
Me3SiO
O2N
HO
OH
9
2.5
2
100
100
O2N
OH
OH
10
OH
OSiMe3
H3C
H3C
H3C
H3C
11
12
2
2
100
100
OSiMe3
OH
OSiMe3
a
Reaction conditions: phenol (1 mmol), HMDS (0.5 mmol), catalyst (1 mol%), CH3CN (1 mL).
GC yield.
Reaction was performed with 0.5 mmol of HMDS per OH group.
b
c

M. Moghadam et al. / Polyhedron 29 (2010) 212–219
217
the crude product, which was confirmed by 1H NMR and IR spectral
data.
H
N
Me
3Si
SiMe3
2.2. Catalyst recovery and reuse
The reusability of [SnIV(TPP)(BF4)2] was investigated in the re-
peated silylation reaction of 4-chlorobenzyl alcohol. The reactions
were carried out as described above. At the end of each reaction,
the solvent was evaporated and n-hexane was added. The catalyst
was filtered, washed with n-hexane (10 mL) and dried before using
in the next run.
H
N
SiMe3
Me3Si
Sn IV
Sn IV
1
3. Results and discussion
ROH
R-O-SiMe3
NH3 +
3.1. Silylation of alcohols and phenols with HMDS catalyzed by
[SnIV(TPP)(BF4)2]
Me
3Si
NH2
Sn IV
Although metalloporphyrins are widely used as oxidation cata-
lysts, there have been few studies on their catalytic activity as Le-
wis acids [27–42]. Since the metalloporphyrins are mild Lewis
acids, we investigated their ability in the silylation of hydroxy
compounds with HMDS. First, in order to choose the appropriate
catalyst, different metalloporphyrins were used as catalysts in
the silylation of 4-chlorobenzyl alcohol with HMDS. These results
(Table 1) showed that amongst them, the [SnIV(TPP)(BF4)2] is the
most efficient catalyst in the silylation reaction. In the case of tin
R-O-SiMe3
ROH
Scheme 2. Proposed mechanism for trimethylsilylation of alcohols and phenols
with HMDS catalyzed by [SnIV(TPP)(BF4)2].
solvent was evaporated, n-hexane (10 mL) was added and the cat-
alyst was filtered. The filtrates were washed with brine and dried
over Na2SO4 and concentrated under reduced pressure to afford
Table 4
Selective silylation of alcohols and phenols catalyzed by [SnIV(TPP)(BF4)2] in CH3CN.a
Row
1
ROH
Silyl ether
Time (min)
1
Yield (%)b
100
CH2OH
OH
CH2OSiMe3
OSiMe3
0
2
1
100
0
CH2OH
CH2OSiMe3
OSiMe3
OH
3
4
1
1
88
CH2OH
CH2OSiMe3
12
OH
OSiMe3
100
CH2OH
OH
CH2OSiMe3
OSiMe3
0

218
M. Moghadam et al. / Polyhedron 29 (2010) 212–219
Table 4 (continued)
Row
5c
ROH
Silyl ether
Time (min)
1
Yield (%)b
100
CH2OH
OH
CH2OSiMe3
100
OSiMe3
a
Reaction conditions for a binary mixture: 1 mmol of each alcohol or phenol, HMDS (0.5 mmol), catalyst (1 mol%), CH3CN (1 mL).
GC yield.
1 mmol of HMDS was used.
b
c
porphyrins, it is clear that introducing of BF4 instead of Cl has in-
3.2. Reusability of catalyst
creased the electron deficiency of [SnIV(TPP)(BF4)2] which in turn
increases its catalytic activity.
The reusability of the catalyst was checked using multiple sily-
lation of 4-chlorobenzylalcohol with HMDS. At the end of the reac-
tion, the solvent was evaporated and n-hexane was added. The
catalyst was filtered and used in the next run. The results showed
that after reusing the catalyst for several times (four consecutive
runs were checked), no change was observed in its catalytic
activity.
The optimized conditions which obtained for silylation of 4-
benzyl alcohol were alcohol, HMDS and catalyst in a molar ratio
of 100:50:1. Under the optimized reaction conditions, a wide vari-
ety of alcohols were converted completely to their corresponding
silyl ethers. The obtained results for silylation of different primary,
secondary (including aliphatic and aromatic alcohols) and tertiary
alcohols showed that the reaction was immediately completed for
all alcohols at room temperature and no alcohol was detected by
TLC or GC (Table 2). These results showed that the nature of sub-
stituents in benzylic alcohols (electron-withdrawing or electron-
releasing) has no significant effect on the yields of silyl ethers.
Blank experiments in the absence of catalyst showed that only
small amounts of the corresponding silyl ethers were produced.
The high catalytic activity of this catalyst in the silylation of
alcohols, prompted us to investigate its ability in the silylation of
phenols. In this manner, different phenols were subjected to silyla-
tion with HMDS. The reaction conditions were as described for
alcohols. These results (Table 3) showed that all reactions were
completed in 1–3 min for all phenols and the desired silyl ethers
were obtained in excellent yields at room temperature. The silyla-
tion of polyhydroxybenzenes such as hydroquinone, pyrocatechol,
resorcinol and pyrogallol was also performed. The results showed
that all hydroxyl groups were silylated and the desired poly(tri-
methylsilyl ether) were obtained in excellent yields (Table 3, en-
tries 5–8).
During the reactions the fast evolution of ammonia gas was ob-
served (the evolution of NH3 was tested by Nessler’s test). On this
basis, a probable mechanism has been shown in Scheme 2. In this
mechanism, the Lewis acid–base interaction between porphyrin
tetrafluoroborate and nitrogen in HMDS polarizes the N–Si bond
of HMDS and a reactive silylating agent (1) is produced, which
effectively silylates the hydroxyl compounds.
The selectivity of this method was also investigated. As shown
in Table 4, primary alcohols were completely converted to the cor-
responding silyl ether in the presence of a secondary or tertiary
alcohols or phenols (Table 4, entries 1, 2 and 4). Also, in a binary
mixture of benzyl alcohol and 1-octanol, the benzyl alcohol was
converted to the corresponding silyl ether in 88% yield, while only
12% of the corresponding silyl ether was observed for the aliphatic
alcohol (Table 4, entry 3). It seems that the higher reactivity of pri-
mary in comparison with secondary and tertiary alcohols is due to
the less steric hindrance of primary alcohols for attacking to sily-
lating agent (1). Selective trimethylsilylation of alcohols in the
presence of phenols is attributed to higher nucleophilicity of alco-
hols. Note that these results have been obtained in the presence of
0.5 mmol of HMDS. When, 1 mmol of HMDS was used both sub-
strates were converted completely to their corresponding TMS-
ethers.
4. Conclusion
In this paper, another application of electron deficient
tin(IV)tetraphenylporphyrinato
tetrafluoroborate,
[SnIV(TPP)-
(BF4)2], which is a stable Sn(IV) compound, is reported. This cata-
lyst was successfully used for rapid, efficient and chemoselective
silylation of primary, secondary and tertiary alcohols and phenols
with 1,1,1,3,3,3-hexamethyldisilazane (HMDS). Short reaction
times, excellent yields, easy work-up and stability and reusability
of the catalyst are noteworthy advantages of this method.
Acknowledgement
We are thankful to the Center of Excellence of Chemistry of Uni-
versity of Isfahan (CECUI)) for financial support of this work.
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