Rapid and efficient protection of alcohols and phenols, and deprotection of trimethylsilyl ethers catalyzed by a cerium-containing polyoxometalate

Article abstract of DOI:10.1002/aoc.1715

Herein, we want to report a simple and convenient way for protection-deprotection of alcohols in the presence of ammonium decatungstocerate(IV) {(NH₄)₈[CeW₁₀O ₃₆]A·20H₂O} as catalyst under ambient temperature in CH₃CN. Using 0.002 mmol of the catalyst, various alcohols and phenols were transformed easily to the corresponding TMS ethers in excellent yields. In the second part, various TMS ethers were successfully converted to the parent hydroxyl compounds in the presence of the ammonium decatungstocerate(IV) catalyst.

Full text of DOI:10.1002/aoc.1715

Full Paper
Received: 16 January 2010
Revised: 29 June 2010
Accepted: 29 June 2010
Published online in Wiley Online Library: 23 August 2010
(wileyonlinelibrary.com) DOI 10.1002/aoc.1715
Rapid and efficient protection of alcohols
and phenols, and deprotection of trimethylsilyl
ethers catalyzed by a cerium-containing
polyoxometalate
Bahram Yadollahi∗, Valiollah Mirkhani, Shahram Tangestaninejad
and Davud Karimian
Herein, we want to report a simple and convenient way for protection-deprotection of alcohols in the presence of ammonium
decatungstocerate(IV) {(NH4)8[CeW10O36]·20H2O} as catalyst under ambient temperature in CH3CN. Using 0.002 mmol of the
catalyst, variousalcoholsandphenolsweretransformedeasilytothecorrespondingTMSethersinexcellentyields. Inthesecond
part, various TMS ethers were successfully converted to the parent hydroxyl compounds in the presence of the ammonium
c
decatungstocerate(IV) catalyst. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: catalysis; polyoxometalate; protection; deprotection; trimethylsilyl ethers
[33]
Introduction
fated SnO2
and LiOAc[34] have been reported for this trans-
formation. However, in some of the reported methods one
or more disadvantages, such as harsh reaction conditions,
high reaction temperatures, long reaction times, high cost
or toxicity of the reagents have been experienced. Conse-
quently, the introduction of efficient and better methods for
the preparation of TMS ethers and their deprotection using
inexpensive, relatively non-toxic, easy handling and environ-
mentally friendly catalysts is of practical importance and is still
in demand.
In recent years, polyoxometalates (POMs) have been taken into
consideration because of their properties and applications. This
class of inorganic compounds is unmatched not only in terms
of molecular structural diversity but also regarding reactivity
and relevance to analytical chemistry, catalysis, medicine and
materials science.[35] The catalytic function of polyoxometalate
compounds which are used in solution as well as in the solid
state as acids and oxidation catalysts is receiving considerable
attention.[36] In the last two decades, the broad utility of
polyoxometalates has been demonstrated in a wide variety
of synthetically useful selective transformations of organic
substrates.[37] Accordingtotheauthor’spreviousstudiesregarding
the use of POMs catalysts, especially those of cerium, in various
organic reactions,[38] and because of the necessity for new
and effective synthetic methodologies, the authors intend for
the first time to report a simple and convenient way for
the protection-deprotection of alcohols in the presence of
ammonium decatungstocerate(IV) {(NH4)8[CeW10O36]·20H2O} as
catalyst under ambient temperature in CH3CN (Scheme 1).
The protection–deprotection of hydroxyl compounds is one
of the most frequently employed transformations in multi-
step organic synthesis.[1] In this connection, silylation of al-
cohols is a commonly used method for their protection.
Generally, formation of trimethylsilyl ethers is carried out by
treating alcohols with trimethylsilyl chloride or trimethylsilyl
triflate in the presence of an organic base,[2] Li2S[3] and some-
times a nonionic super base catalyst.[4] However, some of
these methods have frequently suffered from drawbacks such
as lack of reactivity or the difficulty in the removal of by-
products. 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) is a stable,
commercially available and cheap reagent for trimethylsilyla-
tion of hydrogen labile substrates[5] that gives ammonia as
the only byproduct, which is easily removed from the re-
action medium. The main drawback of HMDS is its poor
silylating power which needs forceful conditions and long re-
action times. Over the years, a variety of catalysts such as
(CH3)3SiCl,[6] silica chloride,[7] ZnCl2,[8] nitrogen ligand complexes
of metal chlorides,[9] zirconium sulfophenyl phosphonate,[10] I2,[11]
LiClO4,[12] K-10 montmorilonite,[13] tungstophosphoric acid,[14]
poly(N-bromobenzene-1,3-disulfonamide),[15] CuSO4·5H2O,[16]
MgBr2·OEt2,[17] LaCl3,[18] Fe(TFA)3,[19] Fe3O4,[20] (n-Bu4N)Br[21] and
[22]
ZrO(OTf)2
have been reported for the silylation of hydroxyl
groups using HMDS. However, in most of those reactions, long re-
action times, drastic reaction conditions and the use of hazardous
solvents or tedious work-up are needed.
Deprotection of TMS ethers to their corresponding alco-
hols under mild reaction conditions is of practical impor-
tance and several methods and catalysts such as bismuth(III)
salts,[23] NaBrO3 –NH4Cl,[24] PhCH2PPh3HSO5,[25] Zr(KPO4)2,[26]
potassium dodecatungstocobaltate(III) trihydrate,[27] cobalt(II)
tetrasulfophthalocyanine,[28] trimethylsilyl bromide,[29] CuBr2,[30]
tetraethylammonium superoxide,[31] DABCO–bromine,[32] sul-
∗
Correspondence to: Bahram Yadollahi, Department of Chemistry, University of
Isfahan, Isfahan 81746-73441, Iran. E-mail: yadollahi@chem.ui.ac.ir
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c
Appl. Organometal. Chem. 2011, 25, 83–86
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

B. Yadollahi et al.
OH
OSiMe3
CH3CN, HMDS, cat, rt
CH3CN, cat, rt
Scheme 1. Protection–deprotection of benzyl alcohol in the presence of (NH4)8[CeW10O36]·20H2O.
Table 1. Catalytic conversion of alcohols to TMS ethers in the
presence of (NH4)8[CeW10O36]·20H2O in CH3CNa
Table 2. Catalytic conversion of TMS ethers into parent alcohols by
(NH4)8[CeW10O36]·20H2O in CH3CN
Entry
Substrate
Time (s)
Yield (%)b
Entry
Substrate
Time (min)
Yield (%)a
1
2
C6H5CH2OH
<5
<5
120
<5
<5
<5
<5
<5
30
100
100
98
1
2
C6H5CH2OSiMe3
30
15
30
120
60
120
30
5
100
90
2-MeC6H4CH2OH
4-O2NC6H4CH2OH
2-FC6H4CH2OH
4-MeOC6H4CH2OH
2-BrC6H4CH2OH
3,4-(Methylenedioxy)benzyl alcohol
4-(Me)3CC6H4CH2OH
Thiophene-2-methanol
3-Pyridyl-1-methanol
PhCH2CH2CH2OH
PhCHMeCH2OH
PhCHCHCH2OH
1-C7H15OH
4-ClC6H4CH2OSiMe3
2-FC6H4CH2OSiMe3
2-MeC6H4CH2OSiMe3
2-O2NC6H4CH2OSiMe3
2-HOC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
2,4-Cl2C6H3CH2OSiMe3
4-HO-3-H3COC6H3CH2OSiMe3
3-Pyridyl-1-methyl-OSiMe3
Anthracene-9-CH2OSiMe3
PhCHCHCH2OSiMe3
PhCHMeCH2OSiMe3
PhCH2CH2CH2OSiMe3
n-C8H17OSiMe3
3
3
95
4
100
100
100
100
99
4
80
5
5
70
6
6
85
7
7
90
8
8
95
9
98
9
60
30
5
100
65
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
30
100
100
100
99
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
<5
<5
<5
60
90
30
15
60
120
60
30
5
100
99
98
90
1-C8H17OH
30
99
60
Cyclohexanol
<5
<5
<5
60
100
100
100
90
n-C7H15OSiMe3
85
2-Methylcyclohexanol
(Ph)2CHOH
Cyclohexyl-OSiMe3
1,3-(Me3SiO)2C4H7
Adamantyl-OSiMe3
Me3COSiMe3
95
100
100
80
PhCH2C(Me)OHCH3
1-Adamantanol
C6H5OH
90
30
30
30
30
90
60
88
30
99
C6H5OSiMe3
90
4-ClC6H4OH
30
100
95
4-ClC6H4OSiMe3
55
4-O2NC6H4OH
60
4-O2NC6H4OSiMe3
1-Naphthyl-OSiMe3
60
1-Naphthol
30
100
99
98c
100c
60
1,3-butanediol
<5
30
a GC yield.
2-HOC6H4CH2OH
4-HO-3-MeOC6H3CH2OH
30
a All products were identified by comparison of their physical and
spectral data with those of authentic samples.[39]
b GC yield.
was confirmed by IR, NMR, thermal gravimetric analysis (TGA)
and elemental analysis. Thermal gravimetric analysis was per-
formed on the catalyst (between 40 and 600 ◦C), and the
results indicated that the hydration numbers are between 18
and 21.
c Reaction was performed with 1.2 mmol of HMDS.
Experimental
General Procedure for Protection of Alcohols into Trimethylsi-
lyl Ethers at Room Temperature
General
All materials were purchased from Merck and Fluka chemical
companies. All products were identified by comparison of their
physical and spectral data with those of authentic samples,[39] and
yields refer to isolated products. GC analysis was performed on
a Shimadzu GC-16A instrument with a flame-ionization detector
using silicon DC-200 or carbowax 20-M columns. Infrared spectra
were recorded on a Nicolet impact 400D spectrophotometer. 1H-
NMRspectrawererecordedonaBruker-AC500 MHzspectrometer.
Mass spectra were recorded on a Shimadzu GCMS-QP 1000
EX. The chemical purities of all silyl ethers were checked
by gas chromatography and confirmed to be higher than
98%.
To a solution of alcohol (1 mmol), and HMDS (hexamethyldisi-
lazane, 0.6 mmol) in CH3CN (3 ml), the catalyst (0.002 mmol) was
added. The mixture was stirred at room temperature for a few
seconds and the progress of the reaction was monitored by GC.
After completion of the reaction, filtration was carried out and
the catalyst, because of its insolubility, was easily removed from
the mixture. After that, evaporation of the solvent under reduced
pressure, gave the pure product (Table 1).
General Procedure for Deprotection of Trimethylsilyl Ethers
To a solution of trimethylsilyl ether (1 mmol) in acetonitrile
(3 ml), ammonium decatungstocerate(IV) was added (0.1 mmol).
The mixture was stirred for an appropriate time at room
The ammonium decatungstocerate(IV) icosahydrate was pre-
pared according to the literature.[40] The structure of the catalyst
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 83–86

Protection/deprotection of trimethylsilyl ethers by Ce polyoxometalate
Table 3. Comparison of results between (NH4)8[CeW10O36]·20H2O and some of the recently reported catalysts for the protection of benzyl alcohol
by HMDS
Entry
Catalyst
Catalyst (mol%)
Time (min)
Yield (%)
Reference
1
2
3
4
5
6
7
8
1,3-dibromo-5,5-diethylbarbituric acid
HClO4 –SiO2
10
30
4
30
2
95
100
90
[41]
[42]
[15]
[43]
[19]
[44]
[45]
–
TBBDA
30
80
5
Nanoporous solid silica sulfonic acid
Fe(F3CCO2)3
2
100
92
2.5
1
H3PW12O40
23
5
90
TiCl2(OTf)–SiO2
1
92
(NH4)8[CeW10O36]·20H2O
0.2
0.05
100
temperature, and the progress of the reaction was monitored
by GC. After completion of the reaction, the mixture was filtered
and evaporation of the solvent followed by chromatography on a
short column of silica gel, gave the pure product (Table 2).
corresponding hydroxyl compounds under this reaction condition
(Table 2, entries 17, 21–23).
In order to show the applicability and efficiency of (NH4)8
[CeW10O36]·20H2O, some of the recently reported methods about
the protection of benzyl alcohol were compared with the present
results. The present method proved to be superior to most of the
previously reported methods, as shown in Table 3.
Results and Discussion
At first, the focus was on the catalytic role of (NH4)8
[CeW10O36]·20H2O. In the initial studies to find the optimized
reaction condition, the reaction of benzyl alcohol (1 mmol) in the
presence of various catalytic amounts of ammonium decatungsto-
cerate(IV)inacetonitrilewascarriedout. Itwasfoundthatexcellent
yields can be obtained with 0.002 mmol of the catalyst. For the
protecting reagent (HMDS), it was observed that 0.6 mmol is the
optimal amount. The protection was efficiently achieved under
the above-mentioned conditions, and the experimental results
showed that the presence of the catalyst is crucial to obtain full
conversion of the alcohols. CH3CN was chosen as the best sol-
vent because we obtained lower yields with other solvents such
as CH2Cl2, CHCl3, THF and n-C6H6. To show the vastness of the
reaction, the study was extended to a wide variety of benzylic and
aliphatic alcohols as well as phenols to afford the corresponding
TMS ethers (Table 1). In all these cases, very clean reactions and ex-
cellent yields were observed. Using the above reaction condition,
various phenols (Table 1, entries 21–23) were transformed easily
to the corresponding TMS ethers in excellent yields. Interestingly,
protection of a phenolic OH group is possible in the presence of an
alcoholic hydroxyl group (Table 1, entries 26 and 27). This method
was also used for protection of secondary and tertiary alcohols
(Table 1, entries 16–20) and excellent yields were observed.
In other investigations, suitable reaction conditions for the
deprotection of TMS ethers to the parent hydroxyl compounds
were studied. At first, various solvents for this reaction were
tested and CH3CN showed the best performance among the other
solvents: i.e. (CH3CH2)2O, CH2Cl2, CHCl3, THF and n-C6H6.
Conclusion
In this study, a convenient and efficient protocol for the protection
of alcohols with HMDS and deprotection of trimethylsilyl ethers in
acetonitrileatroomtemperaturehasbeensuccessfullydeveloped.
High to excellent yields, short reaction times, operational
simplicity,lowcost,easyavailabilityandnon-toxicityofthecatalyst
are noteworthy advantages of the present method, which make
this procedure a useful and attractive addition to the currently
available methods.
Acknowledgments
The authors gratefully acknowledge the financial support of this
work by the University of Isfahan.
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