A mild, efficient, and selective deprotection of tert-butyldimethylsilyl (TBDMS) ethers using dicationic ionic liquid as a catalyst

Article abstract of DOI:10.1016/j.tetlet.2012.07.108

Selective deprotection of alkyl TBDMS ether in the presence of phenolic TBDMS ether using dicationic ionic liquid [tetraEG(mim)₂][OMs] ₂ as a homogeneous catalyst showed significant catalytic activity in methanol at ambient temperature to produce respective alcohol in excellent yield. The present environmentally benign catalytic system is found to be very convenient, fast, high yielding, and clean method for selective desilylation of alkyl silyl ethers even in the existence of other sensitive organic functional groups such as aldehyde, methoxy, and acetate were also achieved.

Full text of DOI:10.1016/j.tetlet.2012.07.108

Tetrahedron Letters 53 (2012) 5338–5342
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Tetrahedron Letters
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A mild, efficient, and selective deprotection of tert-butyldimethylsilyl
(TBDMS) ethers using dicationic ionic liquid as a catalyst
⇑
Arvind H. Jadhav, Hern Kim
Department of Environmental Engineering and Energy, Energy and Environment Fusion Technology Center, Myongji University, Yongin, Kyonggi-do 449-728, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective deprotection of alkyl TBDMS ether in the presence of phenolic TBDMS ether using dicationic
ionic liquid [tetraEG(mim)2][OMs]2 as a homogeneous catalyst showed significant catalytic activity in
methanol at ambient temperature to produce respective alcohol in excellent yield. The present environ-
mentally benign catalytic system is found to be very convenient, fast, high yielding, and clean method for
selective desilylation of alkyl silyl ethers even in the existence of other sensitive organic functional
groups such as aldehyde, methoxy, and acetate were also achieved.
Received 3 June 2012
Revised 21 July 2012
Accepted 25 July 2012
Available online 2 August 2012
Keywords:
Selective deprotection
Desilylation
Ó 2012 Elsevier Ltd. All rights reserved.
tert-Butyldimethylsilyl group
Dicationic ionic liquid
Catalytic cleavage
The strategy of hydroxyl functionality protection and its subse-
quent deprotection at a later stage is usual practice in multistep
synthesis of complex organic and bioorganic molecules.1 The
tert-butyldimethylsilyl (TBDMS) group is used widely for the
protection of hydroxyl group in synthetic organic chemistry due
to their ease of installation, stability toward various conditions
such as Grignard reaction, Wittig reaction, reduction reaction,
etc., and finally deprotect easily without affecting other functional
groups.1,2
The enormous research in silylation chemistry has been re-
sulted in the development of numerous methods of desilylation.
Mainly, the acidic reagents such as CF3COOH/H2O,3 CH3COOH,4
HF,5 Zn(BF4)2,6 Sc(OTf)3,7 SbCl5,8 SnCl2,9 ZrCl4,10 BCl3,11 InCl3,12
ZnBr2,13 Ce(OTf)4,14 Ni(II)Cl2Á6H2O15 have been reported for desily-
lation. The basic reagents like TBAF,2a NaOH/Bu4NHSO4,16 TBATB,17
K2CO3,18 Cs2CO3,19 were also utilized for the deprotection of silyl
ethers. Oxidizing reagents DDQ,20 KMnO4,21 BaMnO4,21 and reduc-
ing agents DIBAL-H,22 LiAlH4,23 NaH,24 have also been reported for
desilylation. However, some of these procedures have difficulties
such as longer reaction time, harsh reaction conditions, use of
expensive and corrosive reagents as well as cumbersome work-
up procedures. Although, discrimination between two silyl ethers
derived from alcohols is common, relatively very few techniques
have been developed to deprotect alkyl silyl ethers in the presence
of phenolic silyl ethers such as oxone,25 N-iodosuccinimide,26 1-
chloroethyl chloroformate,27 and Iron(III) tosylate.28 While, these
protocols are quite moisture sensitive, environmentally hazardous,
and required long reaction time for the completion of reaction.
Therefore, it is essential to develop a mild, eco-friendly, fast, and
selective method for the deprotection of alkyl silyl ethers.
Ionic liquids (ILs) have received great attention in various fields
of science, due to their unique properties such as extremely low
vapor pressure, non-flammability, tunability, and high thermal
stability.29 Especially, imidazolium based ILs played important role
in various organic reactions as a catalyst or alternative for conven-
tional solvents.29,30 More recently, tailor-made ILs [hexaEG-
mim][OMs] or [dihexaEGmim][OMs] with alkali-metal fluoride in
tert-amyl alcohol has been reported as effective catalytic system
for selective deprotection of phenolic silyl ethers.31 However, the
use of basic reagent and harsh reaction condition for desilylation
restrict the use of this protocol in complex organic synthesis.
Recently, we reported short oligo ethylene glycol functionalized
imidazolium dicationic ILs as a catalyst for dehydration of fructose
and sucrose into 5-hydroxymethylfurfural (HMF).32 In continua-
tion of this work, herein we disclose the selective deprotection of
alkyl silyl ethers using [tetraEG(mim)2][OMs]2 as a catalyst in
methanol at mild reaction condition whereas the phenolic silyl
ethers were unaffected.
The task specific dicationic IL [tetraEG(mim)2][OMs]2 was pre-
pared using simple pathway as shown in Scheme 1. In short, tetra
ethylene glycol (1) was mesylated to obtain tetraethylene glycol
dimesylate (2) followed by the addition of N-methylimidazole in
2 to afford tetra ethylene glycol-bis (3-methylimidazolium) dimes-
ylate ([tetraEG(mim)2][OMs]2) (3) as a colorless thick liquid.32
Table 1 demonstrates the deprotection of benzyl silyl ether (4)
as a model compound using various reaction conditions. Initially,
⇑
Corresponding author. Tel.: +82 31 330 6688; fax: +82 31 336 6336.
E-mail address: hernkim@mju.ac.kr (H. Kim).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.108

A. H. Jadhav, H. Kim / Tetrahedron Letters 53 (2012) 5338–5342
5339
-OMs
-OMs
i
ii
N+
O
N+
3
HO
O
OH
3
MsO
O
OMs
3
N
N
(3)
(2)
(1)
Reagents and conditions:
i = Et3N, DCM, CH3SO2Cl, 12 h, rt
ii = CH3CN, mim, 90 oC, 4 days
Scheme 1. Preparation of tetra ethylene glycol-bis (3-methylimidazolium) dimesylate ([tetraEG(mim)2][OMs]2).
Table 1
Deprotection of benzyl silyl ether in presence of [tetraEG(mim)2][OMs]2 using various reaction conditionsa
OTBDMS
[tetraEG(mim)2][OMs]2
OH
Solvent, rt
(4)
(5)
Entry
Solvent
[tetraEG(mim)2][OMs]2 (equiv)
Temperature
Time
Yieldb(%)
1
2
3
4
5
6
7
8
9
10
Methanol
Methanol
Methanol
Methanol
—
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
24 h
1
2.0
1.0
0.5
0.2
0.2
0.2
0.2
0.2
0.2
60 min
70 min
80 min
80 min
12 h
12 h
12 h
12 h
12 h
95
95
95
94
28
15
3
Methanol
Dichloromethane
Acetonitrile
Benzene
Tetrahydrofuran
1,4-Dioxane
6
3
a
All reactions were carried out on 1.0 mmol scale of substrate with 0.2–2.0 equiv of [tetraEG(mim)2][OMs]2, accordingly in 4.0 ml solvent.
Yield refers to the isolated product.
b
we performed catalyst free desilylation of 4 in methanol at room
temperature and we observed that, this reaction proceeded very
slowly and provide only 1% of 5 after 24 h (entry 1). Further, to
determine the catalytic activity of [tetraEG(mim)2][OMs]2 at
room temperature, a number of reactions were performed in
methanol. Firstly, we discovered the minimum amount of [tet-
raEG(mim)2][OMs]2 required for rapid desilylation by carrying
out the deprotection using 2, 1, 0.5, and 0.2 equiv of [tet-
raEG(mim)2][OMs]2 at uniform reaction condition (entries 2–5)
and we found that all reactions were proceeded smoothly and
provide excellent yield of benzyl alcohol within 80 min
(94–95%) showing the significant catalytic activity of [tet-
raEG(mim)2][OMs]2. Considering the mole economy ratio of cata-
lyst we selected to use 0.2 equiv of [tetraEG(mim)2][OMs]2 for
further study of this protocol. In addition, we carried out desilyla-
tion using various solvents to study the effect of these solvents on
desilylation of 4. Entries 6 and 7 show desilylation in dichloro-
methane and acetonitrile were observed very sluggish and provide
only 28% and 15% of desired product even extended time period
after 12 h. However, solvents such as benzene, tetrahydrofuran,
and 1, 4-dioxane were found to be inactive for this protocol (en-
tries 8–10).
Table 2 shows desilylation of structurally varied silyl ethers of
various substrates using optimized reaction condition. Deprotec-
tion of tert-butyl-dimethyl-phenethyloxy-silane and tert-butyl-di-
methyl-(3-phenyl-propoxy)-silane was cleaved very cleanly and
gave excellent yield of respective alcohols (entry 1, 2). Entries 3
and 4 indicate desilylation of substituted benzyl silyl ethers without
affecting the substituted functionality such as methoxy and alde-
hyde to provide 4-methoxy benzyl alcohol and 4-hydroxymethyl-
benzaldehyde in sufficient yield (92% and 89%). However, in entry
5 naphthyl silyl ether was deprotected very smoothly to get naph-
thyl alcohol in quantitative yield. The [tetraEG(mim)2][OMs]2
deprotects all aliphatic silyl ethers excellently to provide desired
alcohols in significant yields (entries 6–8). Furthermore, we tried
to deprotect phenolic silyl ethers using [tetraEG(mim)2][OMs]2 at
uniform reaction condition surprisingly, all phenolic silyl ethers re-
main intact with phenol moieties even after prolonged reaction
time 3 h (entries 9–11). In addition, acetate of benzyl alcohol and
pentyl alcohol were unable to produce respective alcohol in the
presence of IL at similar reaction condition (entries 12, 13). These
observations encourage us to study the possibility of selective
deprotection of alkyl silyl ether in the presence of phenolic silyl
ether using [tetraEG(mim)2][OMs]2 IL as catalyst.
In addition, to go further insight to study the selective cleavage
of alkyl silyl ether we performed desilylation of benzyl silyl ether
(4) and phenolic silyl ether (6) in equimolar ratio in competitive
reaction with 0.2 equiv of [tetraEG(mim)2][OMs]2 in methanol at
room temperature for 3 h and this reaction results in the desilyla-
tion of benzyl silyl ether selectively and provides 100% of benzyl
alcohol (5) whereas phenolic silyl ether (6) was unaffected and un-
able to produce phenol (7). Table 3 shows the results of selective
cleavage of alkyl silyl ether in the presence of phenolic silyl ether
of various substrates using standard condition of this protocol. En-
tries 1–3 provide phenolic silyl ether of different benzyl alcohols
from their di-TBDMS ethers by desilylation of benzyl silyl ethers
selectively in excellent yields (92–94%). Finally, Aromatic silylated
kojic acid was obtained from di-silyl kojic acid by selective cleav-
age of benzylic silyl ether using [tetraEG(mim)2][OMs]2 in high
yield whereas, phenolic silyl ether was unaffected (entry 4).
To compare the remarkable selectivity of dicationic IL with re-
ported monocationic mesylate ILs, we performed desilylation of di-
TBDMS silyl ether 8 using optimized reaction condition and the re-
sults are shown in Table 4. Entries 1 and 2 show deprotection using
[hexaEGmim][OMs] and [dihexaEGim][OMs] monocationic ILs in
methanol proceeded very sluggishly and provide only 42% and 36%
of 9, respectively after 12 h. Next, we carried out desilylation using
potassium fluoride (KF) in MeOH to demonstrate the role of KF in

5340
A. H. Jadhav, H. Kim / Tetrahedron Letters 53 (2012) 5338–5342
Table 2
Desilylation of various substrates using [tetraEG(mim)2][OMs]2 as catalysta
[tetraEG(mim)2][OMs]2
MeOH, rt
OTBDMS
OH
(88-95 %)
R
R
R = alkyl, benzyl, naphthyl
Entry
1
Substrate
Time
Product
Yieldb (%)
91
OH
OTBDMS
80 min
OTBDMS
OH
2
3
80 min
80 min
95
92
OTBDMS
OTBDMS
OTBDMS
OH
MeO
OHC
MeO
OHC
OH
OH
4
5
80 min
80 min
89
89
6
7
75 min
90 min
94
88
OH
OTBDMS
OH
OTBDMS
HO
HO
TBDMSO
TBDMSO
O
O
8
9
85 min
3 h
91
—
OTBDMS
OH
OTBDMS
No reaction
No reaction
No reaction
OTBDMS
10
11
3 h
3 h
—
—
TBDMSO
OTBDMS
OAc
12
13
3 h
3 h
No reaction
No reaction
—
—
OAc
a
All reactions were carried out on 1.0 mmol scale of substrate with 0.2 equiv of [tetraEG(mim)2][OMs]2 in 4.0 ml of methanol.
Yield refers to the isolated product. All products were compared with authentic samples.
b
desilylation under optimized condition of this protocol, we observed
that only KF (0.2 equiv) was unable to cleave any silyl ether even
after prolonged reaction time 12 h (entry 3). While, KF in the pres-
ence of IL [hexaEGmim][OMs] provides 58% of 10 (entry 4). To deter-
mine the possibility of acidic pathway of silyl ether cleavage we
carried out desilylation using methanesulfonic acid (MsOH) in
methanol, this reaction cleaved both silyl ethers efficiently in
80 min and provides 94% of 11. Finally, to confirm the role of MeOH
in optimized condition, we carried out deprotection using IL [tet-
raEG(mim)2][OMs]2 as a solvent in the absence of MeOH and this
reaction was unable to produce any alcohol even after 12 h. These
reactions results clearly indicate that the desilylation occurs by
acidic pathway and [tetraEG(mim)2][OMs]2/methanol system con-
trolled acidity to result in the selective deprotection of alkyl silyl
ether in the presence of phenolic silyl ether.
In conclusion, we have developed a mild, efficient, simple, and
selective method for the deprotection of alkyl TBDMS ether in the
presence of phenolic TBDMS ether using [tetraEG(mim)2][OMs]2
dicationic IL as a catalyst in methanol at room temperature. Fur-
thermore, other functional groups such as aldehyde, acetate, and
methoxy were found to be unaffected using this protocol. The pres-
ent catalytic system [tetraEG(mim)2][OMs]2/methanol showed
higher yield of alcohols in short reaction period, green catalytic sys-
tem, easy work up environmentally favorable reaction procedure,
etc., and therefore, this method has advantages over other desilyla-
tion methods.

A. H. Jadhav, H. Kim / Tetrahedron Letters 53 (2012) 5338–5342
5341
Table 3
Selective desilylation of various substrates using [tetraEG(mim)2][OMs]2 as catalysta
OTBDMS
OTBDMS
OH
OH
[tetraEG(mim)2][OMs]2
Methanol, rt, 3 h
+
+
(100 %)
(0 %)
(7)
(4)
(6)
(5)
Entry
1
Substrate
Time
Product
Yieldb(%)
OTBDMS
OH
85 min
94
92
TBDMSO
TBDMSO
OH
OTBDMS
2
3
85 min
80 min
OTBDMS
OTBDMS
TBDMSO
O
TBDMSO
O
94
91
OH
OH
OTBDMS
OTBDMS
O
O
O
O
4
70 min
TBDMSO
TBDMSO
a
All reactions were carried out on 1.0 mmol scale of substrate with 0.2 equiv of [tetraEG(mim)2][OMs]2 in 4.0 ml of methanol.
Yield refers to the isolated product.
b
Table 4
Comparison of dicationic [tetraEG(mim)2][OMs]2 IL with reported monocationic mesylate IL in TBDMS deprotectiona
OTBDMS
Catalyst
OH
HO
OTBDMS
+
OH
+
Methanol, rt
TBDMSO
TBDMSO
HO
(8)
(9)
(10)
(11)
Entry
Solvent
Catalyst
Time
Yieldb (%)
9
10
11
1
2
3
4
5
6
Methanol
Methanol
Methanol
Methanol
Methanol
[hexaEGmim][OMs]
[dihexaEGim][OMs]
KF
[hexaEGmim][OMs]+KF
MsOH
—
12 h
12 h
12 h
12 h
80 min
12 h
42
36
—
—
—
—
—
—
—
—
—
94
—
—
58c
—
[tetraEG(mim)2][OMs]2
—
—
a
b
c
All reactions were carried out on 1.0 mmol scale of substrate with 0.2 equiv of catalyst in 4.0 ml of methanol at room temperature.
Yield refers to the isolated product.
0.2 equiv of KF.
2.76 (s, 2 Â 3H); 13C NMR (125 MHz, CDCl3): d: 138.00, 123.32,
Preparation of tetra ethylene glycol-bis (3-methylimidazolium)
dimesylate ([tetraEG(mim)2][OMs]2)
123.27, 70.43, 70.34, 69.11, 49.51, 39.83, 36.30; FT-IR (400–
4000 cmÀ1): 3096
[
m
(Ar–H)]; 2973, 2829
(C@N)]; 1568, 1526, 1458 [ (C@C)]; 1183, 1073 [
(C–N)]. LRMS-ESI: m/z [M-OMs]+ calcd: 419.20; found: 419.19.
[
m
(C–H)]; 1612
[m
m
m
m
(S@O)]; 1136
A mixture of tetra ethylene glycol dimesylate (1.0 mmol) and
N-methylimidazole (2.0 mmol) in acetonitrile was refluxed magnet-
ically for 4 days in a two necked round bottomed flask. The reaction
progress was monitored by TLC. After completion of the reaction as
indicated by TLC, the reaction mixture was allowed to cool at room
temperature and the acetonitrile was evaporated under reduced
pressure on rotary evaporator at 55 °C. The reaction mixture was
washed three times usingethyl acetate to remove unreacted starting
materials and the resulting quaternized tetra ethylene glycol-bis (3-
methylimidazolium) dimesylate was obtained.
[
Elem. Anal. Calcd (%) for C18H34N4O9S2: C, 42.01; H, 6.66; N,
10.89. Found: C, 41.85; H, 6.42; N, 10.63.
General procedure for deprotection of TBDMS ether using
[tetraEG(mim)2][OMs]2
A
mixture of TBDMS ether (1.0 mmol) and [tet-
Yield 87%; colorless thick liquid; 1H NMR (500 MHz, CDCl3): d:
9.66 (s, 2 Â H), 7.62 (s, 2 Â H), 7.58 (s, 2 Â H), 4.50 (t, 2 Â 2H),
4.01 (s, 2 Â 3H), 3.90 (t, 2 Â 2H), 3.65 (t, 2 Â 2H), 3.59 (t, 2 Â 2H),
raEG(mim)2][OMs]2 IL (0.2 mmol) in methanol (4 mL) was stirred
at room temperature up to the completion of reaction. Reaction
progress was monitored by thin layer chromatography (TLC). After

5342
A. H. Jadhav, H. Kim / Tetrahedron Letters 53 (2012) 5338–5342
completion of the reaction, methanol was removed under reduced
pressure and the residue was extracted with diethyl ether and
dried over sodium sulfate. The diethyl ether was evaporated under
reduced pressure to afford respective products.
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Acknowledgements
This study was supported by a Grant (#10035574) from the
‘‘Platform Chemical Process Technology from Lignocellulosic Bio-
mass Conversion’’ R&D Program funded by the Ministry of Knowl-
edge Economy and by a grant support by priority Research Centers
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2011-0022968), Republic of Korea.
32. Jadhav, A. H.; Kim, H.; Hwang, I. T. Catal. Commun. 2012, 21, 96.