A mild and efficient approach for the selective deprotection of benzyl and phenyl trimethylsilyl ethers in 1-butyl-3-methyl-imidazolium chloride

Article abstract of DOI:10.1007/s00706-008-0960-y

1-Butyl-3-methyl-imidazolium chloride ([bmim]Cl) in the absence of any catalyst mediated the selective deprotection of benzyl and phenyl trimethylsilyl (TMS) ethers to the corresponding alcohols and phenol in good yields at room temperature even in presence of alkyl silyl ethers. The work-up of reactions is very simple and the products do not require further purification. The ionic liquid (IL) can be recycled and reused for several runs without any significant loss of activity.

Full text of DOI:10.1007/s00706-008-0960-y

Monatsh Chem 139, 1471–1474 (2008)
DOI 10.1007/s00706-008-0960-y
Printed in The Netherlands
A mild and efficient approach for the selective deprotection of benzyl and
phenyl trimethylsilyl ethers in 1-butyl-3-methylimidazolium chloride
Ahmad Shaabani, Ali Hossein Rezayan, Marjan Heidary, Afshin Sarvary
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 21 April 2008; Accepted 11 May 2008; Published online 9 July 2008
# Springer-Verlag 2008
Abstract 1-Butyl-3-methylimidazolium chloride
([bmim]Cl) in the absence of any catalyst mediated
the selective deprotection of benzyl and phenyl tri-
methylsilyl (TMS) ethers to the corresponding alco-
hols and phenol in good yields at room temperature
even in presence of alkyl silyl ethers. The work-up of
reactions is very simple and the products do not re-
quire further purification. The ionic liquid (IL) can be
recycled and reused for several runs without any sig-
nificant loss of activity.
7H2O=NaI [14], TMSOTf [15], BiOClO4 [16], SbCl5
[17], LiCl [18], and BiBr3 [19].
Other reagents, such as carboxylic acid resins
[20], organotin reagents [21], I2 [22, 23], Br2 in
methanol [24, 25], CBr4 in methanol [26], 1-chlor-
oethyl chloroformate in methanol [27], and N-iodo-
succinimide [28] have also been developed to
deprotect silyl ethers under mild reaction conditions.
There are also several reductive methods for
deprotecting silyl ethers including catalytic trans-
fer hydrogenation with palladium [29] and cleavage
using diisobutylaluminium hydride [30] and LiAlH4
[31]. Although many of these methods promote se-
lective deprotection of bis-silyl ethers, some are
strongly reducing and others are oxidizing, thereby
restricting their use. Moreover, many of these meth-
ods suffer from limitations, such as harsh reaction
conditions, the use of expensive reagents, and cum-
bersome work-up procedures. Although discrimina-
tion between two silyl ethers derived from alcohols
is common, relatively few techniques have been de-
veloped to deprotect aryl silyl ethers in the presence
of alkyl silyl ethers [5, 6]. Therefore, it is very im-
portant to develop a novel, neutral, and mild selec-
tive deprotection method for silyl ethers.
Keywords Selective deprotection; 1-Butyl-3-methylimida-
zolium chloride; Trimethylsilyl ethers; Alcohol.
Introduction
Trialkylsilyl-groups have been widely used as hy-
droxyl protecting agents in organic synthesis due
to easy installation, stability to most reaction condi-
tions, and selectivity in cleavage reactions [1–4].
Although protic acids and fluoride sources continue
to be widely used for the removal of silyl protecting
groups, a number of Lewis acids have been intro-
duced to mediate desilylation reactions. Some recent
examples from literature include: BF3–OEt2 [5, 6],
BCl3 [7], decaborane [8], Sc(OTf)3 [9], InCl3 [10],
ZnBr2 [11], Zn(BF4) [4, 12], Ce(OTf)4 [13], CeCl3–
With ever-increasing environmental concerns,
much attention has been directed toward the reduc-
tion or replacement of volatile organic compounds
(VOCs) from the reaction media [32] and a variety
of environmentally benign media, such as water, ion-
ic liquids, immobilized solvents, fluorous solvents,
Correspondence: Ahmad Shaabani, Department of Chemistry,
Shahid Beheshti University, PO Box 19396-4716, Tehran,
Iran. E-mail: a-shaabani@cc.sbu.ac.ir

1472
A. Shaabani et al.
Table 1 Deprotection of 4-methoxybenzyl trimethylsilyl
ether to the corresponding alcohol in various ILs and classical
solvents at room temperature
Entry
Ionic liquid
Time=h
Yield=%
Scheme 1
1
2
3
4
5
[bmim]Cl
[bmim]Br
[bmim]PF6
H2O
0.66
4
24
24
95
80
50
0
and supercritical fluids have been promoted as repla-
cements to VOCs [33]. Ionic liquids, composed en-
tirely of ions, with a melting point below 100ꢀC
[34–36] have been emerging as promising and at-
tractive alternatives due to their unique properties,
including low volatility, high polarity, good thermal
stability, capacity to dissolve various organic, in-
organic, and organometallic compounds, and a fine-
tuning of their miscibility with water and organic
solvents by a proper choice of cation and anion. So
far, ionic liquids have been used as solvents in a wide
range of organic reactions [37–41].
In continuation of our studies on ionic liquids
[42–44], we wish to report herein a new, mild, and
highly efficient method for the selective deprotec-
tion of benzyl and phenyl trimethylsilyl ethers in
[bmim]Cl in the absence of any catalyst at room tem-
perature (Scheme 1).
EtOH
24
0
idazolium bromide ([bmim]Br), 1-butyl-3-methyl-
imidazolium chloride ([bmim]Cl), 1-butyl-3-methyl-
imidazolium hexafluorphosphate ([bmim]PF6), and
classic solvents, such as water and ethanol. Depro-
tection of 4-methoxybenzyl trimethylsilyl ether was
selected as model reaction. The test reaction was
carried out using various ionic liquids (0.20 g) fol-
lowed by addition of 4-methoxybenzyl trimethylsilyl
ether (0.21 g, 1 mmol). As indicated in Table 1,
[bmim]Cl is the best suited ionic liquid for this reac-
tion in terms of yields and easy work-up.
In order to investigate the applicability of this
method we selected a variety of benzyl and phenyl
silyl ethers. As can be seen in Table 2, the present
method general for the cleavage of TMS ethers of
primary and secondary benzyl alcohols (Table 2,
entries 1–5 and 9–11), and phenol (Table 2, entries
12, 13). It is also noteworthy that alkyl silyl ethers
Results and discussion
In order to obtain the best solvent, we examined
various ionic liquids, such as 1-butyl-3-methylim-
Table 2 Deprotection of benzyl and phenyl trimethylsilyl ethers to corresponding alcohols and phenols in [bmim]Cl at room
temperature
Entry
Substrate
Product
Ref.
Time=h
Yield=%a
1
2
3
4
5
6
4-MeCO–Ph–CH2–OTMS
4-Me–Ph–CH2–OTMS
4-F–Ph–CH2–OTMS
4-Br–Ph–CH2–OTMS
4-2ON–Ph–CH2–OTMS
Me(CH2)6-OTMS
4-MeCO–Ph–CH2–OH
4-Me–Ph–CH2–OH
4-F–Ph–CH2–OH
4-Br–Ph–CH2–OH
4-2ON–Ph–CH2–OH
NRc
[46]
[46]
[46]
[47]
[46]
–
0.66
50
0.66
1
2
24
95, 95, 96, 94b
95
95
90
90
0
7
NR
–
24
0
8
9
10
11
12
13
Cyclo–C6H11–OTMS
Ph–CHMe–OTMS
Ph–CHEt–OTMS
Ph2–CH–OTMS
Ph-OTMS
4-SMTO–Ph–CH2–OTMS
NR
–
[27]
[27]
[27]
[27, 46]
[46]
24
0
94
94
90
95
92
Ph–CHMe–OH
Ph–CHEt–OH
Ph2–CH–OH
Ph–OH
1.33
1.33
1.5
12
4-HO–Ph–CH2–OH
12
a
Isolated yields
The same IL was used for each of the four runs
No reaction
b
c

Selective deprotection of benzyl and phenyl trimethylsilyl ethers
1473
Typical procedure for deprotection of 4-methoxybenzyl-
oxytrimethylsilyl ether
To 0.20g of magnetically stirred [bmim]Cl (1.0mmol) at room
temperature 0.21 g 4-methoxybenzyl trimethylsilyl ether
(1.0mmol) [45] was added. The reaction progress was moni-
tored by TLC or GC. After completion of the reaction,
the product was extracted with 3Â5 cm3 diethyl ether.
Evaporation of the solvent gave 0.13 g (95%) colorless 4-
methoxybenzyl alcohol with sufficient purity for most pur-
poses. All reaction products were known and characterized
Scheme 2
1
by IR, H NMR spectra, and melting point as compared with
those obtained from authentic samples [27, 26, 46].
(entries 6–8) are quite stable under these reaction
conditions even after 24 h. Indeed, our results show
that this method is a time-selective method and can
cleave benzylic TMS ethers in the presence of TMS-
protected phenyls by controlling the time of reaction
(Scheme 2). Longer reaction times, however, led to
the formation of more doubly deprotected product
(Table 2, entry 13).
Acknowledgements
We gratefully acknowledge financial support from the
Research Council of Shahid Beheshti University.
References
One of the advantages of ionic liquids is their
ability to function as a recyclable reaction medium.
The product is extracted into diethyl ether and from
the etherical solution the pure product is isolated by
solvent evaporation. The IL is reused for subsequent
reactions without any decrease of reaction yields
(Table 2, Entry 1).
In conclusion, a new, mild, and facile method for
the selective deprotection of benzyl and phenyl silyl
ethers using [bmim]Cl is reported. The reaction con-
ditions allow selective deprotection of silyl-pro-
tected benzyl alcohol and phenyl derivatives in
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reaction. No by-products are formed that require
complicated chromatographic purification; only ex-
traction with n-hexane or diethyl ether to provide
pure products. Most importantly, the [bmim]Cl can
also be recycled and reused in further reactions with-
out any significant loss of catalytic activity.
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