Rice husk: Introduction of a green, cheap and reusable catalyst for the protection of alcohols, phenols, amines and thiols

Article abstract of DOI:10.1016/j.crci.2013.01.018

A mild, efficient and eco-friendly protocol for the chemoselective protection of benzylic and primary and less hindered secondary aliphatic alcohols and phenols as trimethylsilyl ethers and different types of amines as N-tert-butylcarbamates is developed using rice husk (RiH) as the catalyst. This reagent is also able to catalyze the acetylation of alcohols, phenols, thiols and amines with acetic anhydride. Easy work-up, relatively short reaction times, excellent yields and low cost, availability and reusability of the catalyst are the striking features of this methodology, which can be considered to be one of the best and general methods for the protection of alcohols, phenols, thiols and amines. In addition, the use of a green reagent in the above-mentioned reactions results in a reduction of environmental pollution and of the cost of the applied methods.

Full text of DOI:10.1016/j.crci.2013.01.018

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C. R. Chimie xxx (2013) xxx–xxx
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Full paper/Memoire
Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols
*
Farhad Shirini , Somayeh Akbari-Dadamahaleh, Ali Mohammad-Khah, Ali-Reza Aliakbar
Department of Chemistry, College of Science, University of Guilan, Namjoo, Rasht 41335, Islamic Republic of Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A mild, efficient and eco-friendly protocol for the chemoselective protection of benzylic
and primary and less hindered secondary aliphatic alcohols and phenols as trimethylsilyl
ethers and different types of amines as N-tert-butylcarbamates is developed using rice
husk (RiH) as the catalyst. This reagent is also able to catalyze the acetylation of alcohols,
phenols, thiols and amines with acetic anhydride. Easy work-up, relatively short reaction
times, excellent yields and low cost, availability and reusability of the catalyst are the
striking features of this methodology, which can be considered to be one of the best and
general methods for the protection of alcohols, phenols, thiols and amines. In addition, the
use of a green reagent in the above-mentioned reactions results in a reduction of
environmental pollution and of the cost of the applied methods.
Received 22 August 2012
Accepted after revision 30 January 2013
Available online xxx
Keywords:
Rice husk
Heterogeneous catalyst
Trimethylsilylation
N-Boc protection
Acetylation
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
most widely used. Its handling does not need special
precautions, and the work-up is not time consuming, as the
Protection and deprotection of organic functions are
important processes during multi-step organic synthesis
by-product of the reaction is ammonia, which is simple to
remove from the reaction medium. However, the main
drawback is the poor silylating capacity of HMDS which is
a deterrent in its application.
[1,2]. The choice of
a method for the functional
group’s transformations depends on its simplicity, high
yields of the desired products, short reaction times,
low cost of the process and ease of the work-up
procedures.
Between the several methods available for the protec-
tion of amines, N-tert-butylcarbonylation has attracted the
attention of many organic chemists. This considerable
attention can be attributed to the stability of N-tert-
butylcarbamates againt bases, nucleophiles, catalytic
hydrogenation and racemization [16,17], as well as the
easy removal of the Boc moiety by reagents, such as
CF3CO2H, formic acid, (3 N) HCl in ethyl acetate or 10%
H2SO4 in dioxane. Among the different reagents which are
available for the N-Boc protection of amines [18], di-tert-
butoxypyrocarbonate [(Boc)2O] is the most popular one
because of its commercial availability, low cost, stability
and efficiency. However, there are several drawbacks
behind the classical N-Boc protection techniques using this
reagent. The tedious work-up takes time and side products
are generated. Moreover, the catalysts used to promote the
reaction are most of the time very costly and non-
recoverable [19].
Trimethylsilylation of labile hydroxyl groups is one of
the most important strategies for the protection of alcohols
[3]. These functional organosilyl ethers are stable under
various conditions, soluble in non-polar solvents, have
high thermal stability and are easy to remove under acid or
base hydrolytic conditions [4]. They also help to increase
the volatility for analysis in gas-chromatography and mass
spectrometry (GC–MS) [5]. A variety of reagents have been
employed for the introduction of the trimethylsilyl group
[6–13], among which hexamethyldisilazane (HMDS), as a
cheap and commercially available reagent [14,15], is the
*
Corresponding author.
E-mail addresses: fshirini@gmail.com, shirini@guilan.ac.ir (F. Shirini).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.01.018
Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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F. Shirini et al. / C. R. Chimie xxx (2013) xxx–xxx
Among the many protecting groups for alcohols,
phenols, amines and thiols, acetate is used with high
frequency. The acetylation is typically performed using
acetic anhydride in the presence of either base [20–23] or
acid catalysts [24–43]. Although various acetylation
methods are available, most have one or more drawbacks,
including long reaction times, harsh conditions, harmful
organic solvents, and tedious work-up procedures. One of
the most promising solutions to these problems seems to
be the use of green and insoluble catalysts or of eco-
friendly solvent-free conditions. When an insoluble cata-
lyst is used, it can be separated easily by filtration and
recycled. Furthermore, the reported examples have
demonstrated that heterogeneous catalysts typically
require easier work-up procedures. Also, solvent-free
synthetic methods are valuable for environmental and
economical reasons [44–46].
methods reported in the literature [55,56]; silica (14.2%),
cellulose (27.4%), hemicelluloses (18.3%), lignin (25.8%),
inorganic residue (5.8%), solubles (3.5%) and moisture (5%).
The catalyst is also characterized using different
methods, including infrared (IR), scanning electron mi-
croscopy (SEM), X-ray diffraction (XRD) and X-ray
fluorescence (XRF) analysis, as reported in the literature
[57].
2.3. Trimethylsilylation of alcohols and phenols; general
procedure
To a stirring mixture of the substrate (alcohol and/or
phenol) (1 mmol), and RiH (0.08 g) in CH3CN (3 mL), HMDS
(0.75 mmol, 0.120 g) was added at room temperature. The
progress of the reaction was monitored by TLC. After
completion of the reaction, the mixture was filtered and
the residue was washed with acetonitrile (5 mL). Evapora-
tion of the solvent gave almost pure product(s). Further
purification proceeded by bulb-to-bulb distillation under
reduced pressure or re-crystallization to afford pure silyl
ether.
In recent years, the use of green reagents in organic
reactions has attracted the attention of many organic
chemists. This attention can be attributed to the reduction
of environmental pollution and the cost of the applied
methods.
Rice husk, as a thin but abrasive skin in nature, which
covers the edible rice kernel, contains cellulose, hemicel-
lulose, lignin, silica, solubles, and moisture [47,48]. The
worldwide annual rice husk output is about 80 million tons
and over 97% of the husk is generated in developing
countries [49]. In the course of decades, rice husk has found
different applications in chemistry and industry. For
example, unmodified rice husk has been evaluated for
its ability to bind zinc(II) and other metal ions [50,51]. On
the other hand, various modifications have been done on
rice husk in order to enhance its sorption capacities for
metal ions and other pollutants [52,53]. In addition, both
rice husk and rice husk ash are used as potential raw
materials in ceramics, cements and silica-based industries
[54].
2.4. N-Boc protection of amines; general procedure
The substrate (1 mmol) was added to a magnetically
stirred mixture of RiH (0.05 g) and (Boc)2O (1 mmol,
0.218 g) at room temperature. After completion of the
reaction (TLC), the mixture was diluted with EtOAc (10 mL)
and filtered. Evaporation of the solvent by column
chromatography (silica-gel) followed; eluting with EtOAc
in n-hexane (5–15%) gave the desired product in good to
high yields.
2.5. Acetylation of alcohols, phenols and thiols; general
procedure
A mixture of the substrate (alcohol, phenol and/or thiol)
(1 mmol), acetic anhydride (3 mmol) and RiH (0.3 g) was
stirred at 80 8C. After completion of the reaction (TLC),
EtOAc (15 mL) was added and the catalyst was filtered The
organic layer was washed with saturated NaHCO3 and
water (3 Â 15 mL), and dried over anhydrous MgSO4.
Evaporation of the solvent under reduced pressure gave
the almost pure acetates.
2. Experimental
2.1. General
Chemicals were purchased from Fluka, Merck, and
Aldrich chemical companies. All yields refer to the isolated
products. Determination of the purity of the substrate and
monitoring of the reaction were accomplished by thin-
layer chromatography (TLC) on a silica-gel polygram SILG/
UV 254 plates.
2.6. Acetylation of amines; general procedure
A mixture of the substrate (1 mmol), acetic anhydride
(3 mmol) and RiH (0.15 g) was stirred at room temperature.
After completion of the reaction (TLC), EtOAc (15 mL) was
added and the catalyst was filtered. The organic layer was
washed with saturated NaHCO3 and water (3 Â 15 mL), and
dried over anhydrous MgSO4. Evaporation of the solvent
under reduced pressure gave the almost pure acetates.
2.2. Preparation of rice husk
The rice sample which was used in this study was
named as Hassani and was obtained from Rasht (Guilan
Province) in the north of Iran. Prior to use, the rice husk
sample was washed several times with distilled water to
remove any adhering materials and dried at room
temperature for 48 h. The dried RiH was smashed and
sieved (80–170 mesh size), washed with distilled water
and dried at 110 8C for 4 h. We have determined the main
components of the rice husk sample as follows, using the
3. Results and discussion
Very recently, we reported the preparation and
applicability of rice-husk-supported FeCl3 nano particles
in the promotion of the acylation of aldehydes and
Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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3
Table 1
HMDS (0.75 mmol), RiH (0.08 g)
CH3CN, r.t.
ROH
ROSiMe3
Trimethylsilylation of alcohols and phenols catalyzed by rice huska,b
.
Entry Substrate
Product
Time Yield
(min) (%)
Scheme 1. Trimethylsilylation of alcohols and phenols.
1
PhCH2OH
PhCH2OSiMe3
5
15
8
94
91
93
97
92
94
92
91
90
93
95
94
96
2
2-ClC6H4CH2OH
4-ClC6H4CH2OH
3,4-Cl2C6H4CH2OH
4-BrC6H4CH2OH
2-BrC6H4CH2OH
2-NO2C6H4CH2OH
4-NO2C6H4CH2OH
2-MeC6H4CH2OH
2-ClC6H4CH2OSiMe3
4-ClC6H4CH2OSiMe3
3,4-Cl2C6H4CH2OSiMe3
4-BrC6H4CH2OSiMe3
2-BrC6H4CH2OSiMe3
2-NO2C6H4CH2OSiMe3
4-NO2C6H4CH2OSiMe3
2-MeC6H4CH2OSiMe3
3
deprotection of the obtained 1,1-diacetates [57]. In
continuation of this study and on the basis of our ongoing
research program on the development of the application of
silica-based reagents in organic transformations [58–62],
we were interested to investigate the applicability of RiH in
the promotion of the other organic reactions. Our
investigations clarified that RiH is efficiently able to
catalyze the trimethylsilyl protection of alcohols and
phenols with HMDS, Boc protection of amines with (Boc)2O
and acetyl protection of alcohols, phenols, thiols and
amines with Ac2O.
4
9
5
19
19
30
20
26
8
6
7
8
9
10
11
12
13
4-Me2CHC6H4CH2OH 4-Me2CHC6H4CH2OSiMe3
3-MeOC6H4CH2OH
4-MeOC6H4CH2OH
3-MeOC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
12
22
9
OH
OH
OSiMe3
Initially, the standardization of the silylation reaction
conditions was studied with investigating the effect of
different molar ratio of reactants and different solvents
(CH2Cl2, n-hexane, acetone, CHCl3, Et2O, CCl4, H2O and
CH3CN) on the reaction, in terms of time and product yield.
Our investigations clarified that the best results can be
obtained under the conditions showed in Scheme 1.
After optimization of the reaction conditions, different
types of alcohols were subjected to trimethylsilylation
using this method (Table 1).
14
15
96
OSiMe3
15
16
15
30
93
90
OH
Ph
OSiMe3
Ph
OSiMe3
OH
17
18
19
PhCH2CH2OH
PhCH2CH2OSiMe3
15
8
93
97
92
The obtained results showed that the trimethylsilyla-
tion of benzylic alcohols, including different types of
substituents, proceeded efficiently with high isolated
yields under the selected conditions (Table 1, entries 1–
16). Primary and less hindered secondary aliphatic
alcohols were also successfully converted to the corre-
sponding silyl ethers in almost quantitative yields at room
temperature (Table 1, entries 17–21). This method was
also found to be useful for trimethylsilylation of allylic
alcohols, diols and acyloins (Table 1, entries 22 and 23). No
elimination and rearrangement by-products were ob-
served at all. Phenols also undergo silylation easily using
this method, and their corresponding silyl ethers can be
isolated in good to high yields (Table 1, entries 24–33).
As shown in Table 2, more hindered secondary and
tertiary alcohols, amines, and thiols are resistant to this
reagent and remain intact in the reaction mixture (Table 1,
entries 34-43). Therefore, this methodology shows selec-
tivity, and is suitable for the selective trimethylsilylation of
benzylic and primary and less hindered secondary
aliphatic alcohols and phenols in the presence of the
above-mentioned substrates (Table 1, entries 44–46).
To illustrate the efficiency of the proposed method,
Table 2 compares some of our results with some of those
reported for the relevant reagents in the literature, which
demonstrates its significant superiority [73–75].
PhCH(Me)CH2OH
PhCH(Me)CH2OSiMe3
15
OH
OSiMe3
N
H
N
H
20
21
15
45
93
90
OH
OH
OSiMe3
OSiMe3
22
23
PhCH5CHCH2OH
PhCH5CHCH2OSiMe3
15
20
90
65
O
O
Ph
Ph
Ph
Ph
OSiMe3
OH
24
25
16
6
93
80
OSiMe3
OSiMe3
OH
OH
26
10
84
OH
OSiMe3
27
28
10
6
90
98
OH
OH
OSiMe3
RiH was then used for the protection of amines as their
N-tert-butylcarbamates with (Boc)2O under similar reac-
tion conditions. N-Boc protection of aniline was chosen as
a probe reaction. However, the reaction was not as
efficient as the O-silylation of alcohols under the pre-
described conditions. Therefore, the reaction was per-
formed in the absence of solvent at room temperature
using 0.05 g of RiH. In this condition, aniline was N-Boc
OSiMe3
29
30
8
93
95
OH
OH
OSiMe3
OSiMe3
20
Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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Table 1 (Continued )
Table 2
Comparison of some of the results obtained by the silylation of alcohols
Entry Substrate
Product
Time Yield
(min) (%)
with HDMS in the presence of rice husk (1), with some of those reported
by LaCl3a(2) [73], trichloroisocyanuric acidb (3) [74], and PBBSc (4) [75].
31
15
90
Entry
Substrate
Time (min)/Yield (%)
OH
OSiMe3
Me3SiO
1
2
3
4
1
2
3
4
PhCH2OH
5/94
180/91
210/93
300/84
–
240/90
180/95
180/95
180/90
90/90
–
32
4
94c
87
OSiMe3
HO
OH
PhCH(OH)Ph
(-)-Menthol
30/90
45/90
20/91
–
33
18
4-NO2C6H4CH2OH
60/90
OH
OMe
OSiMe3
a
Reaction conditions: room temperature, methylenedichloride as the
OMe
solvent.
b
Reaction conditions: room temperature, dichloromethane as the
solvent.
34
120
0d
c
OSiMe3
OSiMe3
OH
OH
Reaction conditions: room temperature, dichloromethane as the
solvent.
35
36
120
180
0d
them, all of the above-mentioned substrates produced
excellent yields in relatively short reaction times. Howev-
er, the reaction with sterically hindered di-phenylamine
and di-cyclohexyl amine was sluggish (Table 3, entries 8
and 14). No isocyanate or urea formation was observed at
all (IR).
0d
OH
OSiMe3
37
38
39
40
PhCH2NH2
PhNH2
PhCH2NHSiMe3
PhNHSiMe3
120
150
120
120
0d
0d
0d
0d
PhNHMe
PhN(SiMe3)Me
Table 3
NH2
NHSiMe3
N-Boc protection of amines using (Boc)2O in the presence of rice huska,b
.
41
42
PhSH
PhSSiMe3
120
120
0d
0d
EntrySubstrate
Product
Time Yield (%)
SH
SSiMe3
SSiMe3
1
2
3
4
5
6
7
PhNH2
PhNH-Boc
10 min 93
25 min 96
60 min 93
25 min 92
15 min 95
70 min 92
30 min 94
4-EtC6H4NH2
3-MeOC6H4NH2
3-ClC6H4NH2
4-BrC6H4NH2
2-HOC6H4NH2
4-EtC6H4NH-Boc
3-MeOC6H4NH-Boc
3-ClC6H4NH-Boc
4-BrC6H4NH-Boc
2-HOC6H4NH-Boc
43
44
120
7
0d
N
S
N
S
SH
PhCH2OH
+
PhCH2OSiMe3
+
100d
0d
NH2
NH-Boc
OH
OSiMe3
8
Ph2NH
Ph2N-Boc
4 h
92
94
96
9
PhCH2NH2
PhCH2NH-Boc
1 min
1 min
45
46
PhCH2CH2OH
PhCH2CH2OSiMe3
20
12
100d
10
N
N
+
+
NH2
N
NH-Boc
PhNHMe
PhN(SiMe3)Me
4-ClC6H4CH2OSiMe3
+
0d
100d
N
H
H
4-ClC6H4CH2OH
+
PhSH
PhSSiMe3
0d
11
1 min
93
NH-Boc
NH2
a
Products were identified spectroscopically and in comparison with
authentic samples [11–13,61,63–72].
MeO
MeO
OMe
OMe
b
Isolated yield.
c
0.16 g of rice husk was used.
12
13
1 min
1 min
95
92
d
NH2
Identified by thin layer chromatography or gas chromatography.
NH-Boc
NH2
NH-Boc
protected for 10 min to furnish the product in excellent
yield (95%) (Scheme 2).
Then, the protocol was followed using a series of
aliphatic, aromatic and heterocyclic amines and aminols
for generalization and the results are shown in Table 3.
Regardless of the nature of the substituent attached to
14
3.7 h
92
95
H
N
Boc
N
15
16
5 min
NH2
HO
NH-Boc
OH
HO
30 min 94
OH
N
H
N
(Boc)2O (1 mmol), RiH (0.05 g)
Boc
PhNH2
PhNH-Boc
17 PhCH2OH
18
PhCH2O-Boc
2 h
2 h
0c
0c
Solvent free, r.t., 10 min, 95 %
Scheme 2. N-Boc protection of aniline.
O-Boc
OH
Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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5
Table 3 (Continued )
Ac2O (3 mmol), RiH (0.15-0.3 g )
RXH
RXAc
EntrySubstrate
Product
Time Yield (%)
solvent-free, r.t. and/or 80 C
X = O, S, N
19 PhCH2NH2
PhCH2NH-Boc
+
3 min 100c
+
PhCH2OH
20
+
PhCH2O-Boc
0c
2 min 100c
+
Scheme 3. Acetylation reactions catalyzed by rice husk.
NH-Boc
NH2
+
+
0c
Table 5
O-Boc
Acetylation of alcohols, phenols, thiolsa and aminesb in the presence of
rice huskc.
OH
a
Products were identified spectroscopically and in comparison with
Entry Substrate
Product
Time Yield
authentic samples [76–81].
(%)d
b
(h)
Isolated yield.
c
Identified by thin layer chromatography or gas chromatography.
1
2
3
4
5
6
PhCH2OH
PhCH2OAc
1
94
92
4-ClC6H4CH2OH
4-BrC6H4CH2OH
2-BrC6H4CH2OH
4-ClC6H4CH2OAc
4-BrC6H4CH2OAc
2-BrC6H4CH2OAc
1.5
1.25 91
1.25 91
Under the selected conditions, Boc protection of
alcohols and phenols was not successful and the starting
material was recovered unchanged after 2 h (Table 3
entries 17and 18). The selectivity of a method determined
the importance of its application in organic reactions.
Therefore, the chemoselectivity of this method was also
investigated and the results are reported in Table 3 (Entries
6, 15, 16, 19 and 20). The obtained results clearly showed
that this method could be efficiently used for the
chemoselective protection of different types of amines
in the presence of alcohols and phenols.
In order to show the merit of this method, Table 4
compares the results obtained from the N-Boc protection
of aniline by our method with some of those reported in
the literature [82–86].
After the above-mentioned reactions, we have tried to
extend the applicability of RiH in organic transformations
by studying the role of this catalyst in the promotion of the
acetylation reactions. Our investigations clarified that RiH
was also efficiently able to catalyze the acetyl protection of
alcohols, phenols, thiols and amines with Ac2O. All
reactions are performed under mild reaction conditions
in good to high yields (Scheme 3, Table 5).
4-Me2CHC6H4CH2OH 4-Me2CHC6H4CH2OAc
1.5
92
1.75 90
OH
OAc
7
2.5
87
OH
OAc
8
9
PhCH(Me)OH
PhCH(Me)OAc
2
85
90
1.5
OH
OAc
10
1.75 88
1.75 84e
OAc
OH
11
12
OH
OH
Ph
OAc
OAc
Ph
10
60
87
O
O
Ph
Ph
Ph
Ph
OH
OAc
13
14
1.5
OH
OH
OAc
OAc
From the context of green approach, the reusability of
the catalyst was tested in the silylation and acetylation of
4-chlorobenzyl alcohol and N-Boc protection of aniline.
After completion of the reaction, the catalyst was washed
well with acetonitrile, acetone and water, then dried at
100 8C prior to use and tested for its activity in subsequent
run and fresh catalyst was not added. It was seen that the
catalyst displayed very good reusability (Fig. 1).
1.75 85
15
3
80
OH
OAc
16
17
0.8
1
92
83
OH
OH
OAc
OAc
Table 4
Comparison of the results obtained from the N-Boc protection of aniline in
the presence of various catalysts.
Entry Catalyst
Solvent
Time
Yield Reference
(%)
18
1
92
OAc
OH
OH
1
2
3
4
5
6
7
RiH
Neat
CH2Cl2
Neat
H2O
10 min 95
12 h 92
30 min 95
This work
[81]
Zr(ClO4)2Á6H2O
Iodine
[82]
19
20
1
1
90
93
b-Cyclodextrine
2.5 h
8 min
14 h
75
95
90
97
[83]
OAc
Thioglycolic acid EtOH
[84]
Yttria-zirconia
Saccharin
CH3CN
[85]
n-Hexane 1 h
[86]
OH
OAc
sulfonic acid
Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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F. Shirini et al. / C. R. Chimie xxx (2013) xxx–xxx
Table 5 (Continued )
protection of amines and acetylation of alcohols, phenols,
Entry Substrate
Product
Time Yield
thiols and amines, producing remarkable high yields under
very mild conditions. In contrast to some existing methods
using potentially hazardous catalysts/additives, this new
method offers the following advantages: (i) low cost,
availability and reusability of the reagent, (ii) avoidance of
the use of any base, metal, or Lewis acid catalysts, (iii)
relatively short reaction times, (iv) easy and clean work-
up, (v) high chemoselectivity, and (vi) no side reactions.
We are exploring further applications of RiH for the other
types of organic reactions in our laboratory.
(h)
(%)d
21
2
81
OH
OAc
22
0.5
90e
HO
OH
AcO
OAc
23
24
PhCH2SH
PhCH2SAc
0.5
0.5
92
91
Br
SAc
Br
SAc
SAc
SAc
25
26
0.5
93
Me
SAc
SH
Me
0.75 87
Acknowledgments
We are thankful to the University of Guilan Research
Council for the partial support of this work.
27
28
29
PhCH2NH2
PhNH2
PhCH2NHAc
PhNHAc
0.25 95
0.25 93
0.25 93
Br
Br
NH2
NH2
References
NHAc
NHAc
30
31
0.8
0.5
92
94
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Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018

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Please cite this article in press as: Shirini F, et al. Rice husk: Introduction of a green, cheap and reusable catalyst for the
protection of alcohols, phenols, amines and thiols. C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.01.018