Nanoporous Na+-montmorillonite perchloric acid as an efficient and recyclable catalyst for the chemoselective protection of hydroxyl groups

Article abstract of DOI:10.1166/jnn.2019.15524

Nanoporous Na+-montmorillonite perchloric acid as a novel heterogeneous reusable solid acid catalyst was easily prepared by treatment of Na+-montmorillonite as a cheap and commercially available support with perchloric acid. The catalyst was characterized using a variety of techniques including X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), energy dispersive X-ray spectroscopy (EDX), pH analysis and determination of the Hammett acidity function. The prepared reagent showed excellent catalytic activity for the chemoselective conversion of alcohols and phenols to their corresponding trimethylsilyl ethers with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) at room temperature. Deprotection of the resulting trimethylsilyl ethers can also be carried out using the same catalyst in ethanol. All reactions were performed under mild and completely heterogeneous reaction conditions in good to excellent yields. The notable advantages of this protocol are: short reaction times, high yields, availability and low cost of the reagent, easy work-up procedure and the reusability of the catalyst during a simple filtration.

Full text of DOI:10.1166/jnn.2019.15524

Article
Journal of
Nanoscience and Nanotechnology
Vol. 19, 2099–2107, 2019
Copyright © 2019 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Nanoporous Na+-Montmorillonite Perchloric Acid as an
Efficient and Recyclable Catalyst for the Chemoselective
Protection of Hydroxyl Groups
Maryam Mashhadinezhad, Farhad Shirini∗, and Manouchehr Mamaghani
Department of Chemistry, College of Sciences, Guilan University, Rasht, 41335-19141, Iran
Nanoporous Na+-montmorillonite perchloric acid as a novel heterogeneous reusable solid acid cata-
lyst was easily prepared by treatment of Na+-montmorillonite as a cheap and commercially available
support with perchloric acid. The catalyst was characterized using a variety of techniques including
X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron
microscopy (SEM), thermogravimetric analysis (TGA), energy dispersive X-ray spectroscopy (EDX),
pH analysis and determination of the Hammett acidity function. The prepared reagent showed
excellent catalytic activity for the chemoselective conversion of alcohols and phenols to their corre-
sponding trimethylsilyl ethers with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) at room temperature.
Deprotection of the resulting trimethylsilyl ethers can also be carried out using the same catalyst in
ethanol. All reactions were performed under mild and completely heterogeneous reaction conditions
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
in good to excellent yields. The notable advantages of this protocol are: short reaction times, high
Copyright: American Scientific Publishers
yields, availability and low cost of the reagent, easy work-up procedure and the reusability of the
Delivered by Ingenta
catalyst during a simple filtration.
Keywords: Na+-Montmorillonite, Silylation, Protection, Deprotection, Hexamethyldisilazane,
Nanoclay.
1. INTRODUCTION
medium.5 However, the low silylation power of HMDS
is the main drawback to its application.6 To solve this
problem, a variety of catalysts including LaCl3,7 sul-
fonic acid-functionalized nanoporous silica,8 trichloroiso-
cyanuric acid (TCCA),9 HClO4–SiO2,10 Fe(HSO4ꢁ3,11
nanocrystalline TiO2,12 Cu(NO3ꢁ2 · 3H2O,13 saccharin
sulfonic acid,14 tetrabutylammonium Bromide,15 TiO2–
HClO4,16 ZnCl4,17 H-ꢂ zeolite,18 SBA-15-Ph–Pr–SO3H,19
K-10 montmorillionite,20 ꢃ-Fe2O3@SiO2–(CH2ꢁ3NH–
Sn(n-Bu)3,21 sulfamic acid,22 PhMe3N+Br3−,23 iodine,24
ZrO(OTf)2,25 [Ti(salophen)(OTf)2],26 Bi(OTf)3,27 1,3-
dichloro-5,5-dimethylhydantoin (DCH) and trichlorome-
lamine (TCM)28 and InBr239 have been developed for the
promotion of the trimethylsilylation of alcohols and phe-
nols with HMDS. Even though the silylating ability of
HMDS was increased using these catalytic systems, most
of them suffer from disadvantages such as long reaction
times, low yields of the products, tedious work-up, vigor-
ous reaction conditions, non-recyclability of the catalyst
and use of toxic, expensive or moisture-sensitive reagents.
In addition, some of the reagents are not able to catalyze
the reverse reaction.
Protection and deprotection of functional groups are very
important processes, which are widely used in the prepara-
tion of multifunctional target molecules.1 Among different
types of the protecting groups, silylating ones have played
increasingly important roles in the synthesis of compli-
cated molecules, especially in the synthesis of biologically
significant products.2 Several methods are available for
the silylation of the hydroxyl groups, using a variety of
silylating agents. Some silylating agents are not used so
much due to their unavailability, lack of reactivity or the
tedious processes for the purification of the products.3ꢀ4
1,1,1,3,3,3-hexamethyldisilazane has emerged as the most
versatile reagent in recent years for trimethylsilyl (TMS)
protection of alcohols and phenols. HMDS is a stable,
commercially available and inexpensive reagent. Its han-
dling does not require special precautions, and the work-up
is convenient, because the only by-product of the reaction
is ammonia, which is simple to remove from the reaction
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 4
1533-4880/2019/19/2099/009
doi:10.1166/jnn.2019.15524
2099

Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Mashhadinezhad et al.
Acids are widely used as catalysts in industry, producing
more than 1 × 108 mt of products/year. The most com-
monly used acids are HF, H2SO4, HClO4, and H3PO4.30
Perchloric acid is a stronger acid than sulfuric and nitric
acids. When it is hot, it can act as an oxidizing agent,
while in aqueous solutions up to approximately 70% this
reagent only show strong acidic features and no oxidizing
properties. Perchloric acid is also used in the computer
industry for etching liquid crystal displays in critical elec-
tronic applications as well as a solvent for many metals
and alloys and the separation of certain materials.31
solid acids and in continuation of our previous studies
on the preparation and application of new catalysts in
organic functional group transformations,11–17ꢀ41ꢀ42 we dis-
close the preparation of nanoporous Na+-montmorillonite
supported perchloric acid (MMT-HClO4ꢁ and its appli-
cation, as a new, efficient and recyclable catalyst, in
trimethylsilyl protection of hydroxyl groups and depro-
tection of trimethylsilyl ethers at room temperature under
mild and heterogeneous conditions. In this reaction, the
catalyst could be recycled several times without significant
loss of its catalytic activity.
Heterogenization of homogeneous acid catalysts via
their immobilization on various solid supports has been
the subject of the extensive researchs during the past
decade.32–35 This procedure leads to environmentally
friendly industrial manufacturing and improvement of pro-
cessing steps. Solid acids have many advantages such
as simplicity in handling, eliminating toxicity, corrosion
and environmental problems.36 In recent years, clays as
nanostructured materials have been widely used in organic
transformations as solid acid catalysts.32ꢀ34ꢀ37 The main
reasons for use of clays are accessibility, cheapness, non-
corrosiveness, recyclability and easy modification. Mont-
morillonite (MMT) is one of the most widely used clays.
Montmorillonite belongs to the family of 2:1 smectite
minerals. It is basically composed of aluminosilicate lay-
ers, where one octahedral alumina sheet is sandwiched
between two tetrahedral silica sheets (Fig. 1).38
2. EXPERIMENTAL DETAILS
2.1. Material
Chemicals were purchased from Fluka, Merck and Aldrich
Chemical Companies. All yields refer to the isolated prod-
ucts. Products were characterized by their physical con-
stants, comparison with authentic samples. The purity
determination of the substrate and reaction monitoring
were accompanied by thin-layer chromatography (TLC) on
silica-gel polygram SILG/UV 254 plates.
2.2. Instrumentation
The FT-IR spectra were recorded on a FT-IR Bruker
Alpha spectrophotometer. The pristine clay and acid-
treated derivative were dried at 100 ꢀC in the oven
before recording the spectrum. Thermogravimetric analy-
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
ses were conducted by using a TGA PYRIS 1 thermoan-
Copyright: American Scientific Publishers
Montmorillonite minerals have nano-sized particles, and
ꢀ
39
Delivered by Ingenta
alyzer instrument. Samples were heated 25 to 600 C at
they are extremely fine-grained and thin-layered. Layers
ꢀ
ramp 10 C/min under N2 atm. X-ray diffraction measure-
of MMT have a thickness of about 1 nm and a length
of 100 nm or a little more. Broken bonds on the edge of
MMT layers are common phenomena for layered silicates
and lead to the free formation of hydroxyl groups,40 which
can be utilized for chemical modification.
ments were performed at room temperature on a Philips
PW 1840 diffractometer using Ni-filtered Cu-Kꢄ radiation
(ꢅ = 0.15418 nm). Scanning election microphotographs
were obtained on a TESCAN MIRA3 FEG-SEM instru-
ment equipped with EDX. The absorption spectra were
recorded by a scanning UV-3100 spectrophotometer.
Hence, in this study, considering the advantages of
the above mentioned properties of montmorillonite and
2.3. Catalyst Preparation
Al, Fe, Mg, Li
HClO4 (0.12 mL, 2 mmol, as a 70% aq solution) was
added to a suspension of Na+-montmorillonite (Na+-
MMT) (1 g) in Et2O (50 mL). The mixture was refluxed
for 4 h then the suspension was concentrated and the
residue was consecutively washed with Et2O (2 × 5 mL).
After removal of ether using a rotary evaporator, the solid
residue was dried at 100 ꢀC for 10 h to afford MMT-HClO4
(1.11 g) (Scheme 1).
OH
O
Li, Na, Rb, Cs
Tetrahedral
Octahedral
Tetrahedral
2.4. General Procedure for Trimethylsilylation of
Alcohols and Phenols
Cations
Exchangeable
To a stirring mixture of the substrate (1 mmol) and MMT-
HClO4 (7 mg) in CH3CN (3 mL), HMDS (0.75 mmol)
was added at room temperature. After completion of the
reaction, indicated by TLC, ethyl acetate: n-hexane (3:7),
the mixture was filtered to separate the catalyst. The fil-
trate was washed with acetonitrile (5 mL). Removal of the
Figure 1. Structure of montmorillonite.
2100
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019

Mashhadinezhad et al.
Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
H2O+
–
ClO4
–
+OH2
ClO4
OH
OH
OH
OH
–
+ClO4
H3O+
OH
HClO4
–
Na
+OH2
ClO4
Na+
H2O
H2O+
–
ClO4
Et2O,
reflux, 4h
OH H2O
OH
Na+
OH
–O
–
ClO4
H2O+
–
OH
OH
ClO4
–
+OH2
ClO4
OH
MMT-HClO4
Scheme 1. Preparation of the catalyst (MMT-HClO4ꢁ.
solvent under reduced pressure gave almost pure product
in good to high yields. Further purification was carried
out by column chromatography on silica gel (eluting with
EtOAc:hexane = 1:4), if necessary.
absorption bands at 1088, 1111, 1145 [ꢆasym (ClO4ꢁ], 630
[ꢆasym (ClO4ꢁ] and 467 [ꢇsym (ClO4ꢁ] cm−1 which are
assigned to the asymmetric and symmetric ClO4 stretching
and bending vibrations, respectively.45 These results show
modification of nanoporous Na+-MMT using perchloric
acid.
2.5. General Procedure for Deprotection of
Trimethylsilyl Ethers
3.1.2. Thermal Analysis
A mixture of the substrate (1 mmol) and MMT-HClO4
(10 mg) in ethanol (2 mL) was stirred at room temperature.
The progress of the reaction was monitored by TLC, ethyl
acetate: n-hexane (3:7). After completion of the reaction
the catalyst was filtered off and the solvent was evaporated
under reduced pressure. The crude product was purified
by column chromatography on silica gel (EtOAc:hexane =
1:4) to gave the pure alcohol and/or phenol derivative in
good to high yields.
The thermal stability of Na+-MMT and MMT-HClO4 was
determined by thermogravimetric analysis (TGA). Figure 3
provides the TGA curves of pristine clay and catalyst. The
TGA curve of Na+-MMT display a weight loss 11.5%
below 100 ꢀC which is attributed to dehydration of adsorp-
tion water. Another weight loss process occur at the tem-
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:0ꢀ6
perature range 100–600 C, possibly corresponding to the
Copyright: American Scientific Publishers
loss of the bonded H2O within the gallery.
Delivered by Ingenta
MMT-HClO4 indicate four-stage decomposition. The
ꢀ
first weight loss 22% below 150 C which is correspond-
ing to the loss of surface-physisorbed water. The second
weight loss (at around 240–340 C) can be attributed to
3. RESULTS AND DISCUSSION
3.1. Characterization of the Catalyst
3.1.1. FT-IR Analysis
ꢀ
The infrared spectra of Na+-MMT and MMT-HClO4 are
presented in Figure 2. The FT-IR spectra of the pris-
tine and acid-treated clay samples show the structural
modification which occurs owing to acid activation. The
spectrum of montmorillonite exhibits absorption bands at
3432 and 1639 cm−1 which are assigned to the stretch-
ing and bending vibrations of the OH groups of the water
molecules adsorbed on the clay surface, and a band at
3626 cm−1 representing the stretching vibration of the
hydroxyl groups coordinated to the aluminum and/or mag-
nesium in Na+-MMT.43 In this spectrum the bands at 1035
and 919 cm−1 can be attributed to the Si–O stretching
vibrations and bending modes of the hydroxyl groups of
Al2OH, respectively. The bands at 531 and 466 cm−1 are
corresponded to Si–O–Al (octahedral) and Si–O bending
vibrations respectively.44
The spectrum of Na+-MMT supported perchloric acid
displays the broad band around 3418 cm−1 that attributed
to the OH group of perchloric acid and adsorbed water
molecules, whereas deformational vibrations of adsorbed
water molecules caused the absorption band to appear
at 1636 cm−1. MMT-HClO4 also exhibited characteristic
Figure 2. FT-IR spectra of Na+-MMT, HClO4 and MMT-HClO4.
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019
2101

Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Mashhadinezhad et al.
a
b
Figure 3. TGA curves of Na+-MMT and MMT-HClO4.
Figure 5. Absorption spectra of 4-nitroaniline (indicator) (a) and MMT-
HClO4 (catalyst) (b) in CCl4 .
the disintegration of the loaded perchloric acid.45 Heating
to a higher temperature (at around 340–450 C) results in
3.1.4. Hammett Acidity
ꢀ
The Hammett acidity function (H0ꢁ can be used to effec-
tively express the acidity strength of an acid in organic
solvents.47 The acidity of the solution is calculated by
using the following Hammett equation:
the collapse of the clay interlayer structuꢀre as the water is
driven out. Further heating (above 450 C) results even-
tually in complete dehydroxylation of the aluminosilicate
lattice, producing a completely amorphous solid.46 These
results show that the catalyst is thermally-stable up to
H0 = pKꢊIꢁaq +logꢋIꢌs/ꢋHI+ꢌs
(1)
ꢀ
240 C.
Where [Iꢌs and [HI+]s are the molar concentrations of
the indicator and its conjugate acid in the solvent respec-
tively and pK(Iꢁaq is the pKa value of aqueous solution
of indicator. According to the Beer-Lambert law, the value
of [Iꢌ /[HI+] can be determined and calculated using
3.1.3. Powder X-ray Diffraction
The XRD patterns of the pristine and acid-treated clay
samples are given in Figure 4. The interlayer space of
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
Na+-MMT was calculated using Bragg’s equation. Na+-
MMT exhibited a diffraction peak at 2ꢈ = 6ꢉ25ꢀ, with a
corresponding basal spacing of 14.14 Å. As can be seen
in the MMT-HClO4 patterns, the increase in basal spac-
ing from 14.14 Å in Na+-MMT to a 16.02 Å value in
MMT-HClO4 shows the changes in interlayer spaces as
a result of the acid treatment. That is to say, the per-
chloric acid moiety has been intercalated into the mont-
morillonite layers. This increase in the basal spacing
can lead to better interaction between the reactants in
the acid catalysis reactions compared with sodium mont-
morillonite. These results also confirme that nanoporous
Na+-montmorillonite has retained its layered structure and
nanoporosity even after treatment with perchloric acid.
Copyright: American Scientifsic Publishers
s
Delivered by Ingenta
the ultraviolet–visible (UV-Vis) spectrum. For this pur-
pose, 4-nitroaniline (pK(Iꢁaq = 0ꢉ99) as the basic indi-
cator and CCl4 as the solvent were used. As Figure 5
shows, the maximum absorbance of the unprotonated form
of 4-nitroaniline was observed at 330 nm in CCl4. When
MMT-HClO4 as a solid acid catalyst was added to the indi-
cator solution, the absorbance of the unprotonated form of
the indicator decreased, which implied that the indicator
was partially in the form of [HI+]. The results obtained
are listed in Table I, which shows the acidity strength of
MMT-HClO4.
3.1.5. Energy Dispersive X-ray Spectroscopy
Figure 6 shows the EDX spectrum of MMT-HClO4. As
can be seen, the existence of O and Cl peaks indicates suc-
cessful incorporation of HClO4 within the montmorillonite
interlayer spaces.
Table I. Calculation of Hammett acidity function (H0ꢁ for
MMT-HClO4.
Entry
Catalyst
Amax
ꢋIꢌs %
[HI+]s %
H0
1
2
–
1.329
0.334
100
25.132
0
–
MMT-HClO4
74.868
0.516
Notes: Condition for UV–visible spectrum measurement: Solvent: CCl4, indicator:
4-nitroaniline (pK(Iꢁaq = 0.99), 1ꢉ44×10−4 mol/L; Catalyst: MMT-HClO4, 20 mg,
25 ꢀC.
Figure 4. XRD patterns of Na+-MMT and MMT-HClO4.
2102
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019

Mashhadinezhad et al.
Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Figure 6. EDX spectra of nanoporous MMT-HClO4.
3.1.6. Morphological Analysis
of the prinstin clay has remained after acid treatment that
confirm the results of X-ray diffraction analysis and suc-
cessful modification.
Scanning electron microscopy analysis was used to show
the particle size distribution and surface morphologi-
cal changes created on the clay during the modifica-
tion of sodium montmorillonite. In Figures 7(a)–(d) the
SEM pictures of pristine and acid-treated clay are pre-
sented, respectively. SEM images corroborated the vari-
ation of Na+-MMT morphology after acid treatment.
Figures 7(a)–(b) shows that montmorillonite have nano-
sized particles and there is a layer structure in pristine clay
so that particles of montmorillonite are composed of irreg-
ular platelets that stack on top of each other. The SEM
micrographs reveal that MMT-HClO4 has a flaky form
with plate-like particles that imply the layered structure
3.1.7. pH Analysis of Catalyst
To 25 mL of an aqueous solution of NaCl (1 M) with a
primary pH 5.7, MMT-HClO4 (0.5 g) was added and the
resulting mixture was stirred for 2 h at room temperature
after which the pH of solution decreased to 1.66. This is
equal to a loading of 1.09 mmol H+/g.8
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
Copyright: American Scientific Publishers
3.2. Catalytic Activity
Delivered by Ingenta
On the basis of the informations obtained from the
above mentioned studies, we expected that nanoporous
Figure 7. SEM images of Na+-MMT (a, b) and MMT-HClO4 (c, d).
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019
2103

Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Mashhadinezhad et al.
Table II. Continued.
Time Yield
(min) (%)a
Entry
Substrate
Product
Scheme 2. Trimethylsilylation of alcohols and phenols and deprotec-
tion of silyl ethers.
31
32
33
4-MeC6 H4NH2
4-ClC6 H4NH2
4-MeC6 H4NHSiMe3
4-ClC6 H4NHSiMe3
60
60
60
0d
0
0
NHSiMe3
NH
2
MMT-HClO4 can be used as an efficient solid acid catalyst
for the promotion of the reactions which need the use of
an acidic catalyst to accelerate. So we were interested in
investigating the applicability of this catalyst in the pro-
motion of the trimethylsilyl protection of hydroxyl groups
with HMDS. In order to obtain the optimum reaction con-
ditions, the preparation of trimethylsilyl ether derivative of
4-chlorobenzyl alcohol was studied in various conditions
34
35
36
PhCH2NH2
PhCH2NHSiMe3
60
0
0
0
PhCH2CH2CH2NH2 PhCH2CH2CH2NHSiMe3 60
60
NSiMe3
NH
37
60
0e
H
SiMe3
N
N
NH2
NHSiMe3
38
39
40
41
4-BrC6H4SH
4-MeC6H4SH
PhCH2SH
4-BrC6H4SSiMe3
4-MeC6H4SSiMe3
PhCH2SSiMe3
60
60
60
0
0
0
4-ClC6 H4CH2OH
4-ClC6H4CH2OSiMe3
20 Tracef
Notes: aIsolated yield. Products were identified spectroscopically and characterized
by their physical constants, comparison with authentic samples. b1 mmol of HMDS
used for silylation. c1.5 mmol of HMDS used for silylation. dStarting material
was recovered unchanged. e1.5 mmol of HMDS used for silylation. f Reaction was
performed in the presence of Na+-montmorillonite.
Table II. Trimethylsilylation of alcohols with HMDS catalyzed by
MMT-HClO4 at room temperature.
Time Yield
Entry
Substrate
Product
(min) (%)a
1
2
3
4
5
6
7
8
PhCH2OH
PhCH2OSiMe3
2
4
2
2
2
5
7
4
3
2
2
3
2
3
92
92
94
92
94
93
90
91
92
91
90
89
92
90
2-ClC6H4CH2OH
4-ClC6H4CH2OH
3,4-Cl2C6H4CH2OH
4-BrC6H4CH2OH
2-BrC6H4CH2OH
2-NO2C6H4CH2OH
4-NO2C6H4CH2OH
2-MeC6H4CH2OH
4-Me2CHC6H4CH2OH 4-Me CHC H CH OSiMe
4-Me3CHC6H4CH2OH 4-Me2CHC6H4CH2OSiMe3
3-MeOC6H4CH2OH
4-MeOC6H4CH2OH
OH
2-ClC6H4CH2OSiMe3
4-ClC6H4CH2OSiMe3
3,4-Cl2C6H4CH2OSiMe3
4-BrC6H4CH2OSiMe3
2-BrC6H4CH2OSiMe3
2-NO2C6H4CH2OSiMe3
4-NO2C6H4CH2OSiMe3
2-MeC H CH OSiMe
including different amounts of the catalyst, temperature
and solvent. Finally, the best result was obtained using
10 mg of MMT-HClO4 in acetonitrile at ambient temper-
ature (Scheme 2). In order to investigate the applicability
of this method we selected a wide variety of benzylic and
aliphatic alcohols and phenols. As can be seen in Table II,
using this method all of the selected compounds were suc-
cessfully converted to the requested products in high yields
and short times at room temperature in CH3CN under com-
pletely heterogeneous reaction conditions.
9
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
6
4
2
3
10
11
12
13
14
4 Copyrig3ht: American Scientific Publishers
2
6
2
Delivered by Ingenta
3-MeOC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
OSiMe3
As indicated in Table II, in the case of benzylic alco-
hols the nature of the substituents (acid sensitive, electron-
withdrawing or electron-releasing groups) on the aromatic
ring has no significant effect on the product yield (Table II,
entries 1–15). Primary, secondary and tertiary aliphatic
alcohols were also successfully converted to their corre-
sponding trimethylsilyl ethers in high yields under the
same reaction conditions (Table II, entries 16–22). Using
this method unsaturated alcohols (Table II, 23) gave
the corresponding TMS-ethers in high yields. Further-
more, trimethysilylation of different types of phenols with
HMDS under the optimized conditions was performed
cleanly in high yields (Table II, entries 24–29). In gen-
eral, phenols underwent silylation at longer reaction times
compared to benzylic alcohols. It is worth mentioning that
in the presence of Na+-MMT the reactions were so slow
that a considerable amount of the starting material was
recovered unchanged (Table II, entry 41). These results
indicate that the modification of Na+-MMT with perchlo-
ric acid efficiently increased its ability in the promotion
of trimethysilylation reaction and the catalyst is neces-
sary to produce the desired products. Moreover, studies
showed that this method was able to selectively protect
the hydroxyl group against the amino group (Table II, 30).
OH
OSiMe3
15
2
93
16
17
18
PhCH2CH2OH
PhCH2CH2CH2OH
PhCH2CH2OSiMe3
PhCH2CH2CH2OSiMe3
2
2
3
91
92
92
OSiMe3
OH
19
20
10
30
91
90
OH
HO
OSiMe3
Me3SiO
OH
OSiMe3
21
20
91
22
23
25
7
90
94
OH
OH
OSiMe3
OSiMe3
24
25
26
27
28
29
30
4-EtC6H4OH
3,5-Me2C6H3OH
2,4-Me2C6H3OH
2-MeC6H4OH
3-MeC6H4OH
4-MeC6H4OH
4-EtC6H4OSiMe3
3,5-Me2C6H3OSiMe3
2,4-Me2C6H3OSiMe3
2-MeC6H4OSiMe3
3-MeC6H4OSiMe3
4-MeC6H4OSiMe3
7
4
91
10
5
89b
93
7
92
93
92
90c
5
2
OH
OSiMe3
N
N
H
H
2104
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019

Mashhadinezhad et al.
Table III. Chemoselective trimethylsilylation of alcohols.
Time Conver-
Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Table IV. Deprotection of trimethylsilyl ethers catalyzed by
MMT-HClO4.
Entry
Substrate
Product
(min)
sion
Time Yield
Entry
Substrate
Product
(min) (%)a
1
3,4-Cl2C6H3CH2OH 3,4-Cl2C6H3CH2OSiMe3
3
100
0
100
0
100
0
100
0
+ 4-MeC6H4NH2
4-BrC6H4CH2OH
+ 4-ClC6H4NH2
2-ClC6H4CH2OH
+ 4-MeC6H4SH
PhCH2CH2OH
+ 4-MeC6H4NHSiMe3
4-BrC6H4CH2OSiMe3
+ 4-ClC6H4NHSiMe3
2-ClC6H4CH2OSiMe3
+ 4-MeC6H4SSiMe3
PhCH2CH2OSiMe3
1
2
3
4
5
6
7
8
PhCH2OSiMe3
PhCH2OH
2
2
1
2
1
2
3
2
2
2
2
2
3
4
2
2
2
3
10
93
90
92
95
93
91
92
94
93
87
92
94
90
91
90
86
89
90
91
2
3
4
3
5
3
2-ClC6 H4CH2OSiMe3
4-ClC6 H4CH2OSiMe3
3,4-Cl2C6H3CH2OSiMe3
4-BrC6H4CH2OSiMe3
2-BrC6H4CH2OSiMe3
2-NO2C6H4CH2OSiMe3
4-NO2C6H4CH2OSiMe3
2-MeC6H4CH2OSiMe3
4-Me2CHC6H4CH2OSiMe3 4-Me2CHC6H4CH2OH
4-Me3CC6H4CH2OSiMe3 4-Me3CC6H4CH2OH
3-MeOC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
Ph2CHOSiMe3
2-ClC6 H4CH2OH
4-ClC6 H4CH2OH
3,4-Cl2C6H3CH2OH
4-BrC6H4CH2OH
2-BrC6H4CH2OH
2-NO2C6H4CH2OH
4-NO2C6H4CH2OH
2-MeC6H4CH2OH
+4-BrC6H4SH
+ 4-BrC6H4SSiMe3
9
10
11
12
13
14
15
16
17
18
19
Thus, protection of amines and thiols was also inves-
tigated and it was found that amines and thiols were
resistant to this reagent and the starting materials were
recovered intact after 2 h (Table II, entries 31–40). There-
fore, this methodology shows selectivity and is suitable for
the selective trimethylsilylation of alcohols and phenols in
the presence of amines and thiols. The obtained results are
reported in Table III.
Moreover, the results obtained in this study clearly
revealed that nanoporous Na+-montmorillonite perchlo-
ric acid (MMT-HClO4ꢁ can also be an efficient reagent
for the conversion of trimethylsilyl ethers to the par-
ent alcohols and phenols in ethanol at room temperature.
All reactions were performed under mild and completely
heterogeneous reaction conditions with good to excellent
3-MeOC6H4CH2OH
4-MeOC6H4CH2OH
Ph2CHOH
PhCH(OH)Me
PhCH2CH2OH
PhCH(OSiMe3ꢁMe
PhCH2CH2OSiMe3
PhCH2CH2CH2OSiMe3
PhCH(Me)CH2OSiMe3
PhCH2CH2CH2OH
PhCH(Me)CH2OH
OSiMe3
OH
20
21
30
15
10
88
90
85
Me3SiO
OSiMe3
HO
OH
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
Copyright: American S2c2ientific Publishers
yields (Scheme 2, Table IV).
OSiMe3
Delivered by Ingenta
OH
3.3. Reusability of the Catalyst
23
24
25
26
27
28
4-EtC6H4OSiMe3
3,5-Me2C6H3OSiMe3
2,4-Me2C6H3OSiMe3
2-MeC6H4OSiMe3
3-MeC6H4OSiMe3
4-MeC6H4OSiMe3
4-EtC6H4OH
3,5-Me2C6H3OH
2,4-Me2C6H3OH
2-MeC6H4OH
3-MeC6H4OH
4-MeC6H4OH
5
3
5
5
3
3
90
93
92
90
91
90
To examine the reusability of the catalyst, it was recov-
ered by filtration from the reaction mixture, washed with
CH3CN, H2O and acetone and dried in oven. The recov-
ered catalyst was reused under optimized reaction con-
ditions for trimethylsilylation of 4-chlorobenzyl alcohol
which gave the desired product in good yields after four
runs. The obtained results were indicated that the reactions
led to the requested products without significant changes
in terms of reaction time and yield which clearly shows
useful recyclability of MMT-HClO4 (Fig. 8). The IR of
the recovered catalyst showed the same spectra of unused
catalyst. This result indicates that the catalyst remained
unchanged after recovery.
Notes: aIsolated yield. Products were characterized by their physical constants and
comparison with authentic samples.
such as H-ꢂ-zeolite and sulfonic acid-functionalized meso-
porous silica (SBA) in terms of the catalyst amounts, the
reaction times and conditions. According to Table I, MMT-
HClO4 shows better efficiency than other nanoparticles
The evolution of ammonia gas (NH3ꢁ was observed in
all reactions. A conceivable mechanism that confirm the
generation of NH3 and the catalytic role of this catalyst
has been proposed (Scheme 3).
In order to show the effectiveness of the presented
method, we have compared the obtained results in the
trimethylsilylation of alcohols with HMDS catalyzed by
MMT-HClO4 with some of those reported in the litera-
ture. As indicated in Table V, this method avoids disadvan-
tages of other procedures such as excess reagents and long
reaction times. Furthermore, the present method shows a
preference to similar aluminosilicate structural systems,
Figure 8. Recyclability of the catalyst.
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019
2105

Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
Mashhadinezhad et al.
(Me3Si)2NH
although sulfonic acid-functionalized ordered nanoporous
Na+-montmorillonite (SANM) has similar reaction condi-
tions to MMT-HClO4 but SANM catalyzes this reaction
using larger amounts of catalyst and longer reaction times
than MMT-HClO4. On the other hand, for the preparation
of SANM, chlorosulfonic acid is used which is a color-
less liquid, hygroscopic and a powerful lachrymator.48 Fur-
thermore, this method require a proper ventilation and a
gas inlet tube for conducting the produced HCl gas over
an adsorbing solution, but MMT-HClO4 is produced eas-
ily through interaction of clay and perchloric acid without
production of any harmful gas. As the pH analysis indi-
cates the number of H+ sites of SANM arising from
ClSO3H was 0.88 mmol·g−1, but the number of H+ sites
for MMT-HClO4 arising from HClO4 was 1.09 mmol
H+/g. This comparison shows that perchloric acid is sup-
ported on clay more effectively than chlorosulfonic acid.
It is necessary to report that the protection of alcohols
in the presence of Montmorillonite K 10 completed dur-
ing a long time (0.2–8 h) using large amounts of catalyst
(100 mg). This comparison indicates that the modification
of Na+-MMT with perchloric acid efficiently increases its
activity.
MMT-HClO4
[MMT-ClO ]
[(Me3Si)2NH2]
4
NH3
ROH
ROSiMe3
[Me3SiNH3][MMT-ClO4]
[NH4][MMT-ClO4]
ROSiMe3
ROH
Scheme 3. Proposed mechanism of the reaction.
such as nanoporous silica and nanocrystalline TiO2. As
can be seen in Table V, among the reported catalysts the
activity and amount of nano TiO2-HClO4 is near to MMT-
HClO4. But it is noteworthy that clays have a natural ori-
gin and due to their enormous amounts in nature have
cost much less than other nanomaterials and are more
cost effective economically. This comparison also display
4. CONCLUSIONS
In conclusion, in this study we have reported the prepara-
Table V. MMT-HClO4 catalyzed TMS protection of alcohols in com-
parison with the other methods.
tion of MMT-HClO4, as a new, environmentally friendly
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
and stable solid acid catalyst, and its application in the
Copyright: American Scientific Publishers
chemoselective trimethylsilylation of alcohols and phenols,
Delivered by Ingenta
Catalyst
loading Reaction
Time
(h)
as well as in the deprotection of silyl ethers. The low cost
and availability of the reagents, ease of preparation and
handling of the catalyst, chemoselectivity, high yields of
the products, easy procedure and work-up, short reaction
times and the reusability of the catalyst are noteworthy
advantages of this method and we believe it would be
a useful addition to the available organic methodologies.
Further work to explore this catalyst in other organic trans-
formations is in progress.
Entry
1
Catalyst
LaCl3
(mg)
conditions
Ref.
7
24
CH2Cl2,
r.t.
2.5–5
2
Sulfonic acid-
functionalized
ordered
nanoporous
silica
Trichloroiso-
cyanuric acid
(TCCA)
19
23
CH2Cl2,
r.t.
0.5–24
8
3
CH2Cl2,
r.t.
2.5–12
9
Acknowledgments: We are thankful to the University
of Guilan Research Council for the partial support of this
work.
4
SiO2–HClO4
50
87
15
5
CH3CN,
r.t.
CH3CN,
reflux
CH3CN,
r.t.
0.03–0.16
0.25–2.7
0.16–5.16
0.02–0.92
7–30
10
11
12
16
19
20
21
33
5
Fe(HSO4ꢁ3
6
Nanocrystalline
TiO2
Nanocrystalline
TiO2–HClO4
H-ꢂ zeolite
References and Notes
1. P. J. Kocienski, Protecting Groups, Georg Thieme, New York (2000).
2. K. I. Oyama and T. Kondo, Org. Lett. 5, 209 (2003).
3. L. Lombardo, Tetrahedron Lett. 25, 227 (1984).
4. G. A. Olah, B. G. B. Gupta, S. C. Narang, and R. Malhorta, J. Org.
Chem. 44, 4272 (1979).
7
CH3CN,
r.t.
8
10% Toluene,
w/w
19
r.t
9
SBA-15-Ph-Pr-
SO3H
Montmorillonite
K 10
CH2Cl2,
35 ꢀC
CH2Cl2,
r.t.
CH3CN,
r.t.
0.33–24
0.2–8
5. M. Lalonde and T. Chan, Synthesis 1985, 817 (1985).
6. B. Karimi and B. Golshani, J. Org. Chem. 65, 7228 (2000).
7. A. V. Narsaiah, J. Organomet. Chem. 692, 3614 (2007).
8. D. Zareyeea and B. Karimi, Tetrahedron Lett. 48, 1277 (2007).
9. A. Khazaei, M. A. Zolfigol, A. Rostami, and A. G. Choghamarani,
Catal. Commun. 8, 543 (2007).
10
11
12
100
20
7
SANM
0.08–1
MMT-HClO4
CH3CN,
r.t.
0.03–0.5 The present
method
10. H. R. Shaterian, F. Shahrekipoor, and M. Ghashang, J. Mol. Catal.
A: Chem. 272, 142 (2007).
2106
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019

Mashhadinezhad et al.
Nanoporous Na+-Montmorillonite Perchloric Acid as an Efficient and Recyclable Catalyst
11. F. Shirini, M. A. Zolfigol, and A. R. Abri, Monatsh. Chem. 139, 17
(2008).
12. F. Shirini, M. Alipour Khoshdel, M. Abedini, and S. V. Atghia, Chin.
Chem. Lett. 22, 1211 (2011).
30. A. Corma, Curr. Opin. Solid State Mater. Sci. 2, 63 (1997).
31. H. Vogt, J. Balej, J. E. Bennett, P. Wintzer, S. A. Sheikh, and
P. Gallone, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-
VCH, Weinheim (2002).
13. F. Shirini and M. Mashhadi-Nejad, Phosphorus, Sulfur Silicon Relat.
Elem. 187, 376 (2012).
32. F. Shirini, M. Mamaghani, and S. V. Atghia, Appl. Clay Sci. 58, 67
(2012).
14. F. Shirini, M. A. Zolfigol, and M. Abedini, Monatsh. Chem. 140, 61
(2009).
15. F. Shirini and M. Abedini, J. Iran. Chem. Soc. 5, 87 (2008).
16. F. Shirini, S. V. Atghia, and M. Ghazi Jirdehi, Catal. Commun. 8, 5
(2012).
33. K. Khan and Z. N. Siddiqui, Monatsh. Chem. 146, 2097 (2015).
34. S. K. Bharadwaj, P. K. Boruah, and P. K. Gogoi, Catal. Commun.
57, 124 (2014).
35. B. Maleki, H. Keshvari Shirvan, F. Taimazi, and E. Akbarzadeh, Int.
J. Org. Chem. 2, 93 (2012).
17. F. Shirini and E. Mollarazi, Catal. Commun. 8, 1393 (2007).
18. V. H. Tillu, V. H. Jadhav, H. B. Borate, and R. D. Wakharkar, Arkivoc
14, 83 (2004).
19. D. Zareyee, R. Asghari, and M. A. Khalilzadeh, Chin. J. Catal.
32, 1864 (2011).
20. Z. H. Zhang, T. S. Li, F. Yang, and C. G. Fu, Synth. Commun.
28, 3105 (1998).
21. F. Nikbakht, E. Ghonche Poor, H. Ziyadi, and A. Heydari, RSC Adv.
4, 34428 (2014).
22. A. Ghorbani-Choghamarani and N. Cheraghi-Fathabad, Chin. J.
Catal. 31, 1103 (2010).
23. A. Rostami, F. Ahmad-Jangi, M. R. Zarebin, and J. Akradi, Synth.
Commun. 40, 1500 (2010).
36. J. H. Clark, Acc. Chem. Res. 35, 791 (2002).
37. H. S. Nalwa (ed.), Encyclopedia of Nanoscience and Nanotech-
nology, American Scientific Publishers, Los Angeles, CA (2004),
Vols. 1–10.
38. S. Pavlidou and C. D. Papaspyrides, Prog. Polym. Sci. 33, 1119
(2008).
39. E. P. Giannelis, R. Krishnamoorti, and E. Manias, Adv. Polym. Sci.
138, 107 (1999).
40. S. Yariv and H. Cross, Organo-Clay Complexes and Interactions,
CRC Press, Boca Raton, FL (2002).
41. F. Shirini and M. Abedini, J. Nanosci. Nanotechnol. 13, 4838 (2013).
42. F. Shirini, M. Abedini, and M. Seddighi, J. Nanosci. Nanotechnol.
16, 8208 (2016).
24. B. Karimi and B. Golshani, J. Org. Chem. 65, 7228 (2000).
25. M. Moghadam, S. Tangestaninejad, V. Mirkhani, I. Mohammadpoor-
Baltork, S. Chahardahcheric, and Z. Tavakoli, J. Organomet. Chem.
693, 2041 (2008).
26. M. Yadegari, M. Moghadam, S. Tangestaninejad, V. Mirkhani, and
I. Mohammadpoor-Baltork, Polyhedron 31, 332 (2012).
27. T. K. Santosh and K. S. Soo, J. Organomet. Chem. 694, 2562
(2009).
43. B. Tyagi, Ch. D. Chudasama, and R. V. Jasra, Spectrochim. Acta,
Part A 64, 273 (2006).
44. V. C. Farmer, The layer silicates, The Infrared Spectra of Minerals,
edited by V. C. Farmer, Mineralogical Society of Great Britain and
Ireland, London (1974), Vol. 4, p. 331.
45. N. G. Khaligh, J. Mol. Catal. A: Chem. 363, 90 (2012).
46. A. Yahiaoui, M. Belbachir, and A. Hachemaoui, Int. J. Mol. Sci.
4, 548 (2003).
28. A. Ghorbani-Choghamarani, K. Amani, M. A. Zolfigol, M. Hajjami,
47. H. Xing, T. Wang, Z. Zhou, and Y. Dai, J. Mol. Catal. A: Chem.
264, 53 (2007).
48. R. J. Cremlyn, Chlorosulfonic Acid: A Versatile Reagent, The Royal
Society of Chemistry, Cambridge, UK (2002), Chap. 1, pp. 2–5.
IP: 46.148.120.225 On: Tue, 25 Dec 2018 20:08:06
and R. Ayazi-Nasrabadi, J. Chin. Chem. Soc. 56, 255 (2009).
Copyright: American Scientific Publishers
29. J. S. Yadav, B. V. S. Reddy, A. K. Basak, G. Baishya, and A. Venkat
Delivered by Ingenta
Narsaiah, Synthesis 22, 3831 (2006).
Received: 4 April 2017. Accepted: 3 June 2017.
J. Nanosci. Nanotechnol. 19, 2099–2107, 2019
2107