KF/clinoptilolite NPs: An efficient and heterogeneous catalyst for chemoselective silylation of alcohols and phenols

Article abstract of DOI:10.1080/10426507.2021.1904926

Potassium fluoride incorporated on clinoptilolite nanoparticles (KF/CP NPs) by ion exchanging is found to be an effective and inexpensive heterogeneous nanocatalyst for chemoselective silylation of alcohols and phenols with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) at room temperature. Nano-powder of clinoptilolite (CP) was prepared using a planetary ball mill mechanically method and characterized by dynamic light scattering (DLS), X-ray powder diffraction (XRD) and scanning electron microscope (SEM) analyses. Almost all of products were obtained in high yields as well as short reaction times and the catalyst was also reused eight times without loss of its catalytic activity.

Full text of DOI:10.1080/10426507.2021.1904926

Phosphorus, Sulfur, and Silicon and the Related Elements
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KF/clinoptilolite NPs: An efficient and
heterogeneous catalyst for chemoselective
silylation of alcohols and phenols
Razieh Oladee, Daryoush Zareyee & Mohammad A. Khalilzadeh
To cite this article: Razieh Oladee, Daryoush Zareyee & Mohammad A. Khalilzadeh (2021):
KF/clinoptilolite NPs: An efficient and heterogeneous catalyst for chemoselective silylation
of alcohols and phenols, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2021.1904926
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2021.1904926
KF/clinoptilolite NPs: An efficient and heterogeneous catalyst for chemoselective
silylation of alcohols and phenols
Razieh Oladee, Daryoush Zareyee, and Mohammad A. Khalilzadeh
Department of Chemistry, Islamic Azad University, Qaemshahr, Iran
ABSTRACT
ARTICLE HISTORY
Received 28 December 2020
Accepted 14 March 2021
Potassium fluoride incorporated on clinoptilolite nanoparticles (KF/CP NPs) by ion exchanging is
found to be an effective and inexpensive heterogeneous nanocatalyst for chemoselective silylation
of alcohols and phenols with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) at room temperature. Nano-
powder of clinoptilolite (CP) was prepared using a planetary ball mill mechanically method and
characterized by dynamic light scattering (DLS), X-ray powder diffraction (XRD) and scanning elec-
tron microscope (SEM) analyses. Almost all of products were obtained in high yields as well as
short reaction times and the catalyst was also reused eight times without loss of its cata-
lytic activity.
KEYWORDS
Clinoptilolite; KF/CP;
silylation; HMDS;
chemoselective;
alcohols; phenols
GRAPHICAL ABSTRACT
CONTACT Daryoush Zareyee
zareyee@gmail.com; Mohammad A. Khalilzadeh
mkhalil3@ncsu.edu
Department of Chemistry, Islamic Azad University,
Qaemshahr, Iran.
ß 2021 Taylor & Francis Group, LLC

2
R. OLADEE ET AL.
Introduction
Several methods, such as tetrahydropyranylation, acetylation,
methoxymethylation and trimethylsilylation have been used
for the protection of hydroxyl groups.[1] Silylation of alco-
hols and phenols is one of the most commonly and straight-
forward used methods for the protection of hydroxyl
function in organic chemistry.[2] From different silylating
regents, HMDS is a stable, easily handled, commercially avail-
able and inexpensive reagent for silylation of hydroxyl group
and the subsequent workup is not time-consuming because
the byproduct of reaction is ammonia, which is easy to
remove from the reaction medium. The main drawback of
HMDS is its poor silylation power as it needs forceful condi-
tions and long reaction times in many instances.[3] Therefore,
there are various catalysts for activating of HMDS including
ZrCl4,[4] LiClO4,[5] sulfamic acid,[6] Pd,[7] LaCl3, HReO4,[9]
[8]
iodine,[10,11] zirconium sulfophenyl phosphonate,[12] sulfonic
acid-functionalized nanoporous silica,[13] CuSO4.5H2O,[14]
V(IV) porphyrin,[15] (CH3)3SiCl,[16,17] [SnIV(TPP)(OTf)2],[18]
DCH or TCM,[19] CMK-5-SO3H,[20] Trichloroisocyanuric
acid (TCCA),[21] HY Zeolite,[22] H3PW12O40,[23] poly(4-vinyl-
pyridine/styrene) copolymer-supported ytterbium(III) tri-
flate[24] and [TiIV(salophen)(OTf)2].[25] Altough the silylation
ability of HMDS has been improved by these methods, how-
ever, there are also some problems such as the use of non-
recoverable catalyst[4–12,14,16,17,19,21,23] and tedious or time-
consuming work-up.[5,7–10] Moreover, some of these methods
are not chemoselective.[4–12,14,16,17,19,21] Thus, there is consid-
erable scope for development of more applicable heteroge-
neous catalysts.
In heterogeneous catalysis reaction, the catalysts and
reactants exist in different phases which greatly help in
reactant, product, and catalyst separation at the end of the
reaction.[26] During our ongoing efforts to search for envir-
onmentally friendly and reusable nanocatalysts,[27,28] we
have reported KF/CP NPs as a heterogeneous inexpensive
solid catalyst for some organic transformations.[29–31]
Impregnation of potassium fluoride on clinoptilolite as a
natural zeolite with high cation exchange capability particu-
larly for Kþ, is very simple without the need for any pre-
activation.[32,33]
Scheme 1. KF/CP NPs catalyzed silylation of hydroxyl group with HMDS.
Armed with these experiences, in the current study, with
the aim of developing a more convenient and simple
approach toward the construction of silylethers, we wish to
report the use of heterogeneous KF/CP NPs as recyclable
catalyst in the silylation of alcohols and phenols (Scheme 1).
Figure 1. Dynamic light scattering for CP NPs.
Results and discussion
The main reason for selection of KF/CP NPs as catalyst in
silylation of alcohols and phenols with HMDS is because of
abundance, very low-cost and safety of naturally occurring
zeolite CP.[34,35]
Dynamic light scattering technique (Figure 1) verified the
40 nm particle size for CP NPs.
Figure 2 shows the XRD pattern of CP NPs with charac-
teristic peaks in 2§ values of 9.88ꢀ, 11.30ꢀ, 13.54ꢀ, 17.52ꢀ,
19.70ꢀ, 21.47ꢀ, 22.38ꢀ, 25.76ꢀ, 26.72ꢀ, and 30.03ꢀ. The
Figure 2. X-ray diffraction pattern of CP NPs.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
Figure 3. SEM images of CP NPs (a), KF/CP NPs (b).
Table 1. Optimization of amounts of KF/CP NPs and solvents in the silylation of benzyl alcohol.
Entry
Catalyst (%mol)
Solvent
HMDS (mmol)
Time (min)
Yield (%)a,b
1
2
3
20
20
20
Toluene
(Et)2O
H2O
1
1
1
15
15
15
55
68
Trace
4
5
6
7
8
9
10
11
12
13
20
20
20
–
10
30
20
CH2Cl2
CH3CN
DMSO
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
1
1
1
1
1
0.6
1
1
15
15
15
360
120
15
92
65
30
Trace
75
95
92
Trace
15
CP (0.1 g)
KF (20)
KF/CP (20c)
360
360
240
20
55
1
aReaction conditions: benzyl alcohols (1.0 mmol) with HMDS at room temperature.
bIsolated yields.
cBulk KF/CP was used instead of KF/CP NPs.
observed reflection peaks are in accordance with the clinop- with 1 mmol of HMDS (Table 1, entry 4). It is also import-
tilolite crystalline structure data in the library of the instru- ant to mention that when the reaction was performed in the
ment [JCPDS No. 39-1383] and also in the literature.[36] presence of CP or KF, a trace and 20% of the desired prod-
Good agreement between the results shows that the original uct was obtained after 3 h (Table 1, entries 11, 12). The effi-
natural zeolite includes clinoptilolite structure as ciency of KF/CP NPs is obvious when it is compared to
major component.
bulk KF/CP with lower yields and longer reaction time
(Table 1, entry 13). Thus, the best results were obtained in
the presence of 20 mol% of KF/CP NPs in CH2Cl2 at ambi-
ent temperature.
Crystal size distribution and morphology of the KF/CP
NPs were studied by SEM and X-ray diffraction. The SEM
images shown the polydisperse morphology and a wide
range of particle size distributions for CP NPs (Figure 3a)
The general efficiency of this reaction is evident from the
and KF/CP NPs (Figure 3b). These features are influenced reaction of a variety of alcohols and phenols containing
by the milling process and are thus subject to variation.
At the start of our investigations, we selected the reaction
of benzyl alcohol (1.0 mmol) with HMDS (1.0 mmol) as a
model to study the suitability of KF/CP NPs (20 mol%) as a
catalyst in different solvents (Table 1). We found that
CH2Cl2 is the best solvent for the efficient and rapid silyla-
tion of alcohols (Table 1, entry 4). Next, we screened
amounts of catalyst. It should be noted that the reaction did
not yield any product in the absence of KF/CP NPs (Table
both electron-withdrawing and electron-donating groups
which react in excellent yields within a short reaction time
(Table 2). Noteworthy, when employing aliphatic alcohols,
the reaction occurred with a lower yield than aromatic alco-
hols (Table 2, 2j–2m). The results show that the present
methodology uses an inexpensive and environmentally
friendly heterogeneous catalyst and requires relatively short
reaction times.
For the study of chemoselectivity of this method, in a
1, entry 7). The amounts of KF/CP NPs (10 mol%) gave in competitive experiment, an equimolar mixture of benzyl
75% yield after 2 h (Table 1, entry 8), while increasing the alcohol and phenol was allowed to react with HMDS in the
KF/CP NPs to 30 mol% did not notably change the yield of presence of KF/CP NPs under the same reaction conditions
the reaction (Table 1, entry 9). Lower amount of HMDS (Scheme 2a). Interestingly, benzyl alcohol was converted to
(0.6 mmol) was also used in the same reaction (Table 1, the corresponding silylether in 90% yield, while only 8% of
entry 10) and the highest yield of TMS ether was produced the corresponding silylether was observed for the phenol.

4
R. OLADEE ET AL.
Table 2. Silylation of alcohols with HMDS using KF/CP NPs in CH2Cl2 at room temperature.a,b
aReaction conditions: alcohols (1.0 mmol), HMDS (1.0 mmol), KF/CP NPs (20 mol%) in CH2Cl2 (5.0 mL) at room temperature.
bIsolated yields.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
5
Table 3. Comparison the efficiency of KF/CP NPs with recently reported cata-
lysts in trimethylsilylation of benzyl alcohol.
Entry
Catalyst (mol %)
LaCl3 (10)
Sulfonic acid@nanoporous silica (3) r.t.
T (ꢀC) Time (min) Yield (%) [ref]
1
2
3
4
5
6
r.t.
180
55
91[8] a,b
99[13] b
90[21] a,b
80[22] c
90[23] c
92d
Trichloroisocyanuric acid (TCCA) (6) r.t.
240
120
23
HY Zeolite (0.1 g)
H3PW12O40 (1)
KF/CP NPs (20)
55-60
55-60
r.t.
15
aNon-recoverable catalyst.
bLong reaction time.
cHigh temperature.
dThis work.
Figure 4. Recycling experiment of KF/CP NPs for silylation of benzyl alcohol in
CH2Cl2 at room temperature.
Scheme 2. Chemoselective silylation of a) benzyl alcohol in the presence of phenol and b) benzyl alcohol as primary alcohol in the presence of secondary alcohol.
Additionally, a primary benzylic alcohol has chemoselec- Trichloroisocyanuric acid (TCCA)[21] need long reaction
times (Table 3, entries 1-3). Furthemore, HY Zeolite[22] and
tively converted to the corresponding silylether in the pres-
ence of a secondary benzylic alcohol (Scheme 2b).
[23]
H3PW12O40
4, 5).
needs high temperature (Table 3, entries
The possibility of the catalyst reusability was investigated
through a series of sequential synthesis of trimethylsily-
lethers from benzyl alcohol (Figure 4). After completion of
each run of reaction, CH2Cl2 was added and the reaction
mixture was filtered and washed with CH2Cl2. The recov-
ered catalyst then was used in the eight successive runs
without much appreciable loss in its catalytic activity.
Conclusion
Based on the results, heterogeneous KF/CP NPs was a highly
efficient and recoverable catalyst for the chemoselective sily-
lation of alcohols and phenols by HMDS in dichlorome-
thane at room temperature. The reaction can be carried out
under mild conditions with high yields, short reaction times
and easy work-up for the synthesis of the TMS ethers.
Besides, the catalyst could be readily separated without sig-
nificant loss of catalytic activity after eight runs. Moreover,
In order to show the effectiveness of the presented
method in the silylation reactions, we compared the
obtained results in the trimethylsilylation of benzyl alcohol
with HMDS catalyzed by KF/CP NPs with those reported in
the literature (Table 3). Some of catalysts are non-recover-
able (Table 3, entries 1, 3, 5), and some of them such as
LaCl3,[8]
Sulfonic
acid@nanoporous
silica,[13]
and this catalyst is commercially available, recyclable and

6
R. OLADEE ET AL.
[2] Green, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 2nd ed. Wiley: New York, 1999.
[3] Bruynes, C. A.; Jurriens, T. K. Catalysts for Silylations with
1,1,1,3,3,3-Hexamethyldisilazane. J. Org. Chem. 1982, 47,
3966–3969. DOI: 10.1021/jo00141a031.
inexpensive. Further applications of this catalyst to other
synthetically useful transformations are currently being
investigated.
[4] Shirini, F.; Mollarazi, E. Efficient Trimethylsilylation of
Alcohols and Phenols in the Presence of ZrCl4 as a Reusable
Catalyst. Catal. Commun. 2007, 8, 1393–1396. DOI: 10.1016/j.
catcom.2006.11.015.
[5] Saidi, M. R.; Azizi, N. Novel and Efficient Method for the
Silylation of Hydroxyl Groups with Hexamethyldisilazane
(HMDS) under Solvent-Free and Neutral Conditions.
Organometallics 2004, 23, 1457–1458. DOI: 10.1021/
om0341505.
[6] Rostami, A.; Ahmad- Jangi, F.; Zarebin, M. R.; Akradi, J. Green
and Efficient Procedure for the Trimethylsilylation of Hydroxy
Groups and Their Regeneration Using Sulfamic Acid as
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DOI: 10.1080/00397910903097344.
[7] Shirakawa, E.; Hironaka, K.; Otsuka, H.; Hayashi, T. Palladium-
Catalyzed Silylation of Alcohols with Hexamethyldisilane.
Chem. Commun. 2006, 37, 3927–3929. DOI: 10.1039/B608958E.
[8] Narsaiah, A. V. Lanthanum Trichloride: An Efficient Catalyst
for the Silylation of Hydroxyl Groups by Activating
Hexamethyldisilazane (HMDS). J. Organomet. Chem. 2007, 692,
3614–3618. DOI: 10.1016/j.jorganchem.2007.05.002.
[9] Reis, P. M.; Royo, B. Perrhenic Acid as Catalyst for
Hydrosilylation of Aldehydes and Ketones and Dehydrogenative
Silylation of Alcohols. Catal. Commun. 2007, 8, 1057–1059.
DOI: 10.1016/j.catcom.2006.10.023.
[10] Karimi, B.; Golshani, B. Mild and Highly Efficient Method for
the Silylation of Alcohols Using Hexamethyldisilazane
Catalyzed by Iodine under Nearly Neutral Reaction Conditions.
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[11] Jereb, M. Highly Atom Economical Uncatalysed and I2-
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Using HMDS under Solvent-Free Reaction Conditions (SFRC).
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040.
Experimental
The natural clinoptilolite (CP) purchased from Afrand
Touska Company. All analytical grade reagents were pre-
pared from Fluka or Sigma-Aldrich. A silica gel (60 mesh)
was used for product purification. The morphology of KF/
CP NPs was studied using a Philips XL30 scanning electron
microscope (SEM). In order to study the crystallinity of KF/
CP NPs, XRD patterns of the micron sized parent sample
were recorded by X-ray diffraction (XRD) analysis at room
temperature using a Holland Philips Xpert X-ray powder
diffractometer, with CuKa radiation (k ¼ 0.15406 nm).
Preparation of KF/CP NPs
Nano-sized natural clinoptilolite zeolite was generated by
grinding in a planetary ball mill using a zirconia vial set in
dry conditions with a time period of about 20 min. KF/CP
NPs was easily prepared by commercially available starting
materials according to previously reported method[37] by
dissolving KF (1.0 g) in distilled water (10 mL) and nano cli-
noptilolite (9.0 g). The mixture was stirred for 1.0 h and
then water was removed in a rotary evaporator under
reduced pressure at 60–70 ꢀC. The impregnated nano clinop-
tilolite was further dried at 70–80 ꢀC in a vacuum drying
oven for 30 h. The material was powdered with the help of
pestle and mortar. The size of prepared KF/CP NPs was
reported to be 41 nm by X-ray diffraction analysis.
[12] Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O.;
Costantino, U. Heterogeneous Catalysis in Trimethylsilylation
of Alcohols and Phenols by Zirconium Sulfophenyl
Phosphonate. Synth. Commun. 1999, 29, 541–546. DOI: 10.
1080/00397919908085797.
General procedure for trimethylsilylation of alcohols
with HMDS
[13] Zareyee, D.; Karimi, B. A Novel and Highly Efficient Method
for the Silylation of Alcohols with Hexamethyldisilazane
(HMDS) Catalyzed by Recyclable Sulfonic Acid-Functionalized
Ordered Nanoporous Silica. Tetrahedron Lett. 2007, 48,
1277–1280. DOI: 10.1016/j.tetlet.2006.12.030.
To a mixture of alcohols or phenol (1.0 mmol) and HMDS
(1.0 mmol) in CH2Cl2 (5 mL), catalyst KF/CP NPs (0.12 g,
20 mol%) was added. The mixture was stirred at room tem-
perature for the specified time (Table 1). The progress of
the reaction was monitored by GC or TLC. The reaction
mixture was filtered and washed with hexane to recover the
catalyst. The resulting solution was placed in a separating
funnel and washed with water and hexane. The organic layer
was dried with sodium bicarbonate, filtered, and concen-
trated in vacuo. The product was purified by column chro-
matography on silica gel eluting with ethylacetate-
hexane mixtures.
[14] Akhlaghinia, B.; Tavakoli, S. An Efficient Method for the
Protection
of
Alcohols
and
Phenols
by
Using
Hexamethyldisilazane in the Presence of Cupric Sulfate
Pentahydrate under Neutral Reaction Conditions. Synthesis
2005, 2005, 1775–1778. DOI: 10.1055/s-2005-865318.
[15] Moghadam, M.; Mohammadpoor-Baltork, I.; Tangestaninejad,
S.; Mirkhani, V.; Khosropour, A. R.; Taghavi, S. A. Electron-
Deficient Vanadium (IV) Tetraphenylporphyrin: A New, Highly
Efficient and Reusable Catalyst for Chemoselective
Trimethylsilylation
of
Alcohols
and
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Hexamethyldisilazane. Appl. Org. Chem. 2011, 25, 687–694.
DOI: 10.1002/aoc.1828.
Acknowledgements
[16] Langer, S. H.; Connell, S.; Wender, J. Preparation and
Properties of Trimethylsilyl Ethers and Related Compounds. J.
Org. Chem. 1958, 23, 50–58. DOI: 10.1021/jo01095a017.
[17] Gautret, P.; El-Ghammarti, S.; Legrand, A.; Couturier, D.; Rigo,
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The authors acknowledge the Islamic Azad University of Qaemshahr
Research Councils for support of this work.
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