Chlorozincate(II) acidic ionic liquid: Efficient and biodegradable silylation catalyst

Article abstract of DOI:10.1002/aoc.3790

A practical and highly efficient silylation of alcohol and phenol derivatives with hexamethyldisilazane (HMDS) using acidic ionic liquids under mild reaction conditions is described. A series of Br?nsted as well as Br?nsted–Lewis acidic ionic liquids were prepared and their performance investigated for the silylation of a wide variety of alcohols and phenols with HMDS. Imidazole- as well as N-methyl-2-pyrrolidone-based acidic ionic liquids have a higher catalytic activity for the protection of sensitive, hindered alcohols and phenols, thus providing an environmentally begin and versatile alternative to current acid catalysts. In addition, the acidic ionic liquids are reusable, being recovered easily and reused several times without significant deterioration in catalytic activity.

Full text of DOI:10.1002/aoc.3790

Received: 10 January 2017
DOI: 10.1002/aoc.3790
Revised: 6 February 2017
Accepted: 12 February 2017
F U L L PA P E R
Chlorozincate(II) acidic ionic liquid: Efficient and biodegradable
silylation catalyst
Faezeh Abbasi1 | Najmedin Azizi2
| Masumeh Abdoli‐Senejani1
1 Department of Chemistry, Islamic Azad
University‐Arak Branch, PO Box 38135‐
567, Arak, Iran
2 Chemistry and Chemical Engineering
Research Center of Iran, PO Box 14335‐186,
Tehran, Iran
A practical and highly efficient silylation of alcohol and phenol derivatives with
hexamethyldisilazane (HMDS) using acidic ionic liquids under mild reaction condi-
tions is described. A series of Brønsted as well as Brønsted–Lewis acidic ionic liq-
uids were prepared and their performance investigated for the silylation of a wide
variety of alcohols and phenols with HMDS. Imidazole‐ as well as N‐methyl‐2‐pyr-
rolidone‐based acidic ionic liquids have a higher catalytic activity for the protection
of sensitive, hindered alcohols and phenols, thus providing an environmentally
begin and versatile alternative to current acid catalysts. In addition, the acidic ionic
liquids are reusable, being recovered easily and reused several times without signif-
icant deterioration in catalytic activity.
Correspondence
Najmedin Azizi, Chemistry and Chemical
Engineering Research Center of Iran, PO
Box 14335‐186, Tehran, Iran.
Email: azizi@ccerci.ac.ir
KEYWORDS
alcohol, Brønsted–Lewis acidic ionic liquids, hexamethyldisilazane, HMDS, phenol, silylation
1 | INTRODUCTION
hexamethyldisiloxane or allylsilane in the presence of a
nucleophilic catalyst and a stoichiometric amount of a base,
The protection of hetero atoms containing active hydrogens
as well as enolizable carbonyl compounds by silylation is
an important tool in analytical chemistry for gas chromatog-
raphy and mass spectrometry and in synthetic chemistry.[1]
An important protector of the hydroxyl group in multi‐step
synthesis of multifunctional biologically active molecules
and structurally complicated natural products that has gained
recent popularity in the literature is the trimethylsilyl
group.[2] The wide stability range of the various trialkylsilyl
groups under a wide range of reaction conditions as well as
commercial availability of a variety of trimethyl silylating
reagents of different reactivities has become more and more
important for these protecting groups in the last twenty
years.[3] Among the many trialkylsilyl reagents used for the
preparation of silyl ethers, the trimethylsilyl group is the most
popular and versatile. Furthermore, using more sterically
hindered trialkylsilyl groups with different stabilities under
cleavage condition in some processes is necessary.[4] The
most common route for the formation of a silicon–oxygen
bond involves the reaction of a hydroxyl group with a
functionalized silane such as chlorotrimethylsilane,
often a pyridine derivative or tertiary amine.[5] The most
important silazane derivative is hexamethyldisilazane
(HMDS), frequently used for the trimethylsilylation of hydro-
gen‐labile substrates, which is inexpensive, commercially
available and selective.[6–8] Its handling does not require
special precautions and work‐up is not time‐consuming,
because the by‐product of the reaction is ammonia which is
simple to remove from the reaction medium driving the reac-
tion to completion.[9–16] HMDS is a very poor silylating
agent, and that is a main drawback for its application, so there
are a variety of catalysts that have been reported for the acti-
vation of HMDS.[17] However, these methods mainly relate
to protection at elevated temperature, with longer reaction
time and using harmful organic solvents. In view of green
chemistry, there is a global effort to replace conventional
acidic catalysts by eco‐friendly catalysts.
Green solvents have advanced rapidly in recent decades
due to the hazards of many conventional organic solvents,
resulting in a very large reduction in environmental pollution,
and to counteract global warming.[18–20] Among the alterna-
tive green solvents, ionic liquids (ILs) or molten salts with
Appl Organometal Chem. 2017;e3790.
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https://doi.org/10.1002/aoc.3790

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ABBASI ET AL.
a melting point below 100 °C represent a significant propor-
tion of research due to their unique properties in green chem-
istry.[21–29] Furthermore, the characteristics of ILs can be
controlled by the introduction of functional groups to make
task‐specific ILs that opens many new windows for their
application. In this context, introduction of acidic functional
groups into cations or anions of ILs has gained special atten-
tion as catalysts and solvents and may offer a convenient
solution to both the solvent emission and catalytic recycling
problems. Furthermore, acidic ILs possess the advantages
of both liquid acid and solid acid catalysts, such as water sol-
ubility, operationally simplicity, high reactivity, easy separa-
tion and reusability.[30–33]
Initially, we examined the influence of the acidic ILs as
catalysts and reaction media for the reaction of benzyl alco-
hol (1.0 mmol) and HMDS (1.2 mmol) at room temperature.
Selected results from our screening experiments are listed in
Table 1. The protection in the absence of acidic ILs afford the
trimethylsilyl ether in 42% yield after 100 min. Interestingly,
it was found that 1 mol% of all the acidic ILs (IL1–IL11) was
effective for obtaining good to excellent yields within 1 h
(Table 1).
However, Brønsted–Lewis acidic ILs (IL2 and IL10)
were found to be superior to the others (Table 1, entries 2
and 10), the former producing the desired trimethylsilyl ether
in quantitative yields with shorter reaction times.
With the optimal reaction conditions in hand (using
IL10), we extended our studies to the protection of alcohol
substrates to evaluate the generality as well as the effective-
ness of this newly developed protocol. The results are pre-
sented in Table 2. It is apparent that the protection of
various functionalized alcohols proceeded smoothly with
HMDS to afford the corresponding trimethylsilyl ether in
good to excellent yields. A wide range of benzyl alcohol
derivatives having both electron‐donating groups (2‐CH3,
4‐CH3O, 3‐CH3O, 4‐(CH3)3C) and electron‐withdrawing
groups (4‐Br, 4‐Cl, 3‐NO2) were reacted with HMDS using
identical reaction conditions in short reaction times. The
silylation reaction was extended to various secondary and ter-
tiary alcohols (Table 2). Acid‐sensitive and hindered alcohols
such as diarylalkyl carbinols, 4,4‐dichlorobenzhydrol,
benzhydrol and heterocyclic furfuryl alcohol react smoothly
with HMDS, giving good to excellent yields of their
2 | RESULTS AND DISCUSSION
As part of our continuing interest in developing green
solvents for useful organic transformations,[34–37] herein we
report a very effective and green procedure for the silylation
of a wide variety of OH‐containing substrates in the presence
of acidic ILs.
First, various kinds of acidic ILs (Figure 1) based
on N‐methyl‐2‐pyrrolidone (NMP), 1‐methylimidazole,
tetramethylethylenediamine,
triethylamine
and
triphenylphosphine were synthesized according to well‐
known procedures described in the literature.[38–42] With
the acidic ILs in hand, in order to evaluate their catalytic
activity, the reaction of benzyl alcohol and HMDS was
selected as a model reaction.
TABLE 1 Optimization of reaction conditions
Acidic ionic
Entry
liquid
Time (min)
Yield (%)a
1
IL1
IL2
IL3
IL4
IL5
IL6
IL7
IL8
IL9
IL10
IL11
—
60
60
45
20
60
60
60
60
45
25
60
100
68
97
78
95
72
74
70
54
95
100
68
42
2
3
4
5
6
7
8
9
10
11
12
FIGURE 1 Prepared acidic ionic liquids
aGC yield.

ABBASI ET AL.
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TABLE 2 Silylation of alcohols and phenols with HMDS using acidic ionic liquid
Entry
ROH
Time (min)
Temp. (°C)
Yield (%)a
1
25
r.t.
100
2
3
40
60
r.t.
r.t.
84
84
4
5
6
7
8
80
80
80
80
70
r.t.
r.t.
60
r.t.
r.t.
80
82
76
78
80
9
80
r.t.
74
10
11
12
60
60
70
r.t.
60
60
95
95
70
13
14
80
80
60
60
97
95
(Continues)

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ABBASI ET AL.
TABLE 2 (Continued)
Entry
ROH
Time (min)
Temp. (°C)
Yield (%)a
15
80
60
95
16
80
60
72
17
18
80
80
60
60
70
97
19
20
80
80
r.t.
r.t.
76
70
21
22
40
80
r.t.
r.t.
84
68
aGC yield.
corresponding trimethylsilyl ethers. The scope of this protec-
tion reaction was examined with a wide variety of phenols
(mono‐ and bicyclic) bearing a wide range of substituents
and with various electronic as well as steric environments.
As is evident from Table 2, the reaction was quite general
and gave protected phenols in excellent yields at higher tem-
perature. Furthermore, phenols that contained a strongly
deactivating group such as nitro were silylated by the acidic
IL in good yields.
The mechanism of these transformations and the actual
role of IL are not yet clear. A tentative catalytic cycle involv-
ing the activation of HMDS with the acidic IL is proposed,
which has been previously formulated for acid catalysts
(Figure 2). The acid–base interaction of HMDS and with
the acid sites of the IL should enhance the electrophilic char-
acter of Si to produce reactive silylating agent 1. Alcohols
react with activated HMDS to produce O‐silyl ether with
the liberation of ammonia (Figure 2).
The recyclability of acidic ILs is important which helps
address both economic and environmental concerns at an
FIGURE 2 Proposed mechanism of silylation of hydroxyl groups in
the presence of acidic ionic liquid

ABBASI ET AL.
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industrial scale. To demonstrate this issue, the recycling effi-
ciency of the acidic ILs was investigated for the reaction
between benzyl alcohol (10 mmol), HMDS (12 mmol) and
IL10 (10 mol%) under the optimized conditions (Table 2,
entry 1). The spent acidic catalyst was water soluble and
was recovered by simple extraction of products from aqueous
solutions containing acidic catalyst. The results shown in
Figure 3 reveal that the ionic catalyst can be recycled at least
four times without significant loss of its activity.
3.3 | General procedure
A test tube, equipped with a magnetic stir bar, was charged
with alcohols or phenols (1.0 mmol), HMDS (1.2 mmol)
and IL10 (1.0 mol%). The resulting mixture was stirred at
room temperature for alcohols and 60 °C for phenols until
the reaction was complete (60–90 min). The crude reaction
mixture was then diluted with ethyl acetate (5 ml) to remove
the catalyst. The organic solvent was evaporated under vac-
uum. In some cases, the crude products were purified by
flash column chromatography using silica gel or recrystalli-
zation from ethanol or diethyl ether to afford pure products.
All products are known compounds and the characterizations
of these compounds were identical to literature reports.
3 | EXPERIMENTAL
3.1 | General
Melting points were determined with a Buchi B‐545 melting
point apparatus and are uncorrected. Infrared spectra were
obtained with a Bruker Vector‐22 spectrometer in KBr pel-
4 | CONCLUSIONS
A biodegradable reaction protocol has been established for
efficient silylation of hydroxyl groups with HMDS using an
acidic IL under mild conditions. The use of acidic IL for
the protection of sensitive, hindered alcohols and phenols
provides a simple alternative to toxic and precious catalysts.
The acidic IL catalyst could be recycled for at least five con-
secutive runs without any loss in activity.
1
lets with band positions reported in cm−1. H NMR spectra
were recorded at 500 MHz with a Bruker Avance‐500 spec-
trometer in CDCl3/CCl4 or CDCl3/DMSO‐d6 as solvent with
chemical shifts given in ppm relative to tetramethylsilane as
an internal standard. All chemicals such as HMDS, alcohols,
phenols and acidic IL components were obtained from com-
mercial suppliers and used without further purification.
Water and other solvents were distilled before use. All the
reactions were monitored by GC or TLC with UV light as
detecting agent.
ACKNOWLEDGEMENT
Financial support of this work by the Chemistry and Chemi-
cal Engineering Research Center of Iran is gratefully
acknowledged.
3.2 | Acidic IL preparations
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How to cite this article: Abbasi F, Azizi N, Abdoli‐
Senejani M. Chlorozincate(II) acidic ionic liquid:
Efficient and biodegradable silylation catalyst. Appl
Organometal Chem. 2017;e3790. https://doi.org/
10.1002/aoc.3790
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