PEG-DIL-based MnCl42?: A novel phase transfer catalyst for nucleophilic substitution reactions of benzyl halides

Article abstract of DOI:10.1002/aoc.4647

Poly(ethylene glycol) dicationic ionic liquid-based MnCl₄^2? was prepared by nucleophilic substitution of poly(ethylene glycol) dichloride with methylimidazole followed by reaction with MnCl₂. The structural properties of the catalyst were systematically investigated using Fourier transform infrared, UV–visible and Raman spectra and thermogravimetric analysis. The application of this catalyst allows the synthesis of a variety of benzyl thiocyanates and azides in high yield under reflux conditions in water. The main advantages of this method are its easy nature, rapidity, environmental benignity and high yields.

Full text of DOI:10.1002/aoc.4647

Received: 6 July 2018
DOI: 10.1002/aoc.4647
Revised: 11 September 2018
Accepted: 12 September 2018
F U L L P A P E R
2−
PEG‐DIL‐based MnCl4 : A novel phase transfer catalyst for
nucleophilic substitution reactions of benzyl halides
Bijan Mombeni Goodajdar1
| Farideh Akbari1 | Jamal Davarpanah2
1 Department of Chemistry, Islamic Azad
University Omidiyeh Branch, Omidiyeh,
Iran
2 Chemistry Department, Production
Technology Research Institute – ACECR,
Ahvaz, Iran
2−
Poly(ethylene glycol) dicationic ionic liquid‐based MnCl4 was prepared by
nucleophilic substitution of poly(ethylene glycol) dichloride with
methylimidazole followed by reaction with MnCl2. The structural properties
of the catalyst were systematically investigated using Fourier transform
infrared, UV–visible and Raman spectra and thermogravimetric analysis. The
application of this catalyst allows the synthesis of a variety of benzyl thiocya-
nates and azides in high yield under reflux conditions in water. The main
advantages of this method are its easy nature, rapidity, environmental benig-
nity and high yields.
Correspondence
Bijan Mombeni Goodajdar, Department of
Chemistry, Islamic Azad University
Omidiyeh Branch, Omidiyeh, Iran.
Email: bmombini@gmail.com; mombeni.
b@iauo.ac.ir
KEYWORDS
alkyl azide, benzyl thiocyanate, dicationic ionic liquid, green solvent, phase transfer catalyst
1 | INTRODUCTION
along with helping in the transfer of ion pairs across
interfaces, also help in enhancing the reaction rate. This
Benzyl thiocyanates are important precursors in the syn-
thesis of drugs and other organic compounds. Thiocya-
nates have attracted a lot of attention, and are an
important class of compounds in some anticancer natural
products that are formed through deglycosylation of
glucosinolates from cruciferous vegetables. Alkyl thiocya-
nates are used in the preparation of antiasthmatics and
insecticides, and as precursors in the synthesis of hetero-
cycles containing sulfur.[1–7] Moreover, alkyl azides are
useful organic intermediates for the synthesis of various
heterocyclic compounds containing nitrogen which are
important as flexible intermediates for biological, dye,
pharmaceutical and material science.[8,9] Therefore, the
development of a novel catalytic system with environ-
mental benignity in order to synthesize these compounds
is an important task for chemists
mechanistic approach was first proposed by Starks way
back in 1971.[10] This process is called phase transfer
catalysis. It is a green route for the synthesis of fine
chemicals because of the advantages it has over other
methods. For PTCs, mild reaction conditions are
required, the yield of the desired products is very high,
unwanted side reactions are eliminated and there is no
generation of hazardous waste after completion of the
reaction. Today, it seems to be the most important syn-
thetic method in various fields of organic chemistry, as
well as having wide‐ranging industrial applications.
Many works on the use of PTCs have been reported.[11–14]
Selection of a solvent is one of the most important
factors for the successful performance of a reaction. The
solvent can affect the rate, efficiency, selectivity and dis-
tribution of products in the reaction. Problems arising
from solvent separation, complete recovery and environ-
mental effects have drawn the attention of researchers
to the use of green solvents. Water is introduced as the
best alternative for volatile and toxic solvents.[15,16] But
the low solubility of many organic compounds, high
vapour pressure and degradation of some products have
Multiphase reactions are always difficult to conduct
owing to the immiscibility of the phases (basically aque-
ous and organic phases). However, with the help of an
amphiphilic agent, which is soluble in both aqueous
and organic phases, such reactions are now possible.
These agents are phase transfer catalysts (PTCs) that,
Appl Organometal Chem. 2018;e4647.
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https://doi.org/10.1002/aoc.4647

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GOODAJDAR ET AL.
limited the use of water as a solvent in organic reactions.
Dicationic ionic liquids (DILs) contain two symmetric or
asymmetric cationic head groups that are connected to
each other by a flexible or rigid chain. Recently, DILs
have attracted more attention due to their unique and
interesting advantages over traditional ionic liquids, with
their attractive properties, such as higher chemical and
thermal stability, superior heat capacity, excellent struc-
tural tunability, etc. Due to the tunability of DILs, they
are suitable for use in many engineering and scientific
applications such as separation processes, organic synthe-
sis at high temperature, dye‐sensitized solar cells, station-
ary chromatography phases and novel high‐temperature
lubricants.[17–20]
Some ionic liquids that are commonly used have
poly(ethylene glycol) (PEG) chains introduced into cat-
ionic units and are an attractive group of solvents or cat-
alysts that have applications involving various processes
such as extraction, carbohydrate dissolution, gas separa-
tion, organic synthesis and catalysis. Recent studies have
shown that liquids incorporating PEG chains have lower
viscosity and toxicity than their aliphatic‐substituted
counterparts.[21]
SCHEME 1 Synthesis of PEG‐DIL‐MnCl4
Considering the factors mentioned above, we
attempted to prepare PEG‐based DILs (PEG‐DIL) and
used them in organic synthesis as PTCs.
In our ongoing work, and in continuation of our pre-
vious studies in the field of new polymeric dicationic
ionic catalysts, here we report the successful development
of a system for the catalysis of nucleophilic substitution
reactions of aryl halides in water.[22] To the best of our
knowledge, this is the first report on the use of PEG‐
2−
DIL‐based MnCl4 as a new PTC in organic synthesis.
The catalyst has good management and can be stored
for a long time without any reduction in its activity.
2 | RESULTS AND DISCUSSION
2−
PEG‐DIL‐based MnCl4 was prepared according to the
sequence of reactions shown in Scheme 1. The novel
catalyst was carefully characterized using UV–visible,
Fourier transform infrared (FT‐IR) and Raman spectra
as well as thermogravimetric analysis (TGA).
Figure 1 shows the FT‐IR spectrum of the catalyst.
The lack of OH peak in the PEG dichloride spectrum
and appearance of absorption band of C―Cl at 663 cm−1
suggest that the reaction between PEG and SOCl2 had
successfully occurred (Figure 1c). In the next step,
PEG‐based bis(1‐methylimidazolium) was synthesized
by the reaction of PEG dichloride (1 mmol) and
1‐methylimidazole (2 mmol) under neat conditions.[23]
The disappearance C―Cl peak at 663 cm−1, and
FIGURE 1 FT‐IR spectra of (a) Cl‐PEG‐Cl, (b) PEG‐DIL and (c)
PEG‐DIL‐MnCl4
appearance of peaks at 1090, 3108, 1455 and 1555 cm−1
related to N―CH2, C―H and C―C bands of imidazole,
suggest that the reaction had occurred successfully
(Figure 1b).

GOODAJDAR ET AL.
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2−
The U–visible spectrum of PEG‐DIL‐MnCl4 is con-
sistent with the structure composed of imidazolium cat-
2−
ion and MnCl4 anion. The strong band at 230–270 nm
relates to the π–π* transition at the imidazolium cation
and the band at 300–400 nm corresponds to charge trans-
fer of Mn2+ (Figure 2).
The thermal stability of PEG‐DIL‐MnCl4 was studied
by the use of TGA, under a nitrogen atmosphere. From
the TGA curve, it can be concluded that PEG‐DIL‐based
2−
MnCl4 is thermally stable up to about 250°C. Weight
loss up to 200°C is related to evaporation of absorbed
water. Significant weight loss occurs at 250–400°C, which
is related to the degradation of imidazole rings and PEG
chains (Figure 3).
The Raman spectrum was used to prove the presence
of MnCl42− mineral in the structure of the ionic liquid. As
shown in Figure 4, strong and sharp peaks at 1296 and
1590 cm−1 are related to imidazole rings, while the rela-
tively weak peaks at 164.9 and 431.85 cm−1 can be attrib-
uted to the presence of MnCl42− anion in the structure of
the ionic liquid.
After characterization, to evaluate the catalytic activ-
ity of PEG‐DIL‐MnCl4 in organic synthesis, the reaction
of benzyl chloride with sodium azide in water was chosen
as a model study. The model reaction was carried out
FIGURE 4 Raman spectrum of PEG‐DIL‐MnCl4
with varying conditions, such as amount of catalyst, polar
and non‐polar solvents and a wide range of temperature.
As evident from Table 1, the best results (87% yield and
40 min) were obtained at 100°C in the presence of 0.2 g
of catalyst (Table 1, entry 4). It is worth noting that, in
the absence of PEG‐DIL‐MnCl4 as a polymeric PTC, the
reaction was sluggish and, even after long reaction times,
a considerable amount of starting material remained
(Table 1, entry 1).
To show the generality of this new approach, a wide
variety of benzyl halides were reacted with sodium azide
under the optimized conditions to afford the correspond-
ing benzyl azides in good yields (Table 2; Scheme 2).
In a further study with the same method, the reaction
of benzyl halide (1 mmol) and ammonium thiocyanate
TABLE 1 Effect of amount of PEG‐DIL‐MnCl4, temperature and
solvent in the synthesis of benzyl azide
Catalyst
(g)
Temp.
(°C)
Time
(min)
Yield
(%)
FIGURE 2 Visible spectrum of PEG‐DIL‐MnCl4
Entry
Solvent
1
—
100
100
100
100
100
100
100
100
100
100
25
H2O
180
90
—
40
2
0.05
0.1
0.2
0.3
0.2
0.2
0.2
0.2
0.2
0.2
0.2
H2O
3
H2O
90
70
4
H2O
40
87
5
H2O
40
88
6
CH3CH2OH
CH3CN
DMSO
CH3Cl
CH2Cl2
H2O
60
88
7
60
88
8
30
89
9
40
—
10
11
12
180
180
180
—
Trace
30
60
H2O
FIGURE 3 TGA curve of PEG‐DIL‐MnCl4

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GOODAJDAR ET AL.
TABLE 2 Reaction of various benzyl halides with NaN3 in the
presence of PEG‐DIL‐MnCl4 in water
TABLE 3 Reaction of various benzyl halides with NH4SCN in the
presence of PEG‐DIL‐MnCl4
Time
(min)
Yield
(%)
Time
(min) (%)
Yield
Entry
Benzyl halide
Product
Entry Benzyl halide
Product
1
35
30
40
40
84
90
86
87
1
40
35
50
50
86
88
85
86
2
3
4
2
3
4
5
6
50
40
83
84
5
55
83
6
7
40
40
87
85
7
8
35
82
—
300
—
The catalyst has three parts (anionic, cationic and ether
chain). The ether chain operates like a crown ether by
trapping sodium cations. In the anionic part, Mn2+ acts
like a Lewis acid with an empty orbital. This increases
the reaction speed by connecting to the leaving group.
The 1‐methylimidazole units introduced ionic liquid
character to the catalyst (Scheme 4).
(2 mmol) anion under optimal conditions was achieved.
As evident from Table 3, various benzyl halides such as
those bearing electron‐donating and electron‐withdrawing
substituents in water under reflux conditions were con-
verted into the corresponding benzyl thiocyanates in excel-
lent yield (Scheme 3; Table 3).
Primary alkyl halides could be efficiently converted to
the corresponding alkyl thiocyanates, whereas secondary
alkyl halides such as bromocyclohexane did not convert
after 5 h. As a consequence, there is more crowding in
the transition state. The energy barrier for the formation
of the transition state increases with the size of the groups
bonded to the reactive centre, so steric effects are very
important. As the reaction proceeds through the SN2
pathway, the reaction rate depends on the steric effects
of the substrates. Therefore, benzyl halides bearing
electron‐donating and electron‐withdrawing substituents
were converted to the corresponding product at the same
rate.
The advantage of using this PTC for the synthesis of
benzyl azide using azide ion is evident from a comparison
of our results with those previously reported in the litera-
ture (Table 4).
PEG‐DIL‐MnCl4 showed marked activity as a Lewis
acid reagent and considerably accelerated the reactions.
3 | EXPERIMENTAL
All chemical reagents and solvents of high purity were
purchased from Merck and Aldrich and used without
any purification. FT‐IR spectra were recorded with a
Frontier FT‐IR (PerkinElmer) spectrometer using KBr
discs. Electronic spectra were recorded with a Shimadzu
UV 160 spectrophotometer. Reaction monitoring and
determination of purity of products were accomplished
with TLC using silica gel SILG/UV 365 and 254 plates.
Products were characterized by comparing their physical
1
data and FT‐IR, H NMR and 13C NMR spectra with
1
those of known samples. H NMR and 13C NMR spectra
were recorded with a Bruker Advance DPX 400 NMR
instrument, using tetramethylsilane as an internal
reference.
SCHEME 2 Synthesis of benzyl azide
from benzyl halide catalysed by PEG‐DIL‐
MnCl4

GOODAJDAR ET AL.
5 of 6
SCHEME 3 Synthesis of benzyl
thiocyanate from benzyl halide catalysed
by PEG‐DIL‐MnCl4
NaN3/NH4SCN (2 mmol) and Mn‐DIL‐MnCl4 (0.2 g) in
water (5 ml) were magnetically stirred and heated for
an appropriate time (Table 2). The progress of the reac-
tion was monitored using TLC (eluent: n‐hexane–ethyl
acetate 80:20). After completion of the reaction, the prod-
uct was extracted with diethyl ether (2 × 10 ml). The
extracted was dried over anhydrous calcium chloride,
and concentrated under vacuum to afford benzyl
azide/thiocyanate in 82–90% isolated yields (Tables 2
and 3).
TABLE 4 Comparisons of the results of the present system with
those of some recently reported procedures
Time Yield
Entry Product Conditions
(min) (%)
Ref.
[24]
1
Benzyl
azide
β‐CDPU‐MNPs/water/
90°C
15
84
[25]
2
Benzyl
azide
Poly[N‐(2‐aminoethyl)
acrylamide]
60
90
trimethylammonium
chloride/CH3CN/
H2O/100°C
[26]
3
Benzyl
azide
Surfactant pillared clay 150
materials and
87
4 | CONCLUSIONS
sonochemistry/
hexane–water/5°C
In summary, a simple green approach for the preparation
of aryl azides and thiocyanates has been reported. The
main advantages of this method are easy workup, high
efficiency, eliminating toxic solvents and reducing envi-
ronmental impacts. The synthesized polymeric catalyst
also has high chemical and thermal stability and can be
stored for a long time without any significant reduction
in its chemical activity and efficiency. We characterized
the novel catalyst using FT‐IR, UV–visible and Raman
spectra and TGA.
[27]
4
5
Benzyl
azide
MNPs‐βCD/H2O/100°C
35
30
84
90
Benzyl
azide
PEG‐DIL‐MnCl4/H2O/
100°C
This
work
ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support
of this work by the Research Council of Islamic Azad
University of Omidiyeh.
SCHEME 4 Possible mechanism for promotion of reaction by
PEG‐DIL‐MnCl4
ORCID
Bijan Mombeni Goodajdar
8186-9821
http://orcid.org/0000-0001-
3.1 | Procedure for preparation of
PEG‐DIL‐based MnCl4
2−
A solution of (0.2 g, 1 mmol) of MnCl2⋅4H2O in ethanol
(10 ml) was added to a solution (1.0 mmol) of PEG‐
DIL[18] in ethanol (10 ml). After heating under reflux
for 4 h, affording a pale yellow solution, concentration
under reduced pressure led to a viscous liquid (90%).
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How to cite this article: Goodajdar BM, Akbari
F, Davarpanah J. PEG‐DIL‐based MnCl42−: A novel
phase transfer catalyst for nucleophilic substitution
reactions of benzyl halides. Appl Organometal
Chem. 2018;e4647. https://doi.org/10.1002/aoc.4647
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