Phosphonium acidic ionic liquid: An efficient and recyclable homogeneous catalyst for the synthesis of 2-Arylbenzoxazoles, 2-Arylbenzimidazoles, and 2-Arylbenzothiazoles

Article abstract of DOI:10.1039/c8ra01709c

A highly efficient and green strategy for the synthesis of 2-Arylbenzoxazoles, 2-Arylbenzimidazoles, and 2-Arylbenzothiazoles catalyzed by phosphonium acidic ionic liquid has been developed via the condensation of o-Aminophenol, o-phenylenediamines, and o-Aminothiophenol, respectively, with aldehydes. The reaction has a good yield, the broad substrate scope, and mild condition. Triphenyl(butyl-3-sulphonyl)phosphonium toluenesulfonate catalyst was easily obtained from cheap and available starting materials through a one-pot synthesis. Its structure was identified by ¹H NMR, ¹³C NMR, ³¹P NMR, and FT-IR techniques. Other properties including thermal stability and acidity were determined by TGA and Hammett acidity function method. Interestingly, the catalyst can maintain its constantly outstanding performance till the fourth recovery.

Full text of DOI:10.1039/c8ra01709c

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Phosphonium acidic ionic liquid: an efficient and
recyclable homogeneous catalyst for the synthesis
of 2-arylbenzoxazoles, 2-arylbenzimidazoles, and
2-arylbenzothiazoles†
Cite this: RSC Adv., 2018, 8, 11834
Quang The Nguyen,a Anh-Hung Thi Hang,a Thuy-Linh Ho Nguyen,ab
a
Duy-Khiem Nguyen Chaua and Phuong Hoang Tran
*
A highly efficient and green strategy for the synthesis of 2-arylbenzoxazoles, 2-arylbenzimidazoles, and 2-
arylbenzothiazoles catalyzed by phosphonium acidic ionic liquid has been developed via the condensation
of o-aminophenol, o-phenylenediamines, and o-aminothiophenol, respectively, with aldehydes. The
reaction has a good yield, the broad substrate scope, and mild condition. Triphenyl(butyl-3-sulphonyl)
phosphonium toluenesulfonate catalyst was easily obtained from cheap and available starting materials
through a one-pot synthesis. Its structure was identified by 1H NMR, 13C NMR, 31P NMR, and FT-IR
techniques. Other properties including thermal stability and acidity were determined by TGA and
Hammett acidity function method. Interestingly, the catalyst can maintain its constantly outstanding
performance till the fourth recovery.
Received 26th February 2018
Accepted 20th March 2018
DOI: 10.1039/c8ra01709c
rsc.li/rsc-advances
eventually favorable to facilitate the large-scale production of
these important heterocyclic compounds.
Introduction
In recent years, ionic liquids (ILs) have received increasing
interest as environmentally benign media owing to their special
properties such as thermal stability, biodegradability, and non-
volatility.25–29 They have been widely used as catalysts for a large
number of organic transformations.30–39 However, their appli-
cation as catalysts for the synthesis of 2-arylbenzoxazole, 2-
arylbenzimidazoles, and 2-arylbenzothiazoles via the conden-
sation of aldehydes with o-aminophenols, o-phenylenediamine,
and o-aminothiophenols, respectively, has not been known in
the literature. In the continuation of our study in ionic liquids
application, we reported herein the use of phosphonium acidic
ionic liquid as a green and efficient catalyst for the synthesis of
the above mentioned arylated heterocycles. The merits of this
method are short reaction time, high yield, wide substrate
scope, and recyclability of the catalyst.
Benzoxazole, benzimidazole, and benzothiazole are well-known
heterocyclic scaffolds commonly found in various anticancer,
antimicrobial, antidiabetic, anti-inammatory, antioxidant,
anticonvulsant, and analgesic agents due to their wide-
spectrum biological and pharmaceutical properties.1–12 To
date, there have been two pathways for the preparation of these
compounds.13 The rst one relying on the metal-catalyzed cross-
coupling is more competent.14 However, it has several draw-
backs such as expensive catalysts,15 addition of auxiliary
reagents,16 harsh reaction condition,17 prolonged reaction
time,18 low yields,18 volatile organic solvents,19 which cause
a greatly negative impact on the environment as well as an
impediment in the scaling up for industrial production. The
second synthetic pathway involves the condensation of 2-ami-
nophenol with aldehydes or carboxylic acids under acidic
condition.7,20–24 Similarly to metal-catalyzed cross-couplings,
major defects including the use of expensive chemicals and
volatile organic solvents, long reaction times, and tedious work-
up procedures have been still unsolvable. Therefore, the
development of an efficient, reusable, and green catalyst is
Results and discussion
Initially, the condensation of o-aminophenol and benzaldehyde
is chosen as the model reaction for optimization study. The
results were shown in Table 1.
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Science,
Vietnam National University – Ho Chi Minh City, 721337, Viet Nam. E-mail:
thphuong@hcmus.edu.vn
The effect of temperature, reaction time, and phosphonium
ionic liquid loading was investigated by conducting the reaction
ꢁ
at various values of each parameter (r.t. ꢀ120 C, 20–360 min,
bCenter for Innovative Materials and Architectures, Vietnam National University – Ho
and 0–10 mol%). It was indicated that best condition at 100 ꢁC,
20 min, and 7 mol% of catalyst could afford the desired 2-
phenylbenzoxazole in an excellent yield of 91%. In the absence
Chi Minh City, 721337, Viet Nam
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01709c
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Table 1 Optimization data for the synthesis of 2-phenylbenzoxazole
Entrya
Catalyst, (mol%)
Time, (min)
Temperature, (ꢁC)
Yieldb (%)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
7
360
360
90
20
30
40
20
30
30
r.t.
60
80
Trace
47
85
75
92
94
89
91
82
100
100
100
120
100
100
100
100
9
10
11
6
5
0
30
360
75
—
a
b
Reaction conditions: 2-aminophenol (1 mmol), benzaldehyde (1 mmol), solvent-free. Isolated yields by column chromatography (acetone/
petroleum ether ¼ 1/19).
of the catalyst, the failure in the formation of this product was 96% for 2-aryl-5-chlorobenzoxazoles) compared to those re-
observed as a result.
ported in the arylation of non-substituted 2-aminophenol (81–
Next, different catalysts were tested for their performance in 93%). For some arylations in which 2-amino-4-nitrophenol or 4-
the synthesis of 2-phenylbenzoxazole (see detailed experiments nitrobenzaldehyde was employed as starting materials, the
in the ESI†). The desired product was achieved in the best yield unexpected solidication of the ongoing reaction mixture can
using phosphonium acidic ionic liquid while much lower yields slow down the reaction rate as the result of heterogeneity.
were observed in the same reaction mediated by other catalysts. Therefore, adding a small amount of DI water (0.2 mL) to the
With the optimal condition in hand, we explored the reac- reaction mixture in combination with increasing temperature to
tion scope over a large number of aldehydes in the formation of 120 ꢁC can be helpful to signicantly facilitate the reaction
2-arylbenzoxazoles and other analogues (Table 2). In general, (entries 30, 41, 45).
the reactions proceeded smoothly to give the corresponding
As our expectation, this synthetic campaign is well-suited for
products in good to excellent yields. Intuitively, the electronic the preparation of not only 2-arylbenzoxazoles but also other
properties of substituents of benzaldehydes exhibited a little heterocyclic analogues including 2-arylbenzimidazoles and 2-
effect on the reaction. As a common trend, electron-rich alde- arylbenzothiazoles. While 2-aminothiophenol was able to
hydes gave the expected products in slightly better yields than undergo the arylation with the same ease as 2-aminophenol, o-
electron-poor analogues. Only substrates bearing a powerful phenylenediamine required a prolonged reaction time up to
electron-withdrawing group such as p-nitro or o-uoro can give 90 min for a quantitative transformation into expected 2-aryl-
rise to a dramatic yield decrease as low as 68–78% (entry 7, 10, benzimidazoles. Nevertheless, this harsh condition did not
41, 45). Similarly, the substituents on 2-aminophenol have work out in every case. For instance, the arylation of o-phenyl-
certain effect on the yields of the desired products. For 2-ami- enediamine by less reactive 4-nitrobenzaldehyde only afforded
nophenol bearing an electron-donating or a weak electron- the desired product in 68% yield even as a small amount of
withdrawing group such as methyl or chloro, respectively, water was added to the reaction mixture to maintain the
their condensation with a variety of aldehydes could easily homogeneity.
approach the completion with the yields of arylated products
Phosphonium acidic ionic liquid was readily prepared via
above 80%. However, a sharply adverse impact was seen for 2- one-pot two-step procedure in high yield and used as the cata-
amino-4-nitrophenol whose arylated products can be only iso- lyst for esterication.40 However, to the best of our knowledge,
lated in lower yields of 74–81% compared to 81–93% reported no application of this ionic liquid as the catalyst for the
for non-substituted 2-aminophenol (entries 30–35 and 1–6). condensation between 2-aminophenol, o-phenylenediamines or
Meanwhile, on examining the reactivity for a row consisting of 2-aminothiophenol with aldehydes has been reported in the
eight common aldehydes (entries 14–21 and 22–29), both 2- literature so far. Fig. 1 shows the thermal gravimetric analysis
amino-4-methylphenol and 2-amino-4-chlorophenol can be (TGA) of triphenyl(butyl-3-sulphonyl)phosphonium toluenesul-
converted to corresponding heterocyclic adducts in closely fonate. The rst weight loss of 11.64% below 200 ꢁC merely
similar yields (85–92% for 2-aryl-5-methylbenzoxazoles and 86– corresponds to the loss of residual water in the sample. The
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Table 2 Synthesis of 2-arylbenzoxazoles, 2-arylbenzimidazoles, and 2-arylbenzothiazoles catalyzed by [(C6H5)3P(CH2)4SO3H][OTs]a
Paper
Entry
1
RR, X, and Ar
Product
Temperature (ꢁC)
100
Time; (min)
30
Yieldb (%)
91
1a: R ¼ H, X ¼ OH, 2a: Ar ¼ Ph
2
3
4
5
6
7c
2b: Ar ¼ 4-MeC6H4
2c: Ar ¼ 4-t-BuC6H4
2d: Ar ¼ 4-MeOC6H4
2e: Ar ¼ 4-FC6H4
100
100
100
100
100
120
45
45
50
45
45
50
90
92
93
81
91
75
2f: Ar ¼ 4-ClC6H4
2g: Ar ¼ 4-O2NC6H4
8
2h: Ar ¼ 3-FC6H4
2i: Ar ¼ 3-BrC6H4
2j: Ar ¼ 2-FC6H4
2k: Ar ¼ 2-ClC6H4
2l: Ar ¼ 2-HOC6H4
100
100
100
100
120
40
45
45
45
50
80
89
78
90
90
9
10
11
12
13
14
2m: Ar ¼ 4-pyridinyl
100
100
50
35
82
90
1b: R ¼ Me, X ¼ OH, 2a: Ar ¼ Ph
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Table 2 (Contd.)
Entry
15
RR, X, and Ar
Product
Temperature (ꢁC)
120
Time; (min)
40
Yieldb (%)
90
2b: Ar ¼ 4-MeC6H4
16
17
18
19
20
21
22
23
2c: Ar ¼ 4-t-BuC6H4
2d: Ar ¼ 4-MeOC6H4
2e: Ar ¼ 4-FC6H4
120
120
120
120
120
120
100
100
45
50
45
45
50
60
25
45
90
92
87
88
85
81
95
94
2f: Ar ¼ 4-ClC6H4
2m: Ar ¼ 4-pyridinyl
2n: Ar ¼ 4-HOC6H4
1c: R ¼ Cl, X ¼ OH, 2a: Ar ¼ Ph
2b: Ar ¼ 4-MeC6H4
24
2c: Ar ¼ 4-t-BuC6H4
100
50
95
25
26
27
28
29
2d: Ar ¼ 4-MeOC6H4
2e: Ar ¼ 4-FC6H4
100
100
100
120
120
50
45
45
45
50
96
92
95
86
80
2f: Ar ¼ 4-ClC6H4
2m: Ar ¼ 4-pyridinyl
2n: Ar ¼ 4-HOC6H4
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Table 2 (Contd.)
Entry
30
RR, X, and Ar
Product
Temperature (ꢁC)
100, 120
Time; (min)
75, 50
Yieldb (%)
65, 79c
1d: R ¼ NO2, X ¼ OH, 2a: Ar ¼ Ph
2b: Ar ¼ 4-MeC6H4
31
32
33
34
120
120
120
120
50
55
50
60
75
81
81
74
2c: Ar ¼ 4-t-BuC6H4
2d: Ar ¼ 4-MeOC6H4
2e: Ar ¼ 4-FC6H4
35
36
37
38
39
40
41
2f: Ar ¼ 4-ClC6H4
120
120
120
120
120
120
120
60
50
60
60
60
60
70
80
92
88
90
85
91
75c
1e: R ¼ H, X ¼ SH, 2a: Ar ¼ Ph
2b: Ar ¼ 4-MeC6H4
2d: Ar ¼ 4-MeOC6H4
2e: Ar ¼ 4-FC6H4
2f: Ar ¼ 4-ClC6H4
2g: Ar ¼ 4-O2NC6H4
42
43
44
1f: R ¼ H, X ¼ NH2, 2a: Ar ¼ Ph
2d: Ar ¼ 4-MeOC6H4
120
120
120
90
90
90
89
85
90
2e: Ar ¼ 4-FC6H4
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Table 2 (Contd.)
Entry
RR, X, and Ar
Product
Temperature (ꢁC)
120
Time; (min)
90
Yieldb (%)
45
2g: Ar ¼ 4-O2NC6H4
68c
a
Reaction conditions: 2-aminophenol (1 mmol), or 2-aminothiophenol (1 mmol), or o-phenylenediamine (1 mmol); aldehyde (1 mmol); solvent-
b
c
free. Isolated yields by column chromatography (acetone/petroleum ether ¼ 1/19 or ethyl acetate/hexanes ¼ 1/19). Water (0.2 mL) was added
to the reaction mixture.
phosphonium acidic ionic liquid then maintains its stability up a number of C]C stretching vibrations in aromatic rings. The
to 220–225 ꢁC before undergoing two consecutive thermal signals at 1200 cmꢀ1 and 1034 cmꢀ1 are asymmetric and
decomposition steps at 225–400 ꢁC and 400–600 ꢁC corre- symmetric stretching vibrations of S]O bond, respectively.
sponding to the weight loss of 76.75 and 8.81%, respectively.
From the data of TGA as well as latter FT-IR analysis (Fig. 2), it
could be rmly assured that there is no structural deformation
of the ionic liquid at the temperature range of the investigated
arylation of benzoxazole and other analogues (100–120 ꢁC).
The FT-IR spectra of primitively prepared triphenyl(butyl-3-
sulphonyl)phosphonium toluenesulfonate and its recovered
sample are presented in Fig. 2. The broad peak at approximately
3450 cmꢀ1 indicates the presence of hydroxyl group while
overlapped shoulder peaks scattered from 2950 to 3100 cmꢀ1
are assigned to alkyl and aromatic C–H stretching vibrations.
The absorption band at about 1600–1486 cmꢀ1 is paired with
Fig.
2 FT-IR spectra of triphenyl(butyl-3-sulphonyl)phosphonium
toluenesulfonate (a) and its recycled sample after the fourth run (b).
Table 3 Hammett acidity function values of various concentrations of
investigated IL
Entry
IL (mol%)
Amax
[In] (%)
[InH+] (%)
H0
1
2
3
4
5
0
5
6
7
0.361
0.347
0.345
0.329
0.315
100
0
96.20
95.59
91.22
87.28
3.80
4.41
8.78
12.72
ꢀ1.40
ꢀ1.46
ꢀ1.78
ꢀ1.96
Fig.
1 TGA curve of triphenyl(butyl-3-sulphonyl)phosphonium
10
toluenesulfonate.
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Fig. 4 Recycling test of the catalyst.
each other except their cation centers. For the imidazolium-
based sulfonic acid ionic liquid, the delocalization of posi-
tive charge over imidazole ring gives rise to a lessen acidity of
the sulfonic acid moiety. Meanwhile, the sulfonic acid
functional group tethered to a localized positive-charged
center as triphenylphosphonium can behave with more
Fig.
3 The UV/Vis spectra of 4-nitrodiphenylamine indicator
measured in its cosolutions with IL at different concentrations.
acidic property owing to
withdrawing effect of positive charge onto Brønsted
sulfonic acid group.
a better inductive electron-
Finally, the presence of phosphonium functional group is
conrmed by two concomitant absorption bands at 1121 and
1410 cmꢀ1 corresponding with P–Ar and P–CH2 bond defor-
mation, respectively. A close similarity in pattern between two
FT-IR spectra reinforced the above statement on the thermal
stability of phosphonium acidic ionic liquid within the
temperature range of arylation.
The acidity of triphenyl(butyl-3-sulphonyl)phosphonium
toluenesulfonate was examined by means of Hammett
acidity function method using 4-nitrodiphenylamine as an
indicator. H0 values of ionic liquid solutions with a concen-
tration range of 5–10 mol% were calculated from the [In]/
[InH+] ratio which was determined by UV-Vis spectroscopy.
Technically, this ratio is directly proportional to the absor-
bance difference of the indicator in its monocomponent
solution and its cosolution with a given quantity of ionic
liquid. As can be seen from the Table 3 and Fig. 3, the 7 mol%
solution of triphenyl(butyl-3-sulphonyl)phosphonium tolue-
nesulfonate has H0 value of ꢀ1.78 claiming that this ionic
liquid is about 50 times more acidic than [1,2-DiMIMPs][OTs]
analogue with an H0 value of ꢀ0.11 as reported by Yang
et al.41 It could be noted that two analogues almost resemble
The
recyclability
of
triphenyl(butyl-3-sulphonyl)
phosphonium toluenesulfonate was surveyed on the opti-
mized arylation of 2-aminophenol by benzaldehyde. Upon
completion of the reaction, the recovered ionic liquid was
easily separated from other organic matters by washing many
times with diethyl ether. It was subsequently dried in vacuo at
80 ꢁC for 30 min before the reuse for consecutive cycles. It can
be seen from Fig. 4 that only a very minor loss of catalytic
performance was observed over four times of its reuse in the
same condensation of benzaldehyde with 2-aminophenol.
A comparative study of the present method with previous
literatures was reported in Table 4. The phosphonium acidic
ionic liquid-catalyzed condensation between 2-aminophenol
and benzaldehyde afforded the 2-phenylbenzoxazole product
in excellent yield under a mild condition without the demand
for any additives as in previous reports (Table 4). Further-
more, no loss of catalytic activity in the recycling test of this
catalyst is the most prominence of the present method.
Table 4 The comparison of the present method with previous literatures in the synthesis of 2-phenylbenzoxazole
Entry
Catalyst
Condition
Yield (%)
1
2
3
4
5
6
TiO2–ZrO2 (10 mol%), acetonitrile
Hf-MOF (1 mol%), solvent-free
NaCN (10 mol%), DMF, air
Sm(OTf)3 (10 mol%), ethanol–water
Poly(melamine-formaldehyde) (10 mg), oxygen, toluene
Present work: triphenyl(butyl-3-sulphonyl)phosphonium toluenesulfonate (7 mol%), solvent-free
60 ꢁC, 15 min
140 ꢁC, 6 h
91 (ref. 42)
95 (ref. 43)
74 (ref. 44)
92 (ref. 21)
91 (ref. 45)
91
80 ꢁC, 24 h
50–60 ꢁC, 2 h
110 ꢁC, 24 h
100 ꢁC, 30 min
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Scheme 1 The synthetic pathway of triphenyl(butyl-3-sulphonyl)phosphonium toluenesulfonate.
sulphonyl)phosphonium toluenesulfonate (20.5 mg, 7 mol%) in
a 10 mL glass tube at 100 C under solvent-free magnetic stir-
Experimental
Chemicals, supplies, and instruments
ꢁ
ring. Upon completion of the reaction as indicated by TLC aer
30 min, the mixture was diluted and extracted with diethyl ether
(10 ꢂ 5 mL). Then the ethereal solution was washed with water
(2 ꢂ 20 mL) and dried over Na2SO4. The nal product was ob-
tained aer solvent removal by a rotary evaporator followed by
the purication on a silica gel column chromatography using
acetone/petroleum ether (1/19) as an eluent solvent. The struc-
All starting materials were purchased from Sigma-Aldrich and
employed without further purication. Silica gel (230–400
mesh) for ash chromatography was obtained from HiMedia
Laboratories Pvt. Ltd. (India). TLC (silica gel 60 F254) was
purchased from Merck. Ethyl acetate (purity $ 99.5%) and
hexanes ($95%) were obtained from Xilong Chemical Co., Ltd
(China). Chloroform-d (99.8 atom% D, stab. with Ag) was ob-
tained from Armar (Switzerland). GC-MS spectra were taken on
an Agilent GC System 7890 equipped with a mass selective
detector Agilent 5973N and a capillary DB-5MS column (30 m ꢂ
250 mm ꢂ 0.25 mm). FT-IR spectra were recorded in the form of
KBr pellets by a Bruker Vertex 70. 1H and 13C NMR spectra were
recorded on a Bruker Advance II 500 MHz.
1
tural characterization was performed using H, 13C-NMR, and
GC-MS. The recovered catalyst was reactivated by heating under
reduced vacuum at 80 ꢁC for 30 min and reused for next cycles.
Conclusions
In summary, a green and efficient pathway to access to 2-aryl-
benzoxazoles, 2-arylbenzimidazoles, and 2-arylbenzothiazoles
from o-aminophenol, o-phenylenediamines, and o-amino-
thiophenol, respectively, via the condensation with aldehydes
has been successfully developed by outstanding catalytic
performance of triphenyl(butyl-3-sulphonyl)phosphonium tol-
uenesulfonate. Not only did this ionic liquid work out on a wide
range of substrates and reagents, but it also expressed excellent
sustainability of remaining reactivity aer four times of recy-
cling. A simple work-up step allowing to isolate the desired
products as well as simultaneously recover the ionic liquid
catalyst is also an apparent advantage of this process.
Preparation of triphenyl(butyl-3-sulphonyl)phosphonium
toluenesulfonate
Triphenyl(butyl-3-sulphonyl)phosphonium
toluenesulfonate
catalyst was obtained by the one-pot method as depicted in
Scheme 1 (97%).11–14 Its structure was then authenticated by 1H-
NMR, 13C-NMR, FT-IR and TGA. An equimolar mixture of tri-
phenylphosphine and 1,4-butane sultone in toluene solvent (10
mL) was stirred and heated at reux for 12 h to obtain the
zwitterion as a white solid. Then p-toluenesulfonic acid was
added dropwise to the resulting zwitterion until a separated
clear and colorless liquid layer was formed at the bottom of the
round bottom ask aer approximately 6 h. Aer completion of
the reaction, the crude product was washed with diethyl ether (5
ꢂ 20 mL) to remove non-ionic residues. Finally, the ionic liquid
was obtained in 97% yield aer the removal of solvent in vacuo.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
General procedure for the synthesis of 2-arylbenzoxazole
derivatives
This research is funded by Viet Nam National University, Ho Chi
Minh City (VNU-HCM) under grant number 562-2018-18-03. We
2-Aminophenol (109 mg, 1.0 mmol) was treated with benzal- thank Hoang-Tan Le Doan and Ngoc-Mai Hoang Do for their
dehyde (106 mg, 1.0 mmol) in the presence of triphenyl(butyl-3- valuable discussions.
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