Sulfination of acetanilide using liquid coordination complexes as dual catalyst and solvent

Article abstract of DOI:10.1016/j.mcat.2018.10.019

The Freidel-Crafts sulfination reaction of acetanilide with sulfur dioxide was catalyzed by several AlCl₃-based liquid coordination complexes (LCCs), and the results were compared with that catalyzed by traditional ILs. Influences of different catalysts, ligand/AlCl₃ molar ratios, and reaction conditions on their catalytic performance were investigated. The optimal result was obtained using acetamide-AlCl₃-based LCC (molar ratio of acetamide to AlCl₃ was 0.65, marked as 0.65 A A/AlCl₃) as the catalyst, giving nearly 100% conversion of acetanilide within 60 min and 94.07% of selectivity to 4-acetamidobenzenesulfinic acid. AA/AlBr₃ was synthesized and also showed catalytic activity for the sulfination reaction. Then a plausible reaction mechanism was proposed by investigation through the raman spectra.

Full text of DOI:10.1016/j.mcat.2018.10.019

MolecularCatalysis462(2019)56–60
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Sulfination of acetanilide using liquid coordination complexes as dual
catalyst and solvent
Jinghua Shen, Fumin Wang⁎, Tianlei Wang, Haichao Li, Gang Wang, Xubin Zhang⁎
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The Freidel-Crafts sulfination reaction of acetanilide with sulfur dioxide was catalyzed by several AlCl3-based liquid
coordination complexes (LCCs), and the results were compared with that catalyzed by traditional ILs. Influences of
different catalysts, ligand/AlCl3 molar ratios, and reaction conditions on their catalytic performance were investigated.
The optimal result was obtained using acetamide-AlCl3-based LCC (molar ratio of acetamide to AlCl3 was 0.65, marked
as 0.65 A A/AlCl3) as the catalyst, giving nearly 100% conversion of acetanilide within 60 min and 94.07% of selectivity
to 4-acetamidobenzenesulfinic acid. AA/AlBr3 was synthesized and also showed catalytic activity for the sulfination
reaction. Then a plausible reaction mechanism was proposed by investigation through the raman spectra.
Acetanilide
Freidel-Crafts sulfination
Liquid coordination complexes
AlCl3-based
1. Introduction
Lewis acidities and redox potentials [20], which have gained widespread
attention in recent years. Abbott et al. [21–23] reported the mixtures of
Sulfinic acids and their salts are a class of industrially important inter-
mediates extensively used for the preparation of various organic compounds
with sulfonyl motifs (-SO2-). Several methods have been described for the
synthesis of sulfinates, including reduction of the corresponding sulfonyl
chlorides [1,2], the oxidation of thiols [3], iron-catalyzed radical coupling
reaction [4], Freidel-Crafts sulfination reaction [5,6] and so on [7–9].
Freidel-Crafts sulfination reaction-based routes are the most straightforward
approaches to make sulfonates. However, the use of an over stoichiometric
amounts of Lewis acids such as AlCl3 and implemented regulations to limit
the use of volatile organic solvents can induce serious environmental ha-
zards and safety problems, especially in industrial-scale processes. Lewis
acidic ionic liquids (ILs), especially those chloroaluminate ILs, have been
thoroughly studied [10–12] as alternatives to conventional solvents and
catalysts due to their unique physicochemical properties. Some significant
progress has been made in their catalytic applications such as in Friedel-
Crafts chemistry [13–15], Diels-Alder reactions [16], Condensation [17],
Isomerization and Oligomerization [18,19]. Despite their excellent catalytic
performance, they suffer from some inherent drawbacks, such as moisture
sensitivity, high toxicity and cost, and preparation complexity.
simple amides and AlCl3 could form ILs with mental-containing anions and
cations. Coleman et al. [20] studied the speciation of the liquid systems
formed by AlCl3 and donor molecules with different molar ratios, and
identified the existence of equilibrated cationic, anionic and neutral co-
ordination complexes of AlCl3. Hogg et al. [24] and Matuszek et al. [25]
reported that LCCs based on AlCl3 exhibited better catalytic performance
than traditional chloroaluminate ILs in the oligomerization of a-olefins and
Friedel-Crafts alkylation of benzene. Hu et al. [26] used several amide-
based LCCs to catalyze the alkylation of isobutene with 2-butene and in-
vestigated the effects of amide structure, amide/AlCl3 molar ratios, CuCl
modification on their catalytic activities. However, there is very little lit-
erature about the Freidel-Crafts sulfination catalyzed by LCCs.
In the present paper, several AlCl3-based LCCs were used as both solvent
and catalyst in the Freidel-Crafts sulfination of acetanilide with SO2 to form
the 4-Acetamidobenzenesulfinic acid, which could be used for synthesizing
the important intermediate of reactive dyes. The results were compared
with those catalyzed by traditional ILs. The effects of a range of variables on
the sulfination procedure were studied together with the plausible reaction
mechanism.
Less investigated alternatives are liquid coordination complexes (LCCs)
based on the mixtures of metal halides and donor molecules. As a new
generation of liquid Lewis acids, LCCs not only have the potential to in-
herently own all the remarkable properties of ILs, but are superior to tra-
ditional ILs in terms of cost and ease of preparation. Moreover, the much
wide spectrum of species in LCCs provides routes for the broad ranges of
2. Experimental
2.1. Materials
All chemicals were used without further purification unless
⁎ Corresponding author.
E-mail addresses: wangfumin@tju.edu.cn (F. Wang), tjzxb@tju.edu.cn (X. Zhang).
https://doi.org/10.1016/j.mcat.2018.10.019
Received 21 June 2018; Received in revised form 15 September 2018; Accepted 15 October 2018
Availableonline01November2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

J. Shen et al.
MolecularCatalysis462(2019)56–60
otherwise stated. Acetanilide (99%), acetamide (AA, 99%), N-methy-
lacetamide (NMA, 99%), urea (Ur, 99%), thiourea (SUr, 99%), trie-
thylamine hydrochloride (Et3NHCl, 99%), 1-Butyl-3-methylimidazo-
lium tetrafluoroborate ([BMIM]BF4, 99%) and anhydrous AlCl3 (99%)
were purchased from Shanghai Molbase Biological Technology Co., Ltd.
1-Butyl-3-methylimidazolium chloride ([BMIM]Cl, 99%), anhydrous
FeCl3 (> 98%) and AlBr3 (> 98%) were purchased from Aladin
chemistry company. SO2 (99.99%) were obtained from Baoding North
Special Gases Co., Ltd.
Table 1
Influence of various LCCs and ionic liquids on sulfination reaction.
Entry
Catalysts
Time (h)
Acetanilide conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
AA/AlCl3
1
3
2
2
2
2
6
6
100
100
100
0
94.07
93.86
90.06
0
NMA/AlCl3
Ur/AlCl3
SUr/AlCl3
[BMIM]/BF4
[BMIM]Cl/FeCl3
[BMIM]Cl/AlCl3
Et3NHCl/AlCl3
0
0
0
0
42.02
98.94
38.24
90.03
2.2. Preparation of LCCs and ILs
Reaction conditions: catalysts: 43 mmol, acetanilide: 2 mmol, SO2: 2 equiv, li-
LCCs and Et3NHCl/AlCl3 ionic liquid with different ligand/metal
halide molar ratios were synthesized under a nitrogen atmosphere and
typical experiments were described as below. The donor ligands were
slowly added over 30 min to weighed amounts of metal halide in a
100 ml three-neck flask placed in the oil bath with mechanical stirring.
Then the mixture was heated to 80 ℃ and remained constant until
homogeneous liquid was obtained.
gand/ metal halide = 0.65:1, temperature: 90 ℃.
[BMIM]Cl/AlCl3 and [BMIM]Cl/FeCl3 were prepared by slowly
adding weighed individual AlCl3 and FeCl3, respectively to the imida-
zolium salt, then left overnight with stirring at room temperature to
obtain the homogeneous mixture.
2.3. General sulfination reaction procedure
The sulfination of acetanilide with SO2 was carried out in a 50 ml
three-neck flask with magnetic stirrer under a dry nitrogen atmosphere.
In a typical run, 0.27 g (2 mmol) of acetanilide was charged into the
50 ml flask containing 43 mmol solvents with constant stirring. 2
equivalents of SO2 were fed into the solvents after acetanilide was
completely dissolved. The reaction was carried out at 90 ℃ and reacted
for selected period of time with continuous stirring. After completion of
the reaction, the liquid mixture was cooled to room temperature and
immediately analyzed with high performance liquid chromatography
(HPLC). The detailed product isolation procedure and analysis were
described in the Supplementary Material.
Fig. 1. Influence of LCCs and Et3NHCl/AlCl3 on the reaction rate. Conditions:
catalysts: 43 mmol, ligand/AlCl3 = 0.65:1, acetanilide: 2 mmol, SO2: 2 equiv,
temperature: 90 ℃, time: 180 min.
2.4. General solubility measurement procedure of SO2
results are illustrated in Fig. 1. It was observed that reaction rate for the
sulfination reaction catalyzed by LCCs was much faster than that of
Et3NHCl/AlCl3 ionic liquid. AA/AlCl3 showed the superior reactivity over
other catalysts, giving 76.79% conversion of acetanilide over 10 min and
nearly 100% conversion within 60 min. However, Et3NHCl/AlCl3 ex-
hibited a relatively flat reaction rate throughout the reaction and took
more than 3 h to reach the complete conversion of acetanilide. Activity
differences between these catalysts might be explained on basis of their
Lewis acidity. In order to better understand the reaction mechanism and
correlations between the acidity and activity in sulfination reactions, we
used acetonitrile as a probe molecule to determine the Lewis acidity of
these catalysts by infrared spectroscopy [27]. Pure acetonitrile has two
characteristic CN stretching bands at 2293 cm−1 (band 2) and 2252 cm−1
(band 1). The addition of acetonitrile to Lewis acidic ionic liquids results in
the appearance of a new band (band 3) at 2300–2400 cm−1. All the three
bands shift to higher wavenumbers with increasing acidity, with band 2
being more sensitive than others. FIeIR spectra in Fig. 2 show the new
band appearing at 2337 cm−1 when acetonitrile was mixed with the cat-
alysts, indicating the presence of Lewis acidic sites. Moreover, the different
degrees of high wavenumber offset (band 2) of these catalysts demon-
strated that the Lewis acidity increases in the order Et3NHCl/
AlCl3 < NMA/AlCl3 < Ur/AlCl3 < AA/AlCl3 < SUr/AlCl3. The result was
in accord with the reactivity order illustrated in Fig. 1 except for SUr/AlCl3
that interacted with SO2. It also indicated that the acidity of LCCs was
stronger than that of traditional Et3NHCl/AlCl3 ionic liquid, and the strong
Lewis acidity was benefit for sulfination reaction. Hence AA/AlCl3 was
taken up for further investigations.
Measurement of SO2 solubility in the solvents was carried out under
the ambient pressure. The detailed method was described in the
Supplementary Material.
3. Results and discussion
3.1. Acetanilide-SO2 sulfination catalyzed by LCCs and ionic liquids
For both LCCs and ionic liquids, the different ligands can greatly in-
fluence the catalytic performance of catalysts. Various ionic liquids and
AlCl3-based LCCs were used to catalyze the sulfination of acetanilide with
SO2 and the results obtained are presented in Table 1. The most investigated
[BMIM]- and Et3NHCl-based ILs were chosen to represent the traditional
acidic ILs, and donor ligands for LCCs were selected include O-donors (AA,
Ur), N-donor (NMA), S-donor (SUr). [BMIM]BF4 and [BMIM]Cl/FeCl3 were
not active and none of the product was formed in these mixtures (entries 5
and 6). The [BMIM]Cl/AlCl3 with stronger Lewis acidity showed activity,
indicating that the acidity plays a major role in their catalytic performance
(entry 7). The SUr could directly react with sulfur dioxide, leading to the
deactivation of SUr/AlCl3 in the sulfination reaction with no acetanilide
conversion observed (entry 4). Whereas, all other AlCl3-based catalysts ex-
hibited great catalytic activity and excellent SO2 absorption capacity (Fig.
S5) in this reaction, giving almost 100% of acetanilide conversion and
90.03–94.07% yield to desired product (entries 1–3 and 8).
Effect of different ligands on the reaction rate was then studied and the
57

J. Shen et al.
MolecularCatalysis462(2019)56–60
Fig. 2. FT-IR spectra of LCCs and Et3NHCl/AlCl3 (ligand/AlCl3 = 0.65:1): (a)
pure acetonitrile, (b) Et3NHCl/AlCl3, (c) NMA/AlCl3, (d) Ur/AlCl3, (e) AA/
AlCl3, (f) SUr/AlCl3.
Fig. 3. Raman spectra of AA/AlCl3 as a function of the molar ratio.
3.3. Influence of reaction temperature and time
3.2. Influence of AA/AlCl3 molar ratios
Effect of reaction temperature on the sulfination reaction is shown in
Fig. 4. Reaction temperature can markedly affect both the reaction rate and
selectivity of desired product. In addition, TGA curve (Fig. S3) shows the
excellent thermal stability of the liquid. The conversion of acetanilide in-
creased significantly with the reaction temperature increasing from 15 to
110 ℃. The full conversion of acetanilide could be reached within selected
period of time when temperature was higher than 90 ℃. Selectivity was
slightly affected at temperatures below 90 ℃, however, an obvious decrease
of selectivity was observed with the temperature further raised to 110 ℃.
Therefore, 90 ℃ was selected as the optimal reaction temperature.
Fig. 5 shows the effect of reaction time on the sulfination reaction.
The acetanilide sulfination with SO2 is a fast reaction, thus, the com-
plete conversion of acetanilide could be reached within 60 min. In ad-
dition, the reaction rate was very high during the first 20 min of the
reaction and then gradually got slower with reaction time. It was sug-
gested that the reaction might belong to a second order reaction in
kinetics (Fig. S6). Reaction time had a slight influence on the selectivity
which was almost unchanged as the reaction proceeded.
Effect of different AA/AlCl3 molar ratios on the catalytic perfor-
mance is given in Table 2. The catalytic activity of AA/AlCl3 was in-
creased with the AA/AlCl3 molar ratio variation from 0.95 to 0.65.
Reactions could not be observed until the AA/AlCl3 molar ratio reduced
to 0.75. In addition, reaction rate became faster with the 1 h acetanilide
conversion increased from 84.10% to 100% when the molar ratio
changed from 0.75 to 0.65. However, we could not get the homo-
geneous liquid catalyst in LCC synthesis procedure when the AA/AlCl3
molar ratio further dropped to 0.55, which made it unfavorable to the
reaction. These results might be attributed to the different speciation in
AA/AlCl3 system. Two peaks of [Al2Cl7]− (314 cm-1) and [AlCl4]−
(349 cm-1) were observed in the raman spectra of AA/AlCl3 with dif-
ferent molar ratios, as shown in Fig. 3. For the catalysts with AA/AlCl3
molar ratios of 0.95 and 0.85, neutral [AlCl4]− was the dominant
species in the system and made the catalyst exhibited weak Lewis acidic
leading to no reactivity for the sulfination reaction. When AA/AlCl3
molar ratio decreased from 0.95 to 0.65, the asymmetric splitting de-
gree of AlCl3 increased accompanied with more molecular species
transformed into ionic species and more [AlCl4]- transformed into
[Al2Cl7]- (Formulas 1 and 2 [20,28]). Whereas both the cationic Al
species [AlCl2Ln]+ and [Al2Cl7]− were Lewis acidic, and in turn im-
proved activity of the catalysts. These results also suggested that the
catalytic activity of catalyst increased with the increasing of Lewis
acidity. Hence a suitable AA/AlCl3 molar ratio for the sulfination is
suggested to be 0.65.
[AlCl3Ln]+AlCl3
[AlCl2Ln]+ + [AlCl4]
(1)
+2AlCl
+AlCl
3
2L
[AlCl2L2][AlCl4]
3 [AlCl2L2][Al2Cl7]
(2)
Table 2
Influence of different AA/AlCl3 molar ratios on sulfination.
AA/AlCl3 molar ratio
0.65
0.75
0.85
0.95
Acetanilide conversion (%)
Yield (%)
100.00
94.07
84.10
80.91
0
0
0
0
Fig. 4. Influence of reaction temperature on sulfination. Conditions: catalysts:
Reaction conditions: catalysts: 43 mmol, acetanilide: 2 mmol, SO2: 2 equiv,
43 mmol, AA/AlCl3 = 0.65:1, acetanilide: 2 mmol, SO2: 2 equiv, time: 60 min.
temperature: 90 ℃, time : 60 min.
58

J. Shen et al.
MolecularCatalysis462(2019)56–60
Fig. 5. Influence of reaction time on sulfination. Conditions: catalysts:
43 mmol, AA/AlCl3 = 0.65:1, acetanilide: 2 mmol, SO2: 2 equiv, temperature:
90 ℃.
3.4. Influence of AlBr3
Compared with the AlCl3-based systems, aluminum bromide has
much lower bond energy that has to be broken for the symmetric or
asymmetric cleavage, which may greatly improve their catalytic perfor-
mance [29]. Consequently, we synthesized the AA/AlBr3 LCC and ap-
plied it in the sulfination of acetanilide with SO2, as shown in Table 3. To
our delight that AA/AlBr3 also shows activity for the sulfination reaction
and temperature can markedly influence the results of the procedure.
The reaction proceeded much slower at a relatively low temperature, in
comparison to that catalyzed by AA/AlCl3 at the same temperature.
Whereas, none of the desired product was detected in HPLC analysis
when the reaction temperature was further raised up to 90 ℃. No reac-
tion took place at high temperatures suggested that the activation of the
reactants by AA/AlBr3 required a relatively low temperature. The slower
reaction rate exhibited by AlBr3-based system at low temperature might
be mainly attributed to its higher density and viscosity, which was not
beneficial for the dispersion of SO2 in the liquid catalyst.
Fig. 6. Raman spectra of the solvents before and after SO2 absorption.
3.5. A mechanism of sulfination reaction catalyzed by LCCs
indicating the existence of interaction between SO2 and the liquid
system. Moreover, the intensity of Lewis acidic species [Al2Cl7]- de-
creased with the increasing intensity of [AlCl4]- after the mixture of SO2
and the liquid. On the basis of these phenomena mentioned above, a
plausible sulfination mechanism of acetanilide with SO2 catalyzed by
AA/AlCl3 was shown in Scheme 1. The Friedel-Craft sulfination reac-
tion started with the activation of SO2 by Lewis acidic species to form
the AlX3 adduct of SO2 [6] (Scheme 1). The SO2 adduct could be the key
intermediate of Friedel–Crafts sulfination. Then the intermediate at-
tacked acetanilide to generate the coordinated product, which yielded
the corresponding product of 4-acetamidobenzenesulfinic acid.
In order to explore the sulfination mechanism catalyzed by LCCs,
raman spectra of the solvents before and after SO2 absorption were
analyzed, as shown in Fig. 6. It can be seen from Fig. 6a that the spectra
of 1,2-dichloroethane (DCE) saturated with SO2 showed the typical
1
(SO2) S − O symmetric stretching band at 1150 cm−1. The data is
consistent with the value of 1145 cm−1 for the liquid SO2 reported
before [30]. When SO2 was fed into the AA/AlCl3, the typical (SO2)
S − O band appeared but shifted to a higher wavenumber (Fi1g. 6b),
Table 3
Influence of AlBr3 on sulfination.
Catalysts
Temperature(℃)
Time(h)
Yield (%)
4. Conclusions
AA/AlCl3
AA/AlBr3
AA/AlBr3
AA/AlCl3
AA/AlBr3
AA/AlBr3
15
15
15
90
90
110
1
42.14
trace
12.58
93.47
0
1
24
1
Several AlCl3-based LCCs were applied to catalyze the Freidel-Crafts
sulfination reaction of acetanilide with SO2. Compared to the tradi-
tional methods for the synthesis of sulfinates, the reaction could be
carried out more quickly and efficiently without the limitations of vo-
latile organic solvents. The influences of different catalysts, AA/AlCl3
molar ratios, and reaction conditions on their catalytic performance
1
1
0
Reaction conditions: catalysts: 43 mmol, acetanilide: 2 mmol, SO2: 2 equiv, AA/
metal halide = 0.65:1, temperature: 90 ℃, time :60 min.
59

J. Shen et al.
MolecularCatalysis462(2019)56–60
Scheme 1. Proposed mechanism for sulfination reaction of acetanilide with SO2.
were investigated. Under the same ligand/AlCl3 molar ratio, AA/AlCl3
exhibited superior reactivity over other catalysts, giving nearly 100%
conversion of acetanilide within 60 min and 94.07% of selectivity to 4-
Acetamidobenzenesulfinic acid. AA/AlBr3 was synthesized and applied
to catalyze reactions which also showed activity for the sulfination
reaction. Finally, measurement of SO2 solubility indicated LCCs were
excellent solvents for this reaction and a plausible reaction mechanism
was proposed by investigation through the raman spectra.
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